The Near-Earth Objects Survey

Workgroup Report

Table Of Contents

Working Group/Contributions

Executive Summary

I. Introduction

II. Background

III. Approach Of The Current Study

IV. The Near-Earth Objects Population

V. Evaluation of survey systems

VI. Precise Orbit Determination and Physical Observations

VII. Cooperation Between Air Force and NASA

VIII. International Cooperation

IX. Coordination of Observing Program

X. NEO Computational and Data Centers

XI. The Broader Significance of NEOs

XII. Suggested Five-Year Budget for NASA Support of NEO Survey

References

Appendix I. Conclusions of Morrison Committe Report

Appendix II. History of Discovery and Observations of NEOs

Appendix III. Evaluation of CCD Systems for Near-Earth Object

Surverys

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/index.html (1 of 3) [12/5/2001 1:41:12 PM]
Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/index.html (2 of 3) [12/5/2001 1:41:12 PM]
Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/index.html (3 of 3) [12/5/2001 1:41:12 PM]
Working Group Members and Contributions

Eugene M. Shoemaker, Lowell Observatory (Chairman)

Alexander A. Boyarchuk, Russian Academy of Sciences

Gregory Canavan*, Los Alamos National Laboratory

Andrea Carusi, Chair of the IAU NEO Working Group

Marcello Coradini, ESA Headquarters

John Darrah, U.S. Air Force Space Command

Alan J. Harris, Jet Propulsion Laboratory

David Morrison, Ames Research Center

Michael J. Mumma, Goddard Space Flight Center

David L. Rabinowitz, Carnegie Institution of Washington

R. Rikhova, Russian Academy of Sciences

With Contributions from:

Clark R. Chapman, Planetary Science Institute

Brian G. Marsden, Smithsonian Astrophysical Observatory

Steven J. Ostro, Jet Propulsion Laboratory

Simon P. Worden, U.S. Air Force Space Command

Donald K. Yeomans, Jet Propulsion Laboratory

* Dr. Canavan must excuse himself from resource recommendations

involving specific institutions.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/work.html (1 of 3) [12/5/2001 1:41:13 PM]
Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/work.html (2 of 3) [12/5/2001 1:41:13 PM]
Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/work.html (3 of 3) [12/5/2001 1:41:13 PM]
Executive Summary

Approximately 2,000 near-Earth objects (NEOs) larger than one km diameter

revolve around the sun on short-period orbits that can occasionally intersect

the orbit of the Earth. Only about 7% of this estimated population has been

discovered. There is about one chance in a thousand that one of these

undiscovered objects is destined to collide with Earth during the lifespan of

the average American. Such a collision has the potential of injecting

sufficient dark material into the atmosphere to cause a major loss of global

crop production and consequent loss of human life.

NASA's charge to the NEO Survey Working Group was to develop a program

plan to discover, characterize and catalog, within 10 years (to the extent

practicable), the potentially threatening comets and asteroids larger than 1 km

in diameter.

Advances in the last few years in the development of charge-coupled devices

(CCDs) as detectors have led to substantial improvement in the projected

capability to carry out a systematic survey of NEOs. In particular, large

format, high quantum efficiency, fast readout CCDs have been developed

under U.S. Air Force sponsored research. The efficiency of these detectors is

now close to the theoretical limit. Use of these detectors on sufficiently large

telescopes would enable rapid progress to be made in an NEO survey.

We have defined a program that responds immediately to the challenge of

discovering potentially threatening NEOs. It would carry out a census of

short-period comets and asteroids larger than 1 km in diameter and seriously

address smaller NEOs and long-period comets, as well as develop a broad

database of physical observations in order to evaluate the impact hazard. This

program, based on further development of existing efforts within the U.S.

civilian astronomical community, could meet restricted objectives (surveying

the short-period NEO population) in ten years following an initial three-year

development phase.

In order to proceed promptly, maximum use needs to be made of existing

telescopes. In particular:

we encourage collaboration of the U.S. Air Force. The Air Force

facilities and technologies will enhance the undertaking. The Air

Force's continued development of large array imaging cameras will be

of value to all the participants;

● 

we encourage collaboration of the international community, including

further development of programs underway in France, Australia,

● 

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/executive.html (1 of 3) [12/5/2001 1:41:13 PM]
China, Canada, Russia and for the European Southern Observatory.

Recommended Program

The recommended program will accomplish the objective of discovering

60-70% of short-period NEOs larger than 1 km diameter within one decade

(by the end of 2006, for funding beginning in FY96). It will also put into

place the assets that will extend completeness above 90% in the following

five years, and extend it both further and to smaller objects in subsequent

years. Anticipated cooperation from the Air Force and international programs

could shift the attainment of 90% completeness forward to 2006, and

significantly augment capabilities for orbit determination and physical

measurements.

The recommended program requires investment in search telescopes,

detectors, and software to fully utilize current technology. Two dedicated

telescopes of about 2-meter aperture are the core of the search system. One of

these is already under construction. State-of-the-art CCD focal-plane arrays

are required in both telescopes. Acquisition of computers and skilled

personnel is required to bring the CCD systems into full operation within

three years. One or two existing telescopes near 1-meter aperture, with

appropriate advanced focal planes, can round out the survey facilities

(capable of both survey work and astrometric follow-up for orbit

determination). In addition, enhanced funding is necessary to obtain

availability of roughly half time on a 3- to 4-meter class telescope for

physical observations of a representative sample of threatening objects.

Enhancement of the capability of the Minor Planet Center will be necessary

to coordinate the program and handle the enormously increased discovery

rates.

The level of funding required to carry out the recommended program is as

follows:

 Year 1 Year 2 Year 3 Year 4 Year 5

 $4.3M $4.3M $4.7M $6.2M $4.5M

The total cost for 5 years is $24 million. This funding is for the NASA

program; funding for participation of Air Force facilities in NEO surveys is

not addressed in this report. Beyond the first 5 years, the annual costs drop

down to operations costs of about $3.5 million per year.

It must be emphasized that, without an initial investment in large array CCDs

with high-quantum efficiency, only moderate improvements will be made

over the present rate of discovery of NEOs. Continuation of the present level

of NASA supported NEO searches (about $1 million per year) will lead to

discovery of about 25% of NEOs larger than 1 km in 10 years and defer 90%

completeness to some time in the middle of the next century (assuming that

the survey would be continued).

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/executive.html (2 of 3) [12/5/2001 1:41:13 PM]
Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/executive.html (3 of 3) [12/5/2001 1:41:13 PM]
I. Introduction

Since its formation, the Earth has been subject to a continuing bombardment

by cosmic debris in the form of asteroids and comets striking at speeds of

tens of kilometers per second. While the atmosphere protects us from most

of the smaller fragments, larger objects (roughly those bigger than 50 meters)

are capable of reaching the lower atmosphere or the surface where they

explode with an energy greater than that of any but the most powerful

nuclear weapons. Impactors larger than a kilometer or so in diameter have

the potential for still greater damage through global environmental effects;

such impacts could place at risk much of the human population and endanger

the survival of civilization. Geologic evidence suggests that occasional rare,

very large impacts in the past have led to mass extinctions of living species.

The widely observed impacts into Jupiter in July 1994 of the fragments of

Comet Shoemaker-Levy 9 released energy measuring in the millions of

megatons of TNT and generated fireballs and dark clouds on Jupiter about as

large as the Earth. These events provided an object lesson on the effects of

large impacts.

Recognizing the potential hazard of asteroidal and cometary impacts, the

United States House of Representatives wrote in its NASA Multi-year

Authorization Act of 1990: "The chances of the Earth being struck by a large

asteroid are extremely small, but since the consequences of such a collision

are extremely large, the Committee believes it is only prudent to assess the

nature of the threat and prepare to deal with it. We have the technology to

detect such asteroids and to prevent their collision with the Earth." The

Committee directed NASA to study both the detection of potentially

threatening asteroids and the technology for possible mitigation if an object

were found on a collision course. These studies were carried out in 1991-92,

and results were released in written form and discussed with the House

Subcommittee on Space in a hearing held March 23, 1993.

Subsequent to that hearing, and reflecting widespread public interest in the

impact of Comet Shoemaker-Levy 9 with Jupiter, the Committee on Science,

Space, and Technology approved the following additional direction to NASA

in July 1994: "To the extent practicable, the National Aeronautics and Space

Administration, in coordination with the Department of Defense and the

space agencies of other countries, shall identify and catalog within 10 years

the orbital characteristics of all comets and asteroids that are greater than 1

km in diameter and are in an orbit around the Sun that crosses the orbit of the

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/intro.html (1 of 2) [12/5/2001 1:41:13 PM]
Earth." The Committee further requested that the NASA Administrator

should submit to Congress a Program Plan for accomplishing this survey.

The present report is a response to this request for a program plan to carry

out a comprehensive ten-year survey of near-Earth asteroids and comets

along the lines of the earlier recommendations of the Spaceguard Survey

Working Group. The present working group was asked by NASA to assess

the technical requirements for such a survey and propose a plan of action to

implement it. This report summarizes the conclusions of that study.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/intro.html (2 of 2) [12/5/2001 1:41:13 PM]
II. Background

The 1992 proposal for an accelerated search for threatening asteroids in

Earth-crossing orbits (The Spaceguard Survey: Report of the NASA International

Near-Earth Detection Workshop, edited by D. Morrison) provided an analysis of

the nature of the impact hazard and proposed an international telescopic survey to

identify threatening objects. Its primary conclusions taken from the Executive

Summary are given in Appendix I. Since the publication of the Spaceguard Survey

Report, a number of international scientific meetings have been held on subjects

related to the impact hazard, including the Space Science Colloquium on the

Hazards of Impacts by Comets and Asteroids (Tucson, Arizona, January 1993), the

Erice Seminar on Planetary Emergencies: Collision of an Asteroid or Comet with

the Earth (Erice, Italy, May 1993), and the conference on Space Protection of the

Earth SPE-94 (Chelyabinsk-70, Russia, September 1994). The International

Astronomical Union has formed a Working Group on Near-Earth Objects (chaired

by A. Carusi of Rome, Italy) to consider the issue and help coordinate international

survey efforts. Several publications have appeared in the peer-reviewed scientific

literature, including an analysis of the impact hazard (C.R. Chapman and D.

Morrison in Nature, 1994). A comprehensive multi-author book covering a wide

variety of issues associated with asteroid and comet impacts has been published

(Hazards Due to Comets and Asteroids, edited by T. Gehrels, University of

Arizona Press 1994). In addition, an unprecedented concentration of scientific and

public attention focused on the impacts of Comet Shoemaker-Levy 9 with Jupiter

in the summer of 1994.

As a result of these accelerating scientific and technical efforts, the nature of the

impact hazard is better understood, and a variety of technical approaches to

mitigating this hazard have been suggested. In general, the conclusions of the

Spaceguard Survey Report have been confirmed and strengthened by these efforts.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/background.html [12/5/2001 1:41:14 PM]
III. Approach Of The Current Study

While the objectives and strategy of the 1992 report remain valid, the technology

for accomplishing those objectives has been improved substantially. The

development of more sensitive detectors and faster electronics now appears to

enable effective surveys with telescopes that are fewer in number and smaller

than the 2 to 3 meter aperture discussed in the previous report. The current study

applies these advances in detection technology to develop proposals for a less

costly approach to implementing a near-Earth objects survey.

The purpose of this report is to outline a practical, cost-effective approach to

implementing the United States elements of an international survey for

Earth-crossing asteroids and comets. In accord with the Congressional request,

the Survey plan is optimized to permit a complete survey, to the extent

practicable, of the roughly 1000 to 2000 Earth-crossing objects larger than 1 km

in diameter, although a great many smaller objects will also be detected and

tracked.

The objective of a rapid survey for the larger Near Earth Objects (NEOs) is best

achieved if existing or currently planned telescopes can be utilized. The Survey

Working Group has therefore emphasized the use of existing assets from both

NASA and the U.S. Air Force for implementing this survey. While this approach

requires the combination of systems with different configurations and

capabilities into the search network, it takes maximum advantage of past

investments and substantially reduces both the cost and the time needed to

implement the survey.

The proposed implementation approach must be flexible, since it involves the

coordinated use of several different instruments which may become available at

different times. The designs suggested for the NEO Survey telescopes, detectors,

and signal-processing systems permit them to be used in a variety of observing

modes, spanning a range in sky coverage and limiting magnitude. There is a

trade-off between the area of sky that a telescope scans and the distance to which

the telescope can detect objects of a given size. A telescope that monitors the

whole sky will not find objects as far from Earth as one that searches a smaller

area of the sky, using longer exposures. The optimum choice can be made for

each instrument individually or for the network as a whole. As individual

telescopes are added to the net, we expect the operating mode for all of the

existing telescopes to be modified to optimize the efficiency of the entire

operation. This flexible approach also permits us to anticipate future coordination

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/approach.html (1 of 2) [12/5/2001 1:41:14 PM]
with international partners without having to specify the exact nature of this

collaboration at the outset.

Until recently, the best telescope for NEO discovery has been the 46-cm Schmidt

telescope at Palomar Mt. This telescope regularly discovers 1 to 2 NEOs per

month, and has done the most to characterize the threat from km-sized NEOs.

Since 1990, however, the 90-cm Spacewatch Telescope of the University of

Arizona, has been finding 2-5 NEOs per month, and is the first telescope to

probe the entire population of objects ranging from about 10m to 10km. This

telescope has proved the potential of CCD detectors and computer technology for

NEO discovery, and established a bench-mark performance level.

With the new instruments and upgraded capabilities described in this report,

individual telescopes with 1 to 2 meter apertures should be capable of increasing

this discovery rate by at least a factor of ten, and we encourage the

implementation of these improvements. However, the full power of the survey

will be reached only when the telescopes are utilized as part of a coordinated

network rather than as individual search efforts. The objective of this program is

an integrated international survey system in which each instrument is assigned

the tasks that it can do best in order to maximize the discovery rate of the entire

survey ensemble.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/approach.html (2 of 2) [12/5/2001 1:41:14 PM]
IV. The Near-Earth Objects Population

There are two broad categories of NEOs: comets and asteroids. Historically,

asteroids and comets have been distinguished by astronomers on the basis of

their telescopic appearance. If the object is star-like in appearance, it is called

an asteroid. If it has a visible atmosphere or tail, it is a comet. This

distinction reflects in part a difference in composition; asteroids are generally

rocky or metallic objects without atmospheres, whereas comets are

composed in part of volatiles (such as water ice) that evaporate when heated

to produce a tenuous and transient atmosphere. However, a volatile-rich

object will develop an atmosphere only if it is heated by the Sun, whereas a

comet that is far from the Sun, or an old comet that has lost most of its

volatile inventory and is insulated with a regolith deposit, also can look like

an asteroid.

The near-Earth asteroids are categorized as Amors, Apollos, and Atens,

according to whether their orbits lie outside that of the Earth, cross that of

the Earth with period greater than 1 year, or cross that of the Earth with a

period less than 1 year, respectively. Cometary objects are classed as

short-period if their periods are less than 20 years, intermediate-period if

their periods are between 20 and 200 years, and as long-period if their

periods are greater than 200 years.

Even more relevant to this report is the definition of an Earth-crossing

asteroid (ECA). These are the asteroids that have the potential to impact our

planet. An ECA is defined as an asteroid moving on a trajectory that is

capable of intersecting the capture cross-section of the Earth as a result of

on-going long-range gravitational perturbations due to the Earth and other

planets. In this case "long-range" refers to periods of tens of thousands of

years. For any particular NEO, it will not be clear whether it is in fact an

ECA until an accurate orbit is calculated. Further, as we are concerned here

with the near-term hazard of collision with Earth, a survey for hazardous

objects need consider only a subset of the ECAs, those that can pass within a

specified distance of the Earth in the relatively near future (see Section VI).

In 1989 there were 90 known ECAs, while 128 ECAs were known at the

time the Spaceguard Survey Report was written in 1992. Effective January 1,

1995, the number of known ECAs is about 250. The population of

Earth-crossing asteroids can be approximated by several power laws, which

reflect a steep increase in the number of ECAs as we go to smaller and

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/population.html (1 of 3) [12/5/2001 1:41:15 PM]
smaller sizes; from such distributions one can estimate the total number of

asteroids having diameters larger than values of particular interest. Based on

analyses of the discovery statistics of ECAs and also the cratering record on

the Moon (Rabinowitz et al., 1994), there are probably 1 to 2 thousand larger

than 1 km diameter (the size for which this survey seeks completeness), 4 to

8 thousand larger than 500 m ( a size representative of great potential

damage from tsunamis), and 0.5 to 1.5 million larger than 50 m (the

threshold for penetration of the lower atmosphere). Active comets can also

cross the Earth's orbit with the potential for collision. At any given size,

active short-period and intermediate-period comets contribute only an

additional 1 percent or so to the total collision frequency, a value that is

small compared to the estimated uncertainty in the ECA population.

However, recent evidence indicates that inactive short- and

intermediate-period comets may be about 10 to 20 times as numerous as the

active, easily discovered comets (Shoemaker et al., 1994).

Although about 700 longer-period comets are known to have passed through

the inner solar system during recorded history, their total population is

difficult to characterize since the majority remain unobservable in the

outermost regions of the solar system. From their orbital and size

distributions, we estimate that the near-Earth flux of long-period comets is

similar to the flux of active and inactive short- and intermediate-period

comets. The total flux of Earth-crossing comets may be somewhere between

10 and 40 percent that of the Earth-crossing asteroid population.

The well-observed ECAs exhibit a diversity in infrared mineralogy

approaching that in the rest of the asteroid population. The majority are

expected to be similar to the dark C-class asteroids in general properties

(presumably moderately low-density, colored black due to the presence of at

least several percent of opaques). There are also a large number of S-class

asteroids. (S's are thought to be either stony, chondrite-like objects,

stony-iron objects, or a combination of both.) In addition, there are known

examples of metallic bodies (probably like nickel-iron alloy meteorites) and

rocky, monomineralic bodies. ECAs are often quite irregular in shape; they

also tend to have rather rapid spins, but there is a great diversity in such

properties. Radar images of several such objects (Castalia, Toutatis,

Geographos) show that they have a wide diversity of shapes and possible

structure. In the case of Castalia, two fragments appear to be merged to form

a dumbbell-shaped double-lobed object.

It is particularly uncertain what the physical properties of comets (extinct or

active) might be like. Only one comet has been studied in detail: Comet

Halley, which was the target of several flyby spacecraft missions at the time

of its last apparition in 1986. The nucleus of Halley is irregular and dark,

with an average diameter of about 9 km. Like other comets, it is made of a

combination of ices and dust, with much of the atmospheric outgassing near

the Sun confined to discrete plumes or jet-like features ("jets"). The

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/population.html (2 of 3) [12/5/2001 1:41:15 PM]
non-volatiles include both silicates and organic materials, while the primary

ices (with percentages derived for Halley) are water (80%), carbon monoxide

(7% - another 8% comes from organic dust, a distributed source in the

coma), carbon dioxide (3.5%), plus smaller amounts of methyl alcohol,

ammonia, hydrogen cyanide, and hydrogen sulfide. The physical

configuration of comets is even less well understood than that of the small

asteroids, and many comets have been observed to split under rather modest

tidal and thermal forces. A direct estimate of density is derived from the tidal

disruption of Comet Shoemaker-Levy 9, yielding a value near 0.5 g/cm .

For cataloging potential impactors, it is not essential to know a great deal

about the physical nature of comets and asteroids. The most important

properties are simply their mass and impact velocity, although it would make

a difference if the projectile were double or multiple and easily came apart as

it entered the atmosphere. However, a future program for intercepting and

diverting an incoming comet or asteroid probably will require knowledge of

the configuration, density, cohesion, and composition of these objects. For

these reasons, in addition to their significance for basic science, spacecraft

missions to comets and near-Earth asteroids will become essential for

understanding the impact threat.

A program aimed at discovering potential Earth impactors will, by necessity,

be focused on ECAs. Because of their larger orbits and longer periods of

revolution, the discovery of Earth-crossing comets will require a much

longer-term effort and presents a difficult challenge.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/population.html (3 of 3) [12/5/2001 1:41:15 PM]
V. Evaluation Of Survey Systems

Introduction

The evaluation of the performance of a given survey system depends primarily

on only two parameters: the threshold brightness for detection (limiting

magnitude), and the rate of sky coverage. Of much lesser importance are such

factors as the geographic location of the observing site(s), the ability to detect

rapidly moving objects compared to the sensitivity to stationary targets, and the

detailed strategy used to follow up detections to obtain preliminary orbits. For the

proposed NEO survey, we will show that the best strategy for maximizing the

rate of discovery of NEOs is to cover the entire observable sky each month. An

optimum search system should be designed to be capable of fast enough

operation to achieve all-sky coverage, sacrificing limiting magnitude as

necessary to achieve this goal. Thus we can, within the uncertainties of the

models employed, reduce the problem to a single parameter: the limiting

magnitude that a given system can deliver in the mode of covering the whole sky

each month. The practical achievement of this mode has become possible with

the present state of development of CCDs.

In this chapter, we will present the results of a survey simulation to show the

level of completeness that can be expected from putative survey systems as a

function of time (length of survey), area of sky covered per month (from which

we derive the above conclusion for all-sky coverage), and size of NEO. We can

then simply relate these results to specific systems through estimates of the

limiting magnitude achievable with a given system. In Appendix III we give a

more detailed report of the evaluation methods and results for specific systems.

In this chapter, we summarize the search strategies and expected capabilities in

more general terms.

Survey Systems

For the purpose of a quantitative discussion, we shall evaluate survey

completeness for three rather specific systems. However it should be noted that

results can easily be scaled to other systems that might be contemplated. The

three systems are representative in general terms of systems of 0.5-m, 1-m, and

2-m aperture. Following is a brief description of each system.

The Lowell Observatory Near-Earth Object Survey (LONEOS) telescope

is a modified Schmidt telescope of 0.58 m aperture, 1.11 m focal length

(f/1.91), which is under construction at Lowell Observatory. "First light" is

1. 

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (1 of 8) [12/5/2001 1:41:18 PM]
expected during this year. Initially, it will be equipped with a two CCD

chips with 2048 x 2048 pixels, 15 microns square, or a field format of 3

cm by 6 cm. Eventually, it is planned to use two butted 2048 x 4096 chips

with 15 micron pixels, for a 6 cm square format, which yields an angular

field of view 3.17 on a side, or an area of 10.1 square degrees. It is

planned to use front-illuminated, unthinned CCDs with a quantum

efficiency of ~25%. We estimate that this system can reach a limiting

visual magnitude of 19.4 with 68 second exposures. In our evaluations, we

consider the "full-up" system with (4096)2 pixels.

The USAF Space Command currently operates a network of 1-m, f/2

wide-field telescopes, the Groundbased Electro-Optical Deep Space

Surveillance (GEODSS) system, for tracking Earth satellites. The

GEODSS Upgrade Prototype System (GUPS), currently under

development, will employ large format CCD detectors, which with only

minor modifications and changes to the computer software, might be

effectively employed for NEO surveys. The CCD detector under

development at Lincoln Laboratory is a single chip of 1960 x 2560 pixels,

24 microns square, or a total format of 4.7 cm by 6.1 cm. In the GEODSS

telescope, this yields an angular field of 1.23 by 1.61 , or 1.98 square

degrees. The chip is thinned, back-illuminated, with a quantum efficiency

exceeding 75%. We estimate that this system can reach a limiting

magnitude of 20.2 with 20 second exposures.

2. 

The Spacewatch (SW) Telescope on Kitt Peak, Arizona. The present

(operating) system is a 0.9 m telescope of 4.6 m focal length (f/5) with a

single CCD detector with 2048 x 2048 pixels of 24 microns, or a total

format 4.9 cm on a side. The detector is thinned, back-illuminated, with a

quantum efficiency of ~75%. SW has a demonstrated limiting visual

magnitude of ~21.2 with a 147 second exposure covering a 0.57 square

degree field. Since it is the only currently operating system, we have

estimated the limiting magnitudes expected for the other systems by

scaling from the demonstrated performance of SW.

3. 

A second telescope (SW-II) of 1.8m aperture and 4.9m focal length (f/2.7)

is under construction. Initially, it will be equipped with a similar CCD

detector, which will yield a field of view of 0.57 on a side. In a scanning

mode with a 30 second integration time, this system should reach a

limiting visual magnitude of 21.5. With this detector and exposure

arrangement, SW-II cannot achieve all-sky coverage each month (to be

discussed later). It could do so with a mosaic of 4 butted CCD chips,

giving a field of view of 1.14 on a side, and rapid read-out electronics so

that it could take individual exposures as short as 10 seconds. The

telescope is mechanically and optically capable of accommodating this

array and exposure rate. With 10 second exposures, the limiting visual

magnitude would be about 20.9.

4. 

Survey Simulation

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (2 of 8) [12/5/2001 1:41:18 PM]
The approach taken was to generate a set of 1000 synthetic NEO orbital

elements, matching the distribution statistics of the actual NEO swarm as best we

can determine that from the present sample of known NEOs. We imposed one

"unnatural" restriction: we included in the sample only orbits which pass within

0.05 astronomical unit of the Earth's orbit. As a general rule, asteroids whose

orbits do not pass within 0.05 AU of the Earth's orbit pose no threat of collision

on a time scale of a century, as the planetary perturbations necessary to reduce

the miss distance to zero require longer than that to make such a change. Thus we

have limited our sample to a subset of the actual distribution: the ones that

actually pose a potential threat. Our results don't appear to be very much affected

by this restriction, but it is reassuring to know that we have prejudiced the

distribution in favor of the more hazardous objects.

Having created a set of synthetic orbit elements, we then generated a set of

positions for each object, one for each lunation (new moon) for ten years, or 125

positions for each object. For each computed position, we also calculate the rate

of motion on the sky and a relative magnitude which takes into account the

distances from the Earth and Sun, and the solar phase angle (analogous to the

"phase of the moon", which in a like way very much affects the brightness of the

object).

To conduct a survey simulation, we "filter" the file of 125,000 positions to

tabulate which objects are "discovered" and which are not. The various "filter"

elements include limitations on the sky area viewed, either those imposed by the

maximum area the putative system can cover or those naturally existing due to

horizon limits, Sun or Moon in the sky, too close to the galactic plane, where

detections are impossible due to background star confusion, and most important,

object size/system limiting magnitude. On this latter point, we note that the

system limiting magnitude and the absolute magnitude of objects are 100%

correlated parameters. That is, a system capable of detecting objects 4 times

fainter than another system will achieve the same level of completeness of NEOs

at 1/2 the diameter as the other system. Thus in estimating completeness vs. size

of NEOs, we needn't do independent evaluations for different limiting

magnitudes. The same "completeness curve" applies for completeness vs. size at

a given threshold magnitude as applies for completeness vs. threshold magnitude

for a given size of NEO.

Observational Strategy

Even the basic detection of an asteroid requires multiple observations. The

method used by the only operational system, Spacewatch, is to scan the same

area of sky three times, separated by ~1 hour each. The images are compared to

reveal any moving object, with the third scan as a confirmation against erroneous

or confused images in either of the other two scans. It is anticipated that the

systems described above would operate in a similar mode. Some economy could

be achieved by storing a catalog of the sky from past (previous months or years)

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (3 of 8) [12/5/2001 1:41:18 PM]
scans of the same area, so that only two new scans, to be compared against the

archival catalog, would suffice. Thus the first step, detection and confirmation of

a moving NEO, requires taking two or three scans of a given sky area, separated

by an interval of time of the order of an hour or two. This results in a

measurement of the instantaneous position in the sky and a rate of motion, which

is sufficient for finding the object sometime later, for example the next night.

In order to obtain even a preliminary orbit for the object, further observations are

needed. Present practice is to identify NEO candidates on the basis of anomalous

rate of motion compared to main-belt asteroids, as determined on the first night

of observation. For these objects, additional observations are needed, on at least

two more nights, and preferably spaced over an interval of about a week. A one

week "arc" is usually sufficient to make a preliminary estimate of the "minimum

orbit intersection distance" (MOID) from the Earth and determine whether the

object presents any potential hazard to the Earth on a timescale less than a

century. A longer arc is necessary to consider the object reliably "cataloged", but

with only a week arc the number needing further follow-up, on the strict basis of

hazard alone, can be reduced to a small enough number to be accomplished with

modest resources.

With highly automated systems, recording detections at much higher rates than

present systems, it may become more efficient to just cover the sky often enough

that the week-arc follow up occurs automatically, for everything. This has the

advantage that all objects are followed up to the level of a preliminary orbit

determination. Thus the few NEOs which chance to be mimicking main-belt

motion at the time of detection are discriminated and become "discovered." To

operate in this mode requires covering the search area about 4 times each month,

rather than once plus targeted follow up.

In summary, "detection" consists of a sequence of two or three observations on a

single night, which are usually sufficient to distinguish a main-belt object from

an NEO and to find it again the next night. To "discover" the object, in terms of a

preliminary orbit, requires two or three more observations over about a week, and

represents about a doubling of resources over detection alone.

Survey Completeness vs. Area of Sky Coverage

For our first simulations, we specified the area of sky covered per month as the

radius of a circle on the celestial sphere centered on the opposition point, which

is generally the most productive area to search. In this experiment we made no

restrictions for horizon or closeness to the galactic plane. For the detection

threshold, which is a combination of telescope limiting magnitude and size of

object, we chose limiting magnitudes appropriate for a single GEODSS telescope

equipped with the Lincoln Laboratory GUPS CCD chip, with exposure times

appropriate to allow coverage of the area of sky assumed in each case. For size of

object, we took the brightness corresponding to a 1 km diameter object of albedo

0.15 (typical S class albedo), or equivalently, a 2 km object of albedo 0.04

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (4 of 8) [12/5/2001 1:41:18 PM]
(somewhat darker than average C, D, etc. objects). In Figure 1 we plot the rate of

detections of NEOs for three assumed sky areas corresponding to circles of

radius 34 , 65 , and 137 out from the opposition point. For the focal plane

instrumentation assumed, these sky areas correspond to exposure times per single

image of 100 sec, 30 sec, and 10 sec, respectively. With these exposure times, the

specified sky areas can be covered three times (the redundancy required for

detection and confirmation) in ~100 hours of observing time, which is the typical

amount of time available from a given site in a month, allowing for weather and

other types of interruptions. The 34 and 65 sky areas are probably achievable

from a ground-based site. 137 corresponds to covering the whole celestial

sphere down to a solar elongation of only 43 , clearly not possible from

anywhere on the ground without serious losses from atmospheric extinction. The

point of this figure, which is a very robust result and applies for any system we

have evaluated, is that it is better to cover more sky and sacrifice limiting

magnitude as necessary, until all available sky is being surveyed.

All-sky Surveys

Having established that the optimum strategy is always to cover all available sky

each month, we concentrated on this mode of operation in the remaining

analyses. We first evaluated how much sky is accessible and how many hours are

available to cover it for each month of the year. The restrictions applied are:

1. The Sun must be more than 10¡ below the horizon.

2. The moon must be below the horizon.

The target area must be more than 25¡ above the horizon at some time

during the night.

3. 

4. The target area must be more than 20¡ away from the galactic plane.

Subject to these conditions, we determined that, almost independent of station

latitude, the maximum rate of sky coverage required is ~135 square degrees per

hour in order to cover all of the sky once per month. Allowing for duty cycle

losses, cloudy weather, and other down time, the rate of sky coverage should be

~200 square degrees per hour to cover the whole sky once per month. It is

important to note here that any system intended to contribute seriously to the

survey itself, rather than serve as a "test bed", should be designed to cover sky

area at the above rate. Indeed, unless a separate system of astrometric follow-up

is contemplated, the survey system needs to be capable of 2 or 3 times that rate to

assure enough observations to derive preliminary orbits for the discovered

objects.

In Figure 2 we plot the fraction completeness vs. time for each of the three

systems described above. For both LONEOS and Spacewatch - II, we have

assumed the "full-up" configurations described above which would be capable of

all-sky coverage each month. These curves represent the fraction of objects

detected, and do not allow for the necessary work of follow-up observations to

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (5 of 8) [12/5/2001 1:41:18 PM]
determine orbits for detected objects, which will be discussed later in this

chapter.

Completeness as a Function of Size of NEO or Limiting Magnitude of

System

As noted above, the question of whether or not an NEO is detected, given that it

passes within the surveyed area, is a function of only one parameter: brightness

compared to the detection threshold of the survey system. Thus size and albedo

of NEO and threshold limiting magnitude of the detection system all collectively

constitute only a single variable. So we can derive a single "completeness curve"

which can be used to describe completeness as a function of limiting magnitude

of the survey system, for a given size and albedo of object, or equivalently,

completeness as a function of size of body, for a system of specified threshold

detection magnitude.

Figure 3 is a plot of that function derived from the simulated 10-year survey of

1000 objects. The vertical scale is simply the fraction of the 1000 objects

"detected". The horizontal scales are either relative size of object, or threshold

detection limit of the system. We have plotted the curve twice (dashed lines),

offset by a factor of 2 in diameter (1.5 magnitudes brightness), which correspond

the difference of approximately a factor of 4 in albedo between the brighter,

"S-Class" asteroids and darker, "C-class" and related types. Among measured

NEOs, the ratio of high to low albedo objects is approximately 10:1. However

this is strongly affected by the fact that dark objects of a given size are much

more difficult to detect. Thus we suspect the bias-corrected ratio is closer to half

each, at a given size. The solid line curve in Figure 3 is an equally weighted

average of the two dashed curves, and represents the completeness curve for an

NEO population consisting of equal numbers of high and low albedo objects. We

will use this curve for further analyses.

In Fig. 4, we have plotted the completeness curve to represent completeness vs.

diameter of NEO, for various values of system limiting magnitude. In Fig. 5, we

present the completeness curve, this time scaled vs. limiting magnitude of the

system, for various diameters of NEOs. In addition to the three systems discussed

above, we have included curves for the current Palomar 46-cm Schmidt

photographic system and for the suggested "Spaceguard Survey" system (see

Appendix I) of 2-3m telescopes capable of surveying to a threshold magnitude of

22. From these plots, it appears that a system reaching limiting magnitude 20 can

achieve about 80% completeness of NEOs down to a size of 1 km diameter in a

10-year survey.

Strategies for Preliminary Orbit Determination

One can contemplate two strategies to determine orbits rather than merely detect

objects. One way is to do targeted follow-up observations, either by assigning the

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (6 of 8) [12/5/2001 1:41:18 PM]
observations to a second telescope or by taking time from the discovery survey to

make these observations. A second mode is to cover the whole sky so often that

repeated detections of the same object are sufficient to yield orbit solutions from

the regular survey observations. Figure 6 is a comparison of these two follow-up

strategies, which we now describe.

Presently, surveys are done in the first mode, of targeted follow-up. To make the

problem tractable, it is necessary to discriminate NEOs from the much more

abundant main-belt (MB) objects based on motion in the sky, before an orbit is

known. Thus there is a "blind spot" of slow sky motion where an NEO can mimic

a MB object and thus not be discriminated. As we go to surveys reaching to

fainter magnitude, discoveries will be made at greater distances, thus at slower

average motion, and the "blind spot" becomes a more significant loss factor. To

evaluate this mode of follow-up, we have computed a second completeness

curve, this time filtering out objects which, even though they may be in an

observable part of the sky at a given time, are exhibiting main-belt-like motion,

and thus would not be "noticed". From past experience (e.g. Spacewatch,

Palomar photographic), the "overhead" of follow-up of past discoveries appears

to be a task of the same magnitude as the survey itself. Thus a survey telescope

may be occupied about half time taking follow-up observations and half

surveying new sky. Or if two telescopes are available, one could scan while the

other does follow-up. In either case, the "cost" is a factor of two in exposure time

that could be devoted to survey-only, which translates to ~0.4 magnitude in

threshold detection. So we shift the "targeted follow-up" curve 0.4 magnitudes to

the right.

The second possible follow-up mode consists of simply scanning the sky more

often, so that enough positions are obtained of each object to derive a preliminary

orbit of every object detected. Thus even those exhibiting normal MB motion are

discriminated. For the same threshold magnitude, this technique would obviously

discover more objects. However, it is likely that operation in this mode would

require covering the sky many times per month, perhaps 4, to assure that at least

three observations, each separated by several days, would be obtained of a given

object. Thus the "cost" is a factor of 4 in exposure time, or ~0.8 magnitude. So

we shift the other curve in Figure 6, 0.8 magnitude to the right.

The result is that the two curves cross one another, so the strategy of targeted

follow-up is superior except for the very largest objects. On the other hand, it is

the very largest objects which are most important. A pedantic reliance on

anomalous motion leads to a worrisome result that no survey, no matter how

sensitive, can achieve > 90% completeness in 10 years. But the largest

objects are also brighter, and very much less numerous, than smaller objects.

Furthermore, any large object mimicking main-belt motion will be there the next

month for repeat coverage. Thus a hybrid strategy should be possible which

could closely approximate the higher level of the two curves over the entire

range. In any case, the problem of following up to the point of preliminary orbit

determination is roughly a "factor-of-two" complication over bare detection only.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (7 of 8) [12/5/2001 1:41:18 PM]
Returning briefly to Figs. 4 and 5, We have associated "LONEOS" with a

limiting magnitude of 19, whereas we estimate it is capable of reaching 19.4 in

an all-sky survey mode with 68 second exposures. Thus the limiting magnitude

of 19.0 is about correct if that telescope is tasked with doing its own targeted

follow-up, consuming half its time. The limiting magnitude of 20 associated with

GEODSS is about the expected performance of one GEODSS telescope, full time

surveying. Thus in truth, this curve represents the capability of two GEODSS

telescopes, one surveying and a second one doing follow-up, or some similar

combination. The magnitude limit of 21 associated with SW-II is the limit

expected for single-coverage of all sky, so again, to achieve this level of

performance would require a second 2m telescope, or perhaps a highly automated

version of SW-I could keep up with the task. Finally, the limit of 22 associated

with "Spaceguard" is in a sense "by definition." In the Spaceguard Report (see

Appendix I) a requirement was defined to achieve nearly all-sky surveying to

limiting magnitude 22. That requirement was then estimated to correspond to a

system of about five 2-to-3 m telescopes equipped with CCD arrays. We concur

with that scale of instrumentation required to achieve all-sky coverage to

magnitude 22.

Conclusion

If one asks the question, what is the likely largest size of any remaining

undetected object (that is, where completeness equals one over the number of

objects of that size expected), the answer is about 3 km for the evaluated systems,

after 10 years. Pushing this limit down to ~1 km would require a Herculean

effort. Thus we must accept some level of incompleteness. The systems

evaluated can yield completeness in the range 80-90% or better, especially if all

are used in concert. This level of completeness should reduce the threat from

collision by an undetected NEA to less than that posed by impacts from

long-period comets, so in that context, we can declare these systems capable of

achieving the Spaceguard goal of reducing the hazard of asteroid collision by an

unknown object to below that from comets.

The most important lesson which emerges from this analysis is that the best

survey strategy is to cover the entire accessible sky every month, sacrificing

whatever magnitude limit is necessary to accomplish this. A very positive result

is that if that strategy is followed, adopting reasonable and even conservative

limit on sky observability, it is possible to obtain reasonable completeness in ten

years, including objects which never quite reach out to the orbit of the Earth and

hence never come to opposition. Thus the ability to observe closer to the sun or

to remove horizon limitations is not a sufficient justification in itself to move to a

space-based survey system.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/evaluation.html (8 of 8) [12/5/2001 1:41:18 PM]
Figure 1. Rate of discovery vs. time for one GEODSS telescope. Each curve represents a

different choiceof exposure time, and consequently limiting magnitude, and results in a

different area of sky per month that can be covered. The curves represent the discovery

rate for ~1 km diameter objects of moderate albedo (0.15), or ~2 km diameter objects of

low albedo (0.04).

Figure 1

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_01.html [12/5/2001 1:41:18 PM]
Figure 2. Rate of discovery vs. time for each of the three systems evaluated, assuming

that the rateof sky coverage is chosen such that all available sky area is covered each

month. The curves represent the discovery rate for ~1 km diameter objects of moderate

albedo (0.15), or ~2 km diameter objects of low albedo (0.04).

Figure 2

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_02.html [12/5/2001 1:41:19 PM]
Figure 3. Completeness as a function of limiting magnitude of survey systems. Light

refers to a population with albedo equal to average S-type (light) asteroids and dark refers

to asteroids with albedo equal to average C-type (dark) asteroids.

Figure 3

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_03.html [12/5/2001 1:41:19 PM]
Figure 4. Completeness as a function of asteroid diameter for five survey systems

described in text

Figure 4

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_04.html [12/5/2001 1:41:20 PM]
Figure 5. Completeness as a function of limiting magnitude of survey telescopes.

Figure 5

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_05.html [12/5/2001 1:41:20 PM]
Figure 6. Completeness as a function of limiting magnitude of survey systems with two

differentstrategies for preliminary orbit determination (see text).

Figure 6

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_06.html [12/5/2001 1:41:20 PM]
VI. Precise Orbit Determination and

Physical Observations

Additional observations following the discovery of Earth-approaching objects

are required to achieve a more complete understanding of the NEO population

and to assess their threat of impact on Earth. First, the orbital elements of a

potentially threatening object must be obtained with sufficient accuracy to

ensure recovery on its next approach to Earth, and also to determine the

probability of impact at a future date. Any later attempt to mitigate the threat

will depend critically on the accuracy of the observations and either the length

of the observed arc during the discovery apparition, the use of radar range

information, or the recognition of observations obtained on prior apparitions.

Second, a sample of the discovered bodies should be classified by reflectance

and spectral type in order to assess the completeness of the survey as a

function of size and composition. Otherwise we will not know how many

asteroids have been missed, nor the risk we would face of an impact from

these undiscovered bodies. This information will be useful for planning

potential measures to mitigate the threat from such objects, though such

considerations are beyond the scope of this study.

Astrometric Observations

Because NEOs are often discovered at a time when they pass just close

enough for detection, they often become fainter with time. After a few weeks,

and sometimes only a few days, they can not be re-observed with the same

telescope that made the discovery unless longer integration times are used.

Half of the kilometer-sized NEOs that will be detected in an all sky survey

will be visible for no more than one lunation (Table 1). Unfortunately, this

observation window is too short to determine with sufficient certainty the

orbit of a potentially threatening NEO from optical observations alone. Many

NEOs determined to be in Earth crossing orbits could not be ruled out as

threats by such limited observations. Also, the position of such an object

could not be predicted with sufficient accuracy to find it again a few years

later. If the telescopes that conduct the survey are large enough (~2m), they

will have enough light-gathering power to cover the whole sky each month

with short integrations (~10s), and still have time left over for further

astrometric observations of potentially hazardous NEOs discovered in

previous months. However, if meter-class telescopes are used, they will

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/orbit.html (1 of 6) [12/5/2001 1:41:34 PM]
require longer integrations to achieve the same limiting magnitude, and will

not have time left over for this extended astrometric follow-up. Also, they

may not have the pixel resolution needed to determine orbits of adequate

precision for future recovery and hazard prediction. In that case, additional

telescopes of 1-m aperture or larger will be required for astrometric

follow-up.

Physical Observations

There is both a need and an opportunity to make physical observations of a

subset of NEOs discovered by the NEO Survey. In its barest form, the Survey

would identify an enormous number of objects in Earth-approaching orbits,

characterized by their orbital properties and their optical magnitudes.

However, in order to identify the NEOs that would be potentially hazardous

to Earth, sampling of the population's physical properties is essential. The

mass-frequency relation of the NEO population must be defined in order to

reach a specified level of completeness for a threat defined by impact energy.

Asteroid albedos are known to differ by a factor of ~30; some constituent

materials of NEOs differ even more. Without adequate information about

albedos, inferred masses (and hence impact energies) could differ by more

than a factor of 100. Moreover, the population of meter-to km scale objects

could differ greatly from that of the larger objects. Therefore, cataloguing

NEO orbital properties to some limiting magnitude does not meet the minimal

goals of the NEO Survey. Sampling of associated masses (i.e by

determination of diameters and albedos) is also required.

A survey of NEOs made for the purpose of raising our level of understanding

about impact hazards has broader goals than merely obtaining a partial or

complete census of objects of particular sizes. It should seek to obtain data

about physical properties that will help us understand the impact hazard more

generally. For example, it is reasonable to expect that NEOs with different

origins will fragment in different ways. Extinct comet nuclei with tensile

strengths ~100 to 1000 dynes/cm will fragment easily, while metallic bodies

such as are represented by nickel-iron meteorites with tensile strengths

exceeding 10 dynes/cm will not fragment easily. The fragile, cometary

bodies may therefore dominate the population of NEOs smaller than some

transition size (~1 meter to 1 kilometer). Since comets originate in the outer

solar system, such objects might also be expected to dominate NEOs with

higher impact velocities. Thus an understanding of NEO mineralogy will be a

factor in our understanding of impact hazard versus size. Ideally, the

statistical completeness of the survey would be assessed for each

mineralogical class, and this would require knowledge of the mineralogical

type and size for as many NEOs as practicable.

If the survey is conducted with meter-sized telescopes, there is further

impetus to make sample observations that determine the diameter and albedo

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/orbit.html (2 of 6) [12/5/2001 1:41:34 PM]
distributions of the NEOs. Searches by meter-sized telescopes will have

magnitude limits in the range V=20-21. As a result, completeness over a 10

year period drops off rapidly in the size range for which we are hoping to

raise completeness to a maximum. Kilometer-sized, dark NEOs would be

sampled only to a completeness of ~60% (Section VI). However, this estimate

is critically dependent on the assumed albedo distributions. For example, we

currently have measured albedos for only ~40 NEOs, and these are biased by

observational selection. The assumption in Section VI that the ratio of dark (C

type) to light (S type) albedos is ~1 could be off by a factor of 2, and this ratio

may depend on the size of the NEOs. Hence, meter-sized telescopes might

sample bright, km-sized NEOs to completeness level of only 75%, if there are

twice as many dark objects as bright ones at 1 km diameter. We would not

know the true limit unless we made enough physical observations to know the

albedo distribution.

Requirements for Optical Telescopes

Physical observations must be made when an NEO is brightest (usually near

the time of discovery). For the purposes of physical characterization, the

minimum telescope aperture for spectroscopy and light curve measurements

is 2m. Such telescopes are capable of recording visible light spectra with high

signal to noise (high enough for crude spectral classification) down to V 17.

Although most discoveries will be fainter (V 20), a large enough fraction

(~6%) will be discovered with V < 17 to provide a representative sample.

Light curve measurements and filter photometry with the same telescopes will

allow the spectral characterization of fainter discoveries (V~20). These same

telescopes could be used for both astrometric and physical follow-up.

For the purposes of albedo measurements and unambiguous mineralogical

classification, however, a 2m telescope is not large enough. Albedo

measurements require a combination of intensity measurements at both

optical and thermal infrared wavelengths, while classification of mineralogy

requires low resolution (l/d 100 - 1000) spectrophotometry, particularly in

the 1-5 μm region. Such measurements are possible using large format

infrared and visible-light detector arrays. These arrays can acquire the needed

spectrum over a wide spectral range simultaneously, at the required

resolution, by using cross-dispersed diffraction gratings. It is possible to

design a single instrument that acquires thermal infrared, near infrared, and

visible spectra simultaneously, by using several array detectors. This permits

very efficient data collection and greatly reduces the limiting magnitudes.

Despite these modern advances in array technology, telescopes with apertures

several times larger than the survey telescopes will be needed for

mineralogical classification and albedo determination. For equal signal to

noise ratio, the product of telescope area and integration time (on-chip) must

be larger by R (= l/d ), assuming the detection sensitivity is limited by read

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/orbit.html (3 of 6) [12/5/2001 1:41:34 PM]
noise. The on-chip integration time during discovery is typically 10 seconds

and this can be easily increased to several minutes or more during the

follow-up observations, when the NEO is tracked. The telescope diameter

then must be ~3 to 10 times larger than the discovery telescope. For example,

the 3-m IRTF in Hawaii is required to make infrared measurements (J, H, K

wavelengths) for an object with V < 17, but is limited to V < 15 for

infrared spectroscopy. A 3-meter class telescope is a minimum requirement.

In summary, the survey telescopes alone will not provide an adequate data

base for assessing the impact hazard to the Earth. Two dedicated 2-m

telescopes for combined survey and extended astrometric follow-up and/or a

dedicated 3-m class telescope will be needed for follow-up astrometric and

physical observations.

Radar Observations

Radar is the most powerful groundbased technique for post-discovery

reconnaissance of NEOs and is likely to play a central role in identification of

possibly threatening objects during the foreseeable future.

Delay-Doppler measurements are orthogonal to optical angle measurements

and typically have a fractional precision between 10 and 10 , and

consequently are invaluable for refining orbits and prediction ephemerides. A

single radar detection secures the orbit well enough to prevent "loss" of the

object, shrinking the object's instantaneous positional uncertainties by orders

of magnitude with respect to an optical-only orbit, obviating the need for

extensive optical follow-up and greatly improving the accuracy of long-term

trajectory predictions. During the past decade, observations of newly

discovered objects have revealed range errors from ~100 km to ~100,000 km

in pre-radar range predictions. The availability of radar measurements could

be the difference between knowing that an object will pass "within several

Earth-Moon distances of Earth" in a few decades and knowing that it will "hit

the Earth."

The same measurements also provide otherwise unavailable information

about the target's physical properties, including size, shape, rotation,

multiplicity, and surface characteristics. Measurements of the distribution of

echo power in time delay (range) and Doppler frequency (radial velocity)

constitute two-dimensional images that provide spatial resolution as fine as 10

meters. With adequate orientational coverage, such images can be used to

construct geologically detailed three-dimensional models, to define the

rotation state precisely, and to constrain the object's internal density

distribution. Moreover, radar wavelengths are sensitive to near-surface bulk

density and structural scales larger than a few centimeters. For comets, radar

waves can see through optically opaque comas and can also reveal

large-particle clouds. The value of a radar observation increases in proportion

to the echo strength.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/orbit.html (4 of 6) [12/5/2001 1:41:34 PM]
A signal-to-noise ratio (SNR) as large as 20 is usually adequate for detection

and marginal resolution of the echoes. SNRs greater than 100 let one achieve

enough resolution to be able to make simple statements about shape. With

SNRs approaching 1000, the data permit detailed constraints on dimensions,

and with SNRs at least as large as ~3000 one can make images that clearly

show surface features. Crudely, one can expect the number of useful pixels in

a dataset to be of the same order as the SNR. The observation time required to

achieve any given SNR increases with the eighth power of the distance. Even

a modest-SNR radar observation can dramatically shrink the cost and risk of a

spacecraft mission to a near-Earth object, by reducing or eliminating the need

for on-board optical navigation and by providing advance characterization of

the target. The highest-SNR radar investigations can be as informative as the

cheapest NEO flybys under consideration.

The world's two primary facilities used for planetary radar astronomy are the

NAIC Arecibo Observatory in Puerto Rico and the NASA Goldstone Solar

System Radar (part of the Deep Space Network) in California. Arecibo is

being upgraded and by 1996 will be more than 20 times as sensitive as

Goldstone, see twice as far, and cover three times as much volume as

Goldstone. The more fully steerable Goldstone instrument, with a solid angle

window twice the size of Arecibo's and an hour-angle window at least several

times wider than Arecibo's for any given target, will serve a complementary

role, especially for newly discovered objects. Table 2 lists the

radar-astrometric range limits expected for Arecibo and Goldstone by 1996.

Discovery apparition geometry often is exceptionally favorable to radar

reconnaissance, but such windows rarely last more than a week or two. The

optical-astrometric time base needed to secure a newly discovered NEO's

orbit (and to identify the date of subsequent radar opportunities) is generally

at least several months. That is, the radar-targeting decision usually must be

made long before the orbit estimate is good enough to guarantee recovery or

to identify future close approaches accurately. Minimal reconnaissance of a

new NEO requires at least one block of time, probably at least two hours

long, on one of a handful of possible dates, to be scheduled with extremely

short notice (typically on the order of a few days to a few weeks). However,

Arecibo is primarily a national center operated primarily for visitors engaged

in passive radio astronomy and ionospheric physics, and Goldstone's primary

responsibility is spacecraft telemetry. At each site, the total usage for all radar

astronomy has averaged less than 5%; an increase to 10% is unlikely under

current institional constraints. Moreover, each site has limited tolerance for

frequent disruption of schedules to accommodate new targets of opportunity.

Current anticipations are that only those newly discovered NEOs with the

very highest SNRs and/or very long advance notice (probably 5 to 15 objects

per year) can be observed at Arecibo, even if each target is observed only on

one date. Roughly 90% of the opportunities to observe new NEOs from

Arecibo will probably be missed. A dedicated NEO radar facility would cost

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/orbit.html (5 of 6) [12/5/2001 1:41:34 PM]
more than $100M; we deem recommendation of such an expenditure

inadvisable at present, although it is desirable to initiate a preliminary

engineering/cost study to define the capability and optimum design of a

dedicated NEO radar as a function of cost.

On the other hand, it is highly desirable ensure the vitality of current NEO

radar observing programs. It also is desirable to optimize the use of Arecibo

for response to NEO radar opportunities by reducing the time between

identification of a radar opportunity and the initial detection of the target from

several days to several hours, and by taking other steps to minimize

disruption of telescope and personnel schedules. In principle, the following

six actions can triple the number of useful radar observations of newly

discovered NEOs, at a total cost of less than $500K per year:

Expand current capabilities for On-Site Orbit-Determination. The JPL

OSOD program has been in use for Goldstone ECA radar observations

for a year. Simplify its user interface and install it at Arecibo.

1. 

Connect Arecibo and Goldstone to electronic communications lines

with the widest possible bandwidth.

2. 

3. Implement remote and absentee observing protocols at both sites.

Provide support for two full-time technical assistants, one each at

Arecibo and JPL.

4. 

Augment Arecibo's operational support to provide enough transmitter

fuel for dedication of 5% of the telescope time to NEO radar.

5. 

Urge that both NAIC and the DSN adopt flexible policies for

optimizing response to target-of-opportunity situations.

6. 

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/orbit.html (6 of 6) [12/5/2001 1:41:34 PM]
Table 1. Cumulative fraction of NEOs that cannot be re-detected vs. time (t) after discovery. This table

shows the cumulative fraction of NEOs that cannot be re-detected as a function of time, t, after

discovery. V=20 is assumed, but the fractions are nearly the same for V=22. 50% of the discoveries are

visible for no longer than 18 days. Since the moon is dark enough for complete sky coverage only 10

nights in each month, 18 days is typically the time an NEO must be observable if observations are to

cover more than one lunation. It is thus likely that 50% of the discovered NEOs will be observable for

fewer than 10 days.

 t cumulative 

(days) fraction 

 0 0.000

 1 0.000

 2 0.037

 3 0.067 

 4 0.093

 5 0.140 

 6 0.188

 7 0.225

 8 0.258

 9 0.286

 10 0.318 

 11 0.344

 12 0.375

 13 0.407

 14 0.426

 15 0.446 

 16 0.458

 17 0.472

 18 0.504

 19 0.530

 20 0.549 

 21 0.559

 22 0.578

 23 0.587

 24 0.611

 25 0.627

 26 0.642

 27 0.655

 28 0.682

 29 0.696

 30 0.726

 40 0.847

 50 0.946 

 60 0.981

 70 0.988

Table 1

http://impact.arc.nasa.gov/reports/neoreport/figs/table_01.html (1 of 2) [12/5/2001 1:41:34 PM]
 80 0.993

 90 0.996

100 1.000 

Table 1

http://impact.arc.nasa.gov/reports/neoreport/figs/table_01.html (2 of 2) [12/5/2001 1:41:34 PM]
VII. Cooperation Between Air Force and

NASA

Introduction

The Department of Defense has the responsibility for protecting the United States

against catastrophic losses. Those are generally thought to be military threats, but

the possibility of such losses from impacts by celestial bodies is also recognized.

As the primary space arm of the DoD, the USAF and Air Force Space Command

are evaluating the need and steps for defending the planet from such unlikely but

devastating events. Air Force Space Command already maintains a sophisticated

array of ground-based sensors for monitoring the space debris that represent a

hazard to space operations and an extensive array of space-based sensors that can

and have detected the impact of NEOs on the Earth's atmosphere and helped to

diagnose their composition. Air Force Space Command is now in the process of

evaluating whether those sensors should be improved and shared for the defense of

the planet. That evaluation is taking place at the policy and technology levels. The

former should be completed this year; the latter should be completed by

technology developments and field tests next year. If the results of both evaluations

are positive, the Air Force and DoD would be in a position to address this

important mission, which it would do in a manner that would fully integrate the

results and capabilities of civil and international efforts.

The following section describes the capability of the current AF sensors, the

technology programs for their improvement, the plans for testing them, and the

options for integrating them into existing DoD sensor nets. The following section

describes the steps that should and could be undertaken to efficiently integrate civil

and DoD detection efforts.

Background

The Air Force has a space serveillance system which includes both optical and

radar facilities, associated communications and processing which it uses to

maintain the orbits of approximately 7,000 objects which are orbiting the Earth

from very low altitude to geosynchronous and somewhat beyond. The Air Force

maintains for the national and international community a catalog that is used by the

Defense Department, the intelligence community, the civil community and the

commercial community. We currently use approximately 50,000 observations a

day to maintain the on-orbit catalog of all man-made objects down to 5-10 cm in

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/cooperation.html (1 of 3) [12/5/2001 1:41:35 PM]
diameter.

Example Activities of Potential Interest to NEO Search

Optical systems consist of the Ground-based Optical Deep Space Surveillance

(GEODSS) System, plus the large telescopes on Mt. Haleakali, Maui. The

GEODSS main telescope characteristics are described in Figure 7. These

telescopes have low F numbers and large fields of view which are a fundamental

advantage for NEO search because they were designed as search instruments.

These telescopes appear to be quite suited for the search of asteroids down to sizes

on the order of 100 meters in diameter and at ranges out to approximately 1 AU

(see Figure 8).

The Air Force has a research and development program at Lincoln Laboratory to

develop a new advanced CCD imager for GEODSS. Its properties are described in

Figure 9. The GEODSS main telescope and the new Lincoln camera will be in a

testing mode until late spring or early summer 1995. The Air Force's experimental

test site (ETS) is near Socorro, NM. Our current estimate is that the one GEODSS

main telescope at ETS with the new camera should have the capability of

increasing the current world wide discovery rate of Earth-crossing asteroids by a

factor of ten (search coverage approximately 6,000 square degrees per month at

limiting visual magnitude of 22). The Air Force currently has six GEODSS main

telescopes that are not part of the current space surveillance operations.

The Air Force's GEODSS Upgrade Prototype System (GUPS) is focused on

demonstration of new technologies with a plan of incorporating new technologies

in the operational GEODSS. GUPS is a fully transportable GEODSS site, which

would be an advantage in quickly establishing a new site. GUPS is managed by Air

Force Electronic Systems Center and has a three year development schedule

ending 9/96. There are extensive demonstrations to be performed during the

program at the ETS. GUPS is shown in Figure 10.

The Air Force's Phillips Laboratory is developing a 3.7 meter telescope at Maui

known as AEOS. Our analysis suggests that if AEOS is used as a follow-up sensor

it is capable of developing a characterization matrix for each newly discovered

Earth-crossing object (light curve, spectral characterization, albedo) as well as

improving the quality of the orbit for each catalogued object up to approximately

120,000 objects per year, which is the maximum expected from the upgraded

detection suite.

In summary, the advances in computer and focal plane technologies since 1976

have motivated the attempts to improve the GEODSS system. The Lincoln

large-area CCD camera, coupled to a 40-inch GEODSS telescope, will be

field-proven by approximately 1 April 95 at the ETS. The GUPS will be turned

over to the Air Force in very late 1996 at the ETS. The GEODSS sensor upgrades

(Ebiscon to large CCD) may start in FY'1998, but no sooner. The search for NEOs

is not an entirely new mission for there seems the Air Force's ETS has in the past

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/cooperation.html (2 of 3) [12/5/2001 1:41:35 PM]
detected new asteroids; for example 2460 MIT Lincoln, 3343 Nedzel, and 3403

Tammy.

Near Term Cooperative Efforts

We are proposing to pursue within resources a proof of concept capability that will

evaluate the usefulness of GEODSS with the new CCD camera for near-Earth

object searches. We expect such proof of concept work could start late spring/early

summer 1995 with a proposed asteroid tracking demonstration in September 1995

and regular observing schedule proposed to start October 1995. This would be a

research and development activity at the GEODSS ETS optimizing software and

interaction with other site tasking. The Air Force proposes to hold periodic

meetings with the scientific community to discuss the results and provided data to

the scientific community over the next two to three years.

Air Force Space Command (AFSPC) has asked Los Alamos National Laboratory

(LANL) to study the physics of signatures of asteroids impacting Earth's

atmosphere as seen by Defense Department's space sensors, evaluating the

uselfulness of DoD space sensor data on composition of asteroids and measuring

the flux of smaller asteroids passing very near Earth. Classified and unclassified

workshops on these topics are planned for the summer of 1995. AFSPC's Chief

Scientist will be hosting workshops in a number of areas for invited experts from

the scientific community. The first workshop on 12-13 May 1994 covered all

aspects of the threat, detection, and defense. Subsequent workshops will be more

focused. The following three planned workshops will be co-hosted by Dr.

Shoemaker:

A. Evaluating the risk to the planet

B. Evaluating the role of in situ and fly-by missions to future defense, and

C. Evaluating the technical means of defense

(note: The planned international meeting at Lawrence Livermore

National Laboratory during the last week of May 1995 may meet this

need).

Future Decision Making

Air Force Space Command has been tasked by the Air Force Chief of Staff

to do a Mission Area Assessment for defense of the planet. The time scale

for Air Force evaluation in decision making is likely one to two years. The

Air Force Scientific Advisory Board will have an ad hoc study in calendar

year 1995 to review space surveillance/space debris/asteroid warning. Thus,

the Air Force should be able to make firm decisions about significant

commitments in this mission area, although that will not happen in the very

short time scale prior to this report.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/cooperation.html (3 of 3) [12/5/2001 1:41:35 PM]
------------------------------------------------------------------------------------------------------------------

CONFIGURATION Folded Prime Focus

APERTURE DIAMETER (in): 4.0

EFF, CLEAR APERTURE (m2): 0.46

FOCAL LENGTH (in): 8.6

F NUMBER: 2.15

FLAT FOCAL PLANE DIAMETER (mm): 8.0

SPECTRAL BAND: 0.35-0.9 mm

IMAGE QUALITY: 80% of energy from solar pointsource encircled in 2 arc sec diameter

ENCODER ACCURACY (arc sec rms): 1.5

ENCODER RESOLUTION (arc sec): 0.36

----------------------------------------------------------------------------------------------

Figure 7. GEODSS Main Telescope Characteristics

Figure 7

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_07.html [12/5/2001 1:41:35 PM]
Figure 8.

Figure 8

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_08.html [12/5/2001 1:41:35 PM]
----------------------------------------------------------------------------------------------------------------------

LINCOLN CCD IMAGER FOR GEODSS

 PIXELS: 2560 X 1960

 PIXEL WIDTH: 24 mm

 READOUT METHOD: FRAME-TRANSFER

 SPECIAL FEATURE: SEPARATE 32 x 32 PIXEL CCD PHOTOMETER

 FOCAL PLANE AREA: 61.4 x 47.0 mm FOR LARGE CCD

 0.77 « 0.77 mm FOR PHOTOMETER

 # READOUT PORTS: 8 FOR LARGE CCD, 1 FOR PHOTOMETER

 HIGHEST DATA RATE: 2 MHz

 HIGHEST FRAME RATE: 10/s FOR LARGE CCD (2x2 Bin)

 1000/s FOR PHOTOMETER (No Bin)

 READOUT NOISE AT HIGHEST RATE: †10 ELECTRONS RMS FOR LARGE CCD, †6 ELECTRONS RMS FOR PHOTOMETER

 †SOLAR QUANTUM EFFICIENCY : 65% (BACK-ILLUMINATED)

 RESPONSIVITY VARIATION: †2% RMS

 DARK CURRENT AT -50¡ C: †24 ELECTRONS/PIXEL/s

 DARK CURRENT VARIATION: †10%RMS

 CHARGE TRANSFER INEFFICIENCY: †1.0E-5, 1620 electrons, -40¡C

 FULL WELL CAPACITY: 500,000 ELECTRONS PER 2 x 2 BINNED PIXEL

 %PIXELS AFFECTED BY POCKETS: †0.2%, ALL ISOLATED

 %PIXELS WITH BRIGHT DEFECTS; †0.15%, ALL ISOLATED

 FLATNESS: ± 10 mm P-P

----------------------------------------------------------------------------------------------------------------------------

Figure 9. Lincoln CCD Imager Characteristics

Figure 9

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_09.html [12/5/2001 1:41:35 PM]
Figure 10. GEODSS Upgrade Prototype System

Figure 10

http://impact.arc.nasa.gov/reports/neoreport/figs/figure_10.html [12/5/2001 1:41:36 PM]
VIII. International Cooperation

The discovery of asteroids and comets and the determination of their orbits

has been, by long-standing tradition, an international effort. By agreements

through the International Astronomical Union, the maintenance and

publication of ephemerides of all numbered minor planets has been the

responsibility of the Institute for Theoretical Astronomy (ITA) at St.

Petersburg, Russia. The ITA has supported a long-range program of

astrometry to upgrade the orbits of known asteroids. The center for receiving

observations of newly discovered as well as established asteroids and comets

has been the Minor Planet Center, now located at the Smithsonian

Astrophysical Observatory at Cambridge Massachusetts. This center collates

the incoming observations, assigns designations, calculates preliminary and

improved orbits, determines when a minor planet becomes numberable, and

publishes the results on a monthly basis in the Minor Planet Circulars

(MPCs). The work of observers and orbit calculators throughout the world are

published in the MPCs. Most of the immediate dissemination of information

on NEOs is nowadays via the Minor Planet Center's MPECs (Minor Planet

Electronic Circulars), although initial cometary information is communicated

via the IAU Central Bureau for Astronomical Telegrams (IAU Circulars), also

located at the Smithsonian Astrophysical Observatory. This system has served

to coordinate the existing international effort of discovery of asteroids and

comets extremely well, including all work to date on NEOs.

There is a continuing high level of international interest in dedicated NEO

surveys. One of the strongest on-going efforts is the Anglo-Australian

Near-Earth Asteroid Survey (AANEAS). Located at Siding Spring, New

South Wales, AANEAS utilizes several telescopes (Steel, 1995). AANEAS

not only reports numerous discoveries of NEOs but also provides crucial

follow-up astrometry and recovery of NEOs found elsewhere, particularly

when observations are needed in the southern sky. Another important

program of follow-up astrometry is being carried out in Canada.

A new observing system, utilizing an array of CCDs at the focal surface of a

1-m Schmidt telescope is being developed at C·te d'Azure, France. This

system, which will be dedicated primarily to NEO search, will substantially

improve the coverage of the sky needed in NEO surveys.

In Japan, where amateur observers have made significant contributions to

discovery and astrometric observations of NEOs, Syuzo Isobe (1994) of the

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/international.html (1 of 2) [12/5/2001 1:41:36 PM]
National Astronomical Observatory of Japan has proposed that a network of

telescopes belonging to cities and amateurs be equipped with CCD detectors

to provide critical astrometric observations of discovered NEOs and to assist

in the NEO search. A suitable CCD system is being tested; as many as 47

telescopes with apertures larger than 50 cm potentially could participate in

this network.

A working group of the International Astronomical Union, chaired by Andrea

Carusi of Italy, has been established to help formulate an international

program of surveying for NEOs. Investigators and observatories in Europe are

potential participants, and a plan is being developed for use of a 1-m Schmidt

telescope at the European Southern Observatory in Chile in NEO search. In

China, funds have been allocated for a 1-m telescope dedicated to an NEO

survey.

As the hazard of NEO impact is global, the systematic survey for NEOs

should be an international effort. Appropriate institutional mechanisms are

already in place, at the ITA and the Minor Planet Center to coordinate

international participation. It is expected that strong support of an NEO

Survey in the United States by NASA and the Air Force will help foster

support by other nations of the systematic survey work abroad.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/international.html (2 of 2) [12/5/2001 1:41:36 PM]
IX. Coordination of Observing Program

The proposed NEO survey will provide a flexible and cost-effective approach

to meeting the objective of discovering and cataloging NEOs (both

Earth-crossing asteroids and short-period comets) with the goal of complete

coverage of objects 1 km or more in diameter. By upgrading and coordinating

existing telescopes (either presently operational or under development), the

survey reduces start-up costs and accelerates the time-table for NEO

discovery.

Flexibility of operation should allow the most effective use of any particular

configuration of survey instruments that may be operational at any given

time. If only a single telescope is on-line, then it can be operated in a way that

will, for example, maximize the discovery rate for 1-km or larger NEOs by

scanning rather rapidly over a large sky area centered on the opposition point.

If a second identical or similar telescope is added, the two can be used

together to scan complementary sky areas, increasing the total area covered as

well as probing to fainter objects by lengthening exposure. In a similar way,

the observations of different telescopes with dissimilar operating

characteristics can be optimized. The objective of such an approach is to

maximize the total number of discoveries of the target objects by eliminating

redundant discoveries and optimizing the use of each instrument to take

advantage of its particular capabilities. Cooperation replaces competition, to

the advantage of all the parties.

With an adaptable system, it is not necessary to specify detailed instrument

requirements in advance. Within a range of operating parameters (aperture,

focal length, scan rate, number of pixels, etc.), any telescope and detector

system can contribute to the overall NEO survey. It is possible for new

partners to join in the NEO survey effort at any time, as they bring their own

individual systems to an operating state. This adaptability is especially

attractive for an international system, since each partner can decide

independently the nature of any new systems to be developed and the degree

to which such systems must be specially configured for NEO survey work.

While we envision a survey that consists initially of a small number of

existing U.S. telescopes that can be brought into operation quickly, we

anticipate that the survey will grow into an international effort as other

partners choose to join.

As more telescopes become available to the NEO survey, the

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/observing.html (1 of 2) [12/5/2001 1:41:37 PM]
complementarity of their operations can be increased. In the early stages, with

only 2 or 3 systems operating, it may be most efficient to have each system

perform its own confirmation and follow-up, sharing information with others

but operating in a manner not too different from that of current NEO surveys.

At this stage, the primary coordination requirement will be to ensure that the

surveys are complementary and not redundant. But as the network expands, it

might become more efficient for some instruments to concentrate on

discovery, and others to concentrate on follow-up. It will not be efficient to

interrupt the operations of any one telescope for follow-up once the discovery

rate increases by an order of magnitude above current values.

The cross-over into a truly interdependent and fully coordinated system is the

most important operational hurdle for the maturity of the survey. It will

require a fundamental change of attitude, with a high level of communication

and trust among different observers. A coordinating center will need to be

established, possibly to be co-located with the central organization for data

analysis and orbit computation. This center will provide the day-to-day

technical coordination of the effort consistent with guidelines to be

established by the consortium of survey partners, who will meet from time to

time as a steering group to review the performance of the system.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/observing.html (2 of 2) [12/5/2001 1:41:37 PM]
X. NEO Computational and Data Centers

Introduction

As the discovery rate of near-Earth objects accelerates, there will be an

increased need for the close coordination of the various observing sites to

optimize the efficiency of the entire discovery program. An integral part of

this cooperative effort will be the efficient processing of astrometric data, the

computation of orbits and ephemerides, and the rapid dissemination of these

data products to the observing communities. An optimized discovery program

for near-Earth objects will rely upon extremely close cooperation between the

community of observers and those concerned with modeling the past and

future motions of the entire population of the solar system's small bodies.

Once a moving object is detected, telescopic observers will need to be aware

of any previously discovered objects in their fields of view to distinguish new

discoveries from those objects that have been previously observed. In

addition, a central repository, or repositories, will be required to catalog for

each object, the discovery data, the follow-up astrometric data, updated orbits,

and any data that could be used to physically characterize the particular

object. These activities will be the responsibility of the NEO computational

and data centers.

The objectives of the NEO computational and data centers are to:

For all asteroid and comet discoveries, immediately distribute

observations to other data centers. The centers would then coordinate

activities so that NEOs are immediately identified and observers are

notified of the identifications

1. 

Provide discovery and follow-up observers with rapid response orbits,

ephemerides, error analyses, and optimal future observing

opportunities.

2. 

Coordinate the follow-up optical and radar observation programs that

will obtain astrometric observations and the data used to physically

characterize new objects.

3. 

Search existing observation files for pre-discovery astrometric data that

might extend the observation interval for orbit determination work. In

addition, center personnel should encourage the examination of

unmeasured plates for pre-discovery observations.

4. 

Archive astrometric observations, orbits, ephemerides, error analyses,

and the data used to describe the object's physical characteristics.

5. 

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/neocomputational.html (1 of 4) [12/5/2001 1:41:38 PM]
Determine whether each NEO will become a potential threat to Earth in

the future.

6. 

During the establishment of the NEO computational and data centers, care

must be taken to ensure that each center is using common planetary

ephemerides, archives, and data files and that each center's software is

cross-checked against similar software at other centers.

Processing astrometric observations and determining orbits can be separated

into two types of activity that are basically distinct. The first type involves the

recognition of all the observations that belong to individual objects, while the

second involves the handling of objects of special interest.

Separating Observations of Individual Objects

The initial processing of astrometric data and cataloging these data by object

is an activity that is already underway at the Minor Planet Center in

Cambridge Massachusetts. The current activities of the Minor Planet Center

could be extended to meet the demands of the NEO survey described in this

report. As such, it is instructive to begin with a summary of the Minor Planet

Center's current activities.

Astrometric observations are currently received by the Minor Planet Center at

a rate that averages several hundred per day. In a typical year there are

observations of several comets, several tens of near-Earth asteroids and

several thousand asteroids generally. Several tens of observatories distributed

around the world are involved in this effort. A good rule of thumb is that

near-Earth asteroids amount to 1 percent of the total number of observations.

For near-Earth objects, two observing programs currently account for about

55 percent of the data, two more programs add another 10 percent, and

amateur astronomers collectively contribute rather more than 20 percent. For

the most part, it has been expected that observers will arrange their work so

that the same sky fields are covered on two neighboring nights and that on

reporting their astrometric data, they will properly label the positions that

belong to the same objects. On receiving material of this type, the Minor

Planet Center first identifies numbered asteroids, other asteroids previously

observed at multiple oppositions (or for several months during a single

opposition), and asteroids that have been already discovered and observed on

two nights at the current opposition. The remaining two-night sets of

observations are then designated as new discoveries, after which more

sophisticated techniques are used to attempt identification with asteroids

observed only imperfectly in the past; orbits are determined, and advice is

given to the observers as to whether further observations at the current

opposition might be beneficial for securing its orbit. Such communications

are also made when errors--most frequently timing errors--are detected in the

data. Single-night observations may also be included in the identification

process, although discoverers are usually not credited without a second night

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/neocomputational.html (2 of 4) [12/5/2001 1:41:38 PM]
of observations.

While observers have been encouraged to make observations for two-nights

and to participate in a two-way communication to maximize the usefulness of

their observing programs, the Minor Planet Center has also recently worked

with extensive sets of single night observations from the Spacewatch program

with a minimum of communication -- except for the month-by-month

publication (in printed and electronic form) of the orbital information and

astrometric data sets. In a recent experiment, a month's supply of observations

at a daily rate of 500 were completely processed by one person using one

computer in 1.5 days. It is estimated that the current staff and facilities of the

Minor Planet Center could accommodate an order of magnitude increase in

observational throughput if no hardware or software maintenance were

required, the observations are submitted in a businesslike manner without

further communication with the observers, and if the quantity of the formal

published data were sharply reduced.

With a fully operational NEO survey, we estimate that the increase in volume

of observations reaching the Minor Planet Center will be two orders of

magnitude more than the current volume. This increase will also mean that

the number of near-Earth objects detected in the future will be comparable to

the current discoveries of the main-belt objects.

Attending to Objects of Particular Interest

Those asteroids and comets that are potentially Earth-approaching can be

automatically recognized in the complete data set and isolated for further

study. A simple test can easily be conducted to determine whether or not a

particular object's orbit could possibly bring it within close proximity to the

Earth. If this is the case, an effort should be undertaken to determine the

likelihood of the object actually making a close Earth approach (for example,

within the distance of the Moon) in the foreseeable future. The immediate

prediction of actual close Earth approaches is necessary to identify potential

threats and optimal opportunities for ground-based characterization of the

objects by ground-based photometric, spectroscopic, and radar techniques.

During close Earth approaches, the astrometric data (optical and radar) used

to refine the orbital solutions are extremely powerful in driving down

ephemeris uncertainties. In addition, predicted close Earth approaches are

important in identifying future opportunities for spacecraft intercepts because,

during these close approaches, potential mission targets will have greatly

reduced ephemeris uncertainties, short communication distances, and

excellent opportunities for ground-based observations to complement the in

situ spacecraft data.

For the near-Earth objects of special interest as future radar targets, for future

space missions, and for those objects expected to make future close Earth

approaches, personnel at the Jet Propulsion Laboratory (JPL) have carried

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/neocomputational.html (3 of 4) [12/5/2001 1:41:38 PM]
out, or coordinated, the necessary astrometric observing programs, orbit

updates, error analyses and predictions for close approach circumstances. For

these special objects, the necessary astrometric data are either sent directly to

JPL, then relayed to the Minor Planet Center (MPC) or sent first to the MPC

and immediately relayed to JPL. In either case, the astrometric data for these

objects are resident in files at both the MPC and JPL.

For those near-Earth objects that are identified as having the potential to

closely approach the Earth, the following steps would be undertaken:

Additional optical astrometric data (and radar data if possible) would

be solicited to refine the orbit, and forward numerical integrations and

error analyses would be made to identify the times and distances (and

associated errors) for future close Earth approaches.

1. 

For those objects that are predicted to make close Earth approaches

within a few tens of years and are identified as either potential threats

to Earth or suitable spacecraft mission targets, special ground-based

observing programs would be established as follows:

Optical astrometric observations would be taken and reduced

with respect to accurate reference star positions such as those

contained in the Hipparcos and Tycho reference star catalogues.

1. 

If possible, radar astrometric data would be taken to further

refine the object's orbit and to reduce the future ephemeris

uncertainties.

2. 

Photometric, spectroscopic, and radar observations would be

taken to physically characterize the object's size, shape, albedo,

spin state, and infer its chemical composition and mass.

3. 

2. 

As more and more astrometric data are received, the orbit would be

continually improved and forward integrations would be run to refine

the predicted times and circumstances of coming close-Earth

approaches. For each new orbit, error analyses and impact probabilities

would be computed to define the object's position and position

uncertainties for future close Earth approaches.

● 

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/neocomputational.html (4 of 4) [12/5/2001 1:41:38 PM]
XI. The Broader Significance of NEOs

The NEO survey recommended here constitutes a fiscally sensible first step in

assessing and reducing the risk of a globally catastrophic asteroid or comet

collision with Earth. The most likely, and certainly the most desirable

outcome of the survey, will be that none of the discovered NEOs will turn out

to be hazardous during at least the next century. Regardless of the outcome,

the survey will identify tens of thousands of small worlds that, at least for a

century, are not on collision course with Earth.

The potential promise of these "friendly" NEOs is highly significant for a

variety of reasons, most of which stem from their closeness to Earth and their

accessibility. The NEO population includes the cheapest targets of human and

robotic exploration beyond the Earth-Moon system. In fact, many of the

objects likely to be discovered will be accessible with very low delta V (i.e.,

less than required to land on the Moon and for only those with small orbital

eccenticities) and low roundtrip mission times (6 months or less). These

represent by far the easiest targets for human exploration (far easier than

landing on the Moon - no landing module required, minimal delta V for

rendezvous and escape).

Therefore, expeditions to NEOs are logical next steps in an evolutionary

program of human exploration of the solar system: The NEOs are humanity's

stepping stones to Mars and ultimately deeper space. Of course, any

economically viable vision of space exploration will place the highest

immediate priority on inexpensive robotic including multiple flyby,

rendezvous, and sample return. High-speed intercept of numerous NEOs by

microsatellites also appears feasible and cost-effective.

The rationale for telescopic and robotic reconnaissance of NEOs is

multifaceted. Pieces of NEOs that have fallen to Earth as meteorites show a

wide diversity of composition. Most contain at least some free metal

(iron-nickel alloy and small portions of platinum-group metals and gold) and

chemically-bound oxygen. Many contain organic chemicals and water of

hydration. The NEOs thus constitute key space resources with considerable

potential in the future exploration of space. In principal, NEO minerals could

be used to provide oxygen, water, biomass, and fuel to help sustain human

colonies. Many NEOs may be loosely bound aggregates that would facilitate

construction of a human habitat within a shell of material to secure shielding

from potentially lethal high-energy particles. In this light, NEOs may

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/significance.html (1 of 3) [12/5/2001 1:41:38 PM]
ultimately become the first long term human outposts beyond the Earth-Moon

system.

Meanwhile, it is desirable to begin to build up a base of knowledge for

deflecting or destroying a threatening object; thorough understanding of the

internal structure of NEOs will require robotic exploration and may require

geophysical reconnaissance by humans. Fortunately, the likely sizes of

accessible NEOs are well suited to the capabilities of human explorers.

The recent discovery that we exist in an asteroid swarm has enormous long

term consequences, and its historical importance may someday be seen to

rank with Columbus's discovery of the New World. Collisions with NEOs

have played a prominent role in the geological and biological evolution of

Earth, and one way or another, NEOs probably will play a role in the long

term future of human civilization. Our knowledge of the NEO population

(and of its influence on the origin and evolution of Earth and life) is in its

infancy, and detailed information about individual objects is extremely sparse.

However, a science of NEOs has emerged, and can be expected to mature in

parallel with the discovery and exploration of the NEO population.

It is thought that the NEO population is derived from a great range of sources

-- primitive asteroids, differentiated asteroids, and extinct comets. Probably

they include everything from returning fragments of Earth-Venus

planetesimals to Uranus-Neptune planetesimals and Kuiper belt objects.

Because of their frequent close passes by the terrestrial planets, the dynamical

evolution of NEOs is extremely complex. Definition of the orbits of a

sufficiently large sample of NEOs is likely to elucidate mechanisms for their

delivery into Earth-crossing orbits and should also refine statistical

assessments of collision probabilities. This work is coupled to determination

of cratering rates and the chronology of evolution of the terrestrial planets. It

also should constrain ideas about delivery of volatiles to Earth and the

terrestrial planets and the influence of this process on the early history of life.

During the past few decades, NASA's discovery and exploration of other

worlds has captured the imagination of a generation of young people who

decided to embark on careers in science and engineering. These achievements

of America's space program have truly elevated the human spirit. History tells

us that the need to explore is built into the human psyche. Most would agree

that an exciting, yet affordable, domestic space program can help to ensure

the vitality of our society as we begin the next millennium. All indications

from telescopic observations are that many NEOs are unlike any objects yet

seen by spacecraft, extraordinarily strange places that pose unique challenges

for investigations by robots and humans.

Accordingly we recommend that NASA view the NEO survey as part of a

broad-based program of NEO research focused on robotic and human

exploration objectives. NASA should embrace discovery and exploration of

Earth-crossing asteroids as a cornerstone of the space program, as a fiscally

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/significance.html (2 of 3) [12/5/2001 1:41:38 PM]
responsible initiative with virtually enormous guaranteed return.

The United States could celebrate the dawn of the Third Millennium by

declaring its intention to land humans on an asteroid to conduct intensive

scientific exploration and to return them safely to Earth in the first decade of

the next century.

Note Added in Press by Near-Earth Objects Survey Working Group on 30

June 1995: This study was conducted under severe time restrictions, thus it

was not possible to consider certain aspects that would otherwise have been

explored more thoroughly. In particular, the Working Group did not consider

surveys from space. Proper study of space-based surveys would have required

more resources and time than were available to the Working Group, and are

therefore deferred to a future study.

Likewise, it must be stated that the plan outlined in this report does not fully

address the hazards posed by comets, especially long-period ones and

dormant nuclei. This matter deserves further study, both in terms of the level

of hazard posed and in effective means of detection of such objects.

This report deals primarily with scientific and technical issues related to

near-term assessment of the impact hazard, within the constraints imposed by

a severe fiscal environment. However, in this section we briefly consider

certain other aspects of near-Earth objects, primarily from the perspective of

future exploration and exploitation. The Study Group presents these as

interesting possibilities with varying degrees of practicality, ranging from

`presently economically and technically feasible' in the case of robotic

exploration to `provocative and interesting' in the case of resource extraction.

Clearly, the most significant dividend from near-term exploration of

near-Earth objects will be improved understanding of the roles played by such

objects in the origin and evolution of our planetary system, and their influence

on humans and other living organisms.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/significance.html (3 of 3) [12/5/2001 1:41:38 PM]
XII. Suggested Five-Year Budget for NASA Support of NEO

Survey

The level of funding required to carry out the recommended program is as follows:

Years from Beginning of Funding

 Year 1 Year 2 Year 3 Year 4 Year 5

 Optical Survey

 Equipment 2.5 0.5 0.2 1.2 1.0

 Development &

 Operations 1.0 1.0 1.3 1.5 1.7

 New 2-m

 Telescope 0.3 2.0 2.0 2.0 0.1

 Data Centers 0.2 0.2 0.4 0.5 0.6

 Physical

 Observations 0.3 0.6 0.8 1.0 1.1

 

 TOTAL 4.3 4.3 4.7 6.2 4.5

The total cost for five years is $24 million. Beyond the first 5 years, the annual costs drop down to operational

costs of about $3.5 million per year. Major investments are needed in advanced focal-plane arrays, supporting

computers, and one new dedicated 2-meter telescope in the first five years. The survey cannot come close to

meeting a goal of 90% completeness down to 1 km diameter NEOs without such an investment.

Along with the acquisition of the necessary equipment, substantial development of computer programs to handle

the data flow from large format CCDs and the maintenance and augmentation of skilled observing teams is

required. The costs of operation of the observing facilities will increase as these facilities become fully

instrumented.

A huge increase in the number of discovered asteroids will require support for centers that receive the positional

observations, collate and coordinate observations, and complete preliminary and final orbits. The level of support

needed will increase as the discovery rate ramps up to a plateau in about five years.

We recommend that physical observations of NEOs be increased from the present modest level of effort as the

rate of discovery ramps up. Both optical (including infrared) and radar observations are needed. To obtain a good

understanding of a representative sample of NEOs, an appropriate level of effort would be about $1.1 million per

year, when discovery rate reaches a plateau.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/fiveyear.html [12/5/2001 1:41:38 PM]
References

Isobe, S., 1994, Proposed observational network to detect near-Earth objects

in Japan: in Seventy-Five Years of Hirayama Asteroid Families, Kozai, Y.,

and Binzel, R. P., eds., Astronom. Soc. Pacific Cont. Ser., v. 63, p. 220-222.

Rabinowitz, D., Bowell, E., Shoemaker, E., and Muinonen, K., 1944, The

population of Earth-crossing asteroids: in Hazards Due to Comets and

Asteroids, T. Gehrels, ed., Univ. of Arizona Press, p. 285-312.

Shoemaker, E. M., Weissman, P. R. and Shoemaker, C. S., 1994, The flux of

periodic comets near Earth: in Hazards Due to Comets and Asteroids, T.

Gehrels, ed., Univ. of Arizona Press, p. 313-335.

Steel, D., 1995, Asteroid discovery efficiencies for telescope systems at

Siding Spring (unpublished report).

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/ref.html [12/5/2001 1:41:39 PM]
Appendix I

Conclusions of Morrison Committee

Report

Impact Hazard.

The greatest risk from cosmic impacts is associated with objects large enough to

disturb the Earth's climate on a global scale by injecting large quantities of dust

into the atmosphere. Such an event would depress temperatures around the

globe, leading to massive loss of food crops and possible breakdown of society.

Various studies have suggested that the minimum mass impacting body to

produce such global consequences is several billions of tons. The corresponding

threshold diameter for Earth-crossing asteroids or comets is between 1 and 2 km.

Smaller objects (down to tens of meters diameter) can cause severe local

damage but pose no global threat.

Search Strategy.

Current technology permits us to discover and track nearly all asteroids and

short-period comets larger than 1 km in diameter (estimated population about

2000) that are potential Earth-impactors. These objects are readily detected with

moderate-size ground-based telescopes. What is required is a systematic survey

that effectively monitors a large volume of space around our planet and detects

these objects as their orbits repeatedly carry them through this volume of space.

In addition, long-period comets could be detected with advance warning of

several months before impact, using the same telescopes used for the asteroid

survey.

Spaceguard Survey Network

The survey involves a coordinated international network of specialized

ground-based telescopes for discovery, confirmation, and follow-up

observations, monitoring about 6000 square degrees of sky per month with

automated signal processing and detection systems that recognize asteroids and

comets from their motion against the background stars. The telescopes should

reach astronomical magnitude 22. For purposes of the Spaceguard Survey

Report, we focus on a network of six 2.5-m aperture, f/2 prime focus reflecting

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append1.html (1 of 2) [12/5/2001 1:41:39 PM]
telescopes, each with four 2048x2048 CCD detectors in the focal plane.

Follow-up and Coordination

In addition to discovery and confirmation, the Spaceguard Survey program will

require follow-up observations to refine orbits, determine the size of

newly-discovered objects, and establish the physical properties of the asteroid

and comet population. Observations with large planetary radars are an especially

effective tool for the rapid determination of accurate orbits. The survey program

also requires rapid international electronic communications and a central

organization for coordination of observing programs and maintenance of a

database of discovered objects and their orbits.

Expected Survey Results

Numerical modeling of the nominal survey indicates that about 500

Earth-crossing asteroids will be discovered each month. Over a period of 25

years we will identify more than 90 percent larger than 1 km in diameter. The

advantage of this survey approach is that it achieves the greatest level of

completeness for the largest and most dangerous objects; however, if continued

for a long time, it will provide the foundation for assessing the risk posed by

smaller objects as well. Continued monitoring of the sky will also be needed to

provide an alert for potentially dangerous long-period comets.

Conclusions:

The Spaceguard Survey is an essential step toward a program that can reduce the

risk associated with an unforeseen cosmic impact by more than 75 percent over

the next 25 years.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append1.html (2 of 2) [12/5/2001 1:41:39 PM]
 Appendix II

History of Discovery and Observations of

NEOs

The recognition of Earth-crossing comets occurred during the founding period of

modern science in the 17th century. Edmond Halley, who carried out the first

systematic determination of comet orbits, found that the orbits overlapped the orbit

of the Earth. He recognized immediately that comets could collide with the Earth.

Over the course of the intervening centuries, hundreds of Earth-crossing comets

have been discovered. Awareness of the possibility of comet impact with Earth

was heightened by the near miss of Comet Lexell in 1770, which passed within

three million km of the Earth. Pierre Simon de LaPlace, among others, speculated

on the consequences of comet impact for mankind. However, this awareness faded

during the 19th century, owing, perhaps, to a widespread opinion that comets were

entirely diffuse objects and, therefore, not threatening. This misconception was

decisively challenged in 1950 by Fred Whipple, who showed that the diffuse coma

of a comet must originate from a solid nucleus. Whipple's conclusion has since

been confirmed by infrared and radar studies of comet nuclei and by imaging of

the nucleus of Halley's comet from flyby spacecraft. Impact of the nuclei of

Shoemaker-Levy 9 on Jupiter in 1994 provided a direct demonstration of the

effects of cometary collision with a planet.

The first discovered Earth-approaching asteroid was (433) Eros, found in 1898 by

G. Witt at Berlin. (1862) Apollo, discovered by K. Reinmuth at Heidelberg in

1932, was the first recognized Earth-crossing asteroid, although (887) Alinda,

found by M. Wolf at Heidelberg in 1918, is now recognized as an Earth crosser.

Two more Apollos, (2101) Adonis and 1937 UB (Hermes) were discovered in the

1930's. On the basis of these early discoveries, the population and impact rate of

Earth-crossing asteroids larger than 1 km in diameter was correctly inferred by

Watson (1941). By 1972, a total of 13 recognized Earth crossers had been

discovered, chiefly in the course of stellar proper-motion surveys and a broad

survey of the sky with the 1.2-m Schmidt telescope at Palomar Observatory. Four

of these objects that were considered lost have been recovered and are now

numbered asteroids.

In 1973, the first dedicated survey for near-Earth asteroids was initiated by Helin

and Shoemaker (1979) at Palomar Observatory, California. This survey used the

46-cm Schmidt telescope for photographing a region near opposition (the direction

opposite from the Sun) each month. A second survey with the Palomar 46-cm

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append2.html (1 of 2) [12/5/2001 1:41:40 PM]
Schmidt, started by E. M. Shoemaker and C. S. Shoemaker in 1983 and continued

through 1994, photographed a much broader region of the sky on a monthly basis.

A parallel effort on the same telescope was carried out by Helin. T. Gehrels began

a development effort to utilize CCD detectors to scan the sky for NEOs in 1981.

By 1989, a 2048 x 2048 pixel CCD was installed in the Spacewatch telescope at

Kitt Peak, Arizona, and the Gehrels team began discovering a substantial number

of near-Earth asteroids with this system. A fruitful photographic search program in

the southern hemisphere was begun in 1990 at Siding Spring Observatory, New

South Wales, under the direction of D. I. Steel. This program uses plates taken

with a 1.2-m Schmidt for NEO detection and two other telescopes for follow-up. In

addition to the dedicated surveys, summarized by Carusi et al. (1994), discoveries

of NEOs have been reported from many other observatories world-wide, including

amateur observatories in Japan. Significantly increased attention has been given to

Earth-approaching asteroids since the beginning of the first systematic survey

effort at Palomar. About 250 Earth-crossing asteroids have been discovered to date

from the combined effort of all observers.

Photometric, spectrophotometric, infrared observations all made with optical

telescopes, and radar observations are the entire basis for our understanding of the

sizes, shapes, rotation states and mineral composition of the near-Earth asteroids.

Observations of (433) Eros and the Apollo asteroid (1685) Toro were carried out in

1972. The discovery at Palomar of (2062) Aten, in 1976, initiated a period in

which physical observations of many near-Earth asteroids were obtained during the

discovery apparition. The first radar detection of an NEO was made of the Apollo

asteroid (1566) Icarus in 1968 by R. M. Goldstein; since 1975, radar observations

have been made of numerous NEOs by S. J. Ostro and his colleagues. To date,

physical observations of one type or another have been obtained for more than 80

near-Earth asteroids (Chapman, et al.1994)

References

Carusi, A., T. Gehrels, E. F. Helin, B. G. Marsden, K. S. Russell, C. S. Shoemaker,

E. M. Shoemaker, and D. I. Steel, 1994, Near-Earth objects: present search

programs: in Hazards Due to Comets and Asteroids, T. Gehrels, ed., Univ. of

Arizona Press, pp. 127-147.

Chapman, C. R., A. W. Harris, and R. Binzel, 1994, Physical properties of

near-Earth asteroids: Implications for the hazard issue: in Hazards Due to Comets

and Asteroids, T. Gehrels, ed., Univ. of Arizona Press, p. 537-549.

Helin, E. F., and E. M. Shoemaker, 1979. Palomar Planet-Crossing Asteroid

Survey 1973-1978. Icarus 40, 321-328.

Watson, F., 1941. Between the Planets. Blakiston Co., Philadelphia.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append2.html (2 of 2) [12/5/2001 1:41:40 PM]
Appendix III

Evaluation of CCD Systems for Near-Earth Object Surveys

Alan W. Harris*

A commonly stated goal to address the hazard of a possible impact of a

Near-Earth Object (NEO) with the Earth is to conduct a nearly complete survey

of all NEOs larger than ~1 km within a time of ~10 years. Current photographic

surveys have a completion time scale of more than a century. Several automated

systems using CCD detectors and computer searching algorithms promise much

better performance. In this paper, we consider three such systems which are in

various stages of development to evaluate how well they can meet the above

stated goal. The systems are Spacewatch II, a 1.8 m telescope currently under

construction to be installed on Kitt Peak, Arizona; LONEOS (Lowell

Observatory Near-Earth Object Survey), a 0.6 m Schmidt telescope system under

construction at Lowell Observatory, Arizona, and the GEODSS (Groundbased

Electro-Optical Deep Space Surveillance) system of 1m telescopes currently

operated by the USAF for tracking Earth satellites.

To evaluate these systems, we generated a synthetic set of monthly positions of

1000 NEOs chosen to mimic the expected distribution of real objects, and

estimated the fraction which could be detected by each of these systems under

expected levels of performance for an interval of 10 years. A general result from

these simulations is that the best strategy is to cover the entire accessible sky

every month, rather than cover a smaller area to a fainter threshold brightness.

We find that each of these systems could potentially achieve a level of

completeness in the range of 80% or more, for all objects larger than 1 km. For a

variety of practical reasons, it may require a system of two or more telescopes to

actually achieve this level of completeness, including reliable orbit determination

of the objects detected.

INTRODUCTION

In recent years, it has become evident that the Earth is moving through a swarm

of asteroids and comets in orbits which come very close to the Earth's, and that

these objects, collectively referred to as Near-Earth Objects (NEOs) pose a

hazard to life, if one were to impact the Earth. Compared to other natural

disasters such as floods, earthquakes, volcanic eruptions, etc., impacts by

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (1 of 10) [12/5/2001 1:41:41 PM]
asteroids or comets are far less frequent, but potentially the most severe of

natural disasters . Thus a prudent first step in addressing this hazard is to catalog

most such objects large enough to cause global disaster (objects of diameter

> 1 km) in the relatively near future. Present surveys, most notably that done

at Palomar Observatory using a photographic Schmidt telescope of 18" aperture,

would require more than a century to reach a substantial level of completeness of

the 1 - 2 thousand such objects estimated to exist.

Several telescope systems are under development which could substantially

increase the rate of coverage, and offer the possibility to obtain a fairly complete

inventory on the time scale of a decade. In this paper, we evaluate three such

systems:

The Spacewatch (SW) Telescope on Kitt Peak, Arizona. The present

(operating) system is a 1.2 m telescope with a single CCD detector. A

second telescope (SW-II) of 1.8 m aperture is under construction. Since

SW-I is the only currently operating system, we use its demonstrated

performance to scale capabilities of the other planned systems. For SW II,

we consider briefly the system in its start-up configuration, but more

thoroughly with more capable focal plane instrumentation, comparable to

that contemplated for the other systems evaluated. We call this system

"Super-Spacewatch II".

1. 

The Lowell Observatory Near-Earth Object Survey (LONEOS) telescope

is a modified Schmidt telescope of 0.58 m aperture, which is under

construction at Lowell Observatory. "First light" is expected during this

year. As with SW-II, we evaluate the system capability as it will be when

upgraded to its full capability, rather than the initial "testbed" system.

2. 

The USAF Space Command currently operates a network of 1 m, f/2

wide-field telescopes, the Groundbased Electro-Optical Deep Space

Surveillance (GEODSS) system, for tracking Earth satellites. Current

plans call for upgrading these telescopes to operate with large format CCD

detectors, which with only minor modifications and changes to the

computer software, might be effectively employed for NEO surveys.

3. 

The evaluation of the various planned or possible survey systems requires several

steps. First, we evaluate the limiting magnitude expected for a given system, as a

function of rate of motion in the sky. A moving object will record as a streak,

and thus the limiting magnitude decreases with increasing motion. In addition,

we define the rate of sky coverage. For most planned systems, this is variable.

Covering larger sky area per month requires decreasing exposure time, so we

define the limiting magnitudes for several rates of sky coverage. In order to

estimate the effectiveness of a given system and coverage strategy, we generated

a synthetic sample of 1000 NEO orbital elements, randomly chosen to fit

expected distributions of orbital elements of actual objects. We then generated

"predictions" of position, motion, and magnitude for each object for each of 125

lunations (10 years). The final step in the evaluation is to "filter" the set of

125,000 positions according to the sky area coverage and magnitude limits of

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (2 of 10) [12/5/2001 1:41:41 PM]
each given system to estimate the fraction "discovered" by a given system. It

should be emphasized that this algorithm defines "discovery" as a detection.

Since several passes (detections) are required to confirm a moving object, a

crude rate of motion is implicitly included, but full orbit determination is not.

This could be accomplished employing a follow-up telescope, taking some time

from the survey mode to do follow-ups, or by simply surveying at several times

the rate of sky coverage so that multi-day arcs are obtained of each object in the

survey area.

LIMITING MAGNITUDE

The limiting visual magnitude for an untrailed image, m , is scaled from the

empirical limit derived for Spacewatch :

A is the effective light collecting area of the telescope, that is, the area of the

primary mirror, less obstructed area, times the transmission factor of the optics.

The above relation yields a limiting magnitude ~0.8 less than the theoretical limit

based on photon statistics, as noted by Rabinowitz, and also confirmed by the

theoretical magnitude threshold computed by Tennyson for GEODSS. We

adopt the above formula for the limit for all systems evaluated, but recognize that

this is conservative, with ~0.5 magnitude or more improvement being

theoretically possible.

In Table 1 and Figure 1, we evaluate the limiting magnitude vs. rate of motion of

an object in the sky for the current Spacewatch, Spacewatch II, LONEOS and

GEODSS, scaling from the above relation. We also tabulate and plot the

performance of the original Spacewatch, as given by Rabinowitz2, which was

used for scaling to the other systems.

Q is the quantum efficiency of the CCD detector. For unthinned,

front-illuminated detectors (Original Spacewatch and LONEOS) we have taken

this to be an average of 0.25 over the spectral band 0.50 to 0.85 m. For thinned,

anti-reflection coated, back-illuminated detectors (Current Spacewatch,

Spacewatch II and GEODSS), we have taken Q to have an average value of 0.75

over the spectral band 0.45 to 0.80 m.

t is the integration time. For sidereal scanning (with the telescope clamped down,

as with Spacewatch) this is fixed by the drift rate. Scanning can also be

accomplished at a faster rate by driving the telescope in declination and clocking

the CCD chip at a faster rate, e.g. Spacewatch II or LONEOS. In the "stare

mode", with the telescope tracking at sidereal rate, the exposure time may be

chosen to be almost any value. Any of the evaluated systems can operate in this

mode. It is planned as the only mode for GEODSS.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (3 of 10) [12/5/2001 1:41:41 PM]
a is the sky area (in arcsec ) which contains the image of a point source.

Generally this is the area of a square array of pixels needed to span the image

width plus some margin. for the Spacewatch cameras, it is 3x3 pixels; for

LONEOS and GEODSS it is 2x2 pixels.

(S/N) is the limiting signal-to-noise level that is deemed detectable. We take

this to be 4 for all systems evaluated, except for Spacewatch, where Rabinowitz

uses a value of 6.

 is the rate of motion in the sky at which a point image trails by the width

of the image during the exposure time t. For GEODSS, the planned mode of

operation is to take ~10 short exposures for each set position, and then co-add

them with all possible offsets to eliminate trailing. Thus in effect, , where t is the

total integration time of the ten exposures. The limiting magnitude for a short

trailed image of an object moving at rate r > r is:

m = m -1.25 log (r / r ).

r is the rate of motion at which the trail becomes too long to reconstruct as a

single signal. This limit occurs for a variety of reasons, including limitations of

computer capability, long trails crossing confusing images, or just the physical

dimensions of the CCD chip. We take r to correspond to a trail length of 100

pixels for all systems. For r > r 2, the limiting magnitude is given by:

m = m -2.5 log (r / r ),

where m = m -1.25 log (r / r ) is the limiting magnitude at the rate r .

The present Spacewatch system uses a strategy of three passes over the same

area of sky to achieve detections. The three passes are necessary to eliminate

false images and chance coincidences from genuine objects. Spacewatch II and

LONEOS will employ the same strategy. It is expected that GEODSS can

achieve reliable detections with only two passes, since the 10 sub-exposures in

each pass can serve to eliminate false images (cosmic ray hits, for example) and

establish rough rates of motion which must correlate between passes to be

counted as real detections.

RATE OF SKY COVERAGE AND

AVAILABLE SKY AREA

The rate of sky coverage is just the FOV of the instrument, divided by the

exposure time, divided by the number of passes per area (3, or 2 for GEODSS).

Roughly speaking, there are ~150 hours of darkness (dusk to dawn, and moon

below the horizon) per month. Assuming weather, duty cycle losses, etc. can be

held to less than 1/3, the annually averaged time available is about 100 hours per

month, so the rate of sky coverage per month is ~100 times the rate per hour. For

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (4 of 10) [12/5/2001 1:41:41 PM]
Spacewatch I and II and LONEOS, we assume one telescope dedicated full time.

For GEODSS, one can contemplate less than full time dedication, or more than

one telescope. Thus the rates of sky coverage might be adjusted accordingly.

Note that 10,000 sq. deg. corresponds to a circle of ~60 radius.

After some initial evaluations, to be discussed later, it became clear that the

generally best strategy is to cover all available sky each month. Thus we

evaluated that matter in some detail. In addition to limitations due to darkness

and horizon, it is not practical to observe too close to the galactic plane, because

the number of confusing stars becomes so great that images of asteroids have an

increasing probability of being obscured by such stars. This limit is of course

magnitude dependent: the fainter the limiting magnitude, the farther from the

galactic plane one must stay. In the range of the systems evaluated, m ~

19-21, the limiting galactic latitude is ~20. We also required that objects be >25

above, and the sun be >10 below, the horizon. Imposing these limits, we

calculated, as a function of station latitude, the total available sky area, the

number of hours of darkness each month, and the ratio of the two, which is the

required rate of sky coverage (Figure 2). From these calculations, we conclude

that a net rate of sky coverage of ~130 sq. deg./hour is necessary to cover all

available sky each month. Since the total available sky area is correlated with the

length of the night, the required scan rate is not as variable throughout the year as

the length of night.

SURVEY SIMULATION

In order to evaluate the effectiveness of each of the systems for carrying out an

inventory of NEOs, we simulated a 10 year survey in the following way. We first

generated a set of 1000 orbital elements, chosen to match the statistical

distributions expected of NEOs. For eccentricity and inclination, we used

two-dimensional Maxwellian distributions with dispersions chosen to match

those of known NEAs: RMS (e) = 0.54 and RMS (i) = 21 . In the case of

eccentricity, we had to truncate the distribution at 0.95, since one or two

members of the distribution tended to be >1.0 if unconstrained. For semi-major

axis, we used a uniform distribution, subject to the following limitations: the

perihelion must be less than 1.05 AU [a(1-e) < 1.05]; and the aphelion must be

greater than 0.95 AU but less than 4.8 AU [0.95 < a(1+e) < 4.8]. Thus all orbits

are capable of approaching within 0.05 AU of the Earth's orbit, but are not

capable of crossing Jupiter's orbit. In making this exclusion, we may be

removing from the sample "dead comet" nuclei in Earth and Jupiter crossing

orbits. There are presently no such objects in our NEA catalogs. If they exist,

they are no more difficult to detect than other low-albedo NEAs, except for the

fact that they come close enough to be discovered less frequently. But by the

same token, they have an exactly proportionate lower probability of collision

with the Earth, so they are just as easy to detect in proportion to the risk

associated with them. Thus we feel that the simulation with this limitation is

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (5 of 10) [12/5/2001 1:41:41 PM]
valid. We can also note that in the sample generated according to the above

constraints, we did obtain a number of orbits which have aphelia inside the orbit

of the Earth, i.e. in the range 0.95-1.0 AU. These are the interior equivalents to

Amor asteroids, and are theoretically expected. To date, none have been

detected, but that is likely do to the fact that present searches are mostly confined

to less than 90 from the opposition point, where such bodies can never be

found. Such a body could, however be perturbed by the Earth or Venus into an

Earth-crossing orbit in a relatively short time period, thus constituting an impact

hazard. Thus we consider it prudent to include a hypothetical population of such

bodies to be sure that our survey designs are capable of detecting them. Figure 3

is a a set of plots of a vs. e vs. i for the set of 1000 elements.

Having chosen a, e and i for each object, we then assigned values of the

argument of ascending node and mean anomaly at epoch to be uniformly

distributed over 360 . The argument of perihelion was chosen uniformly within

a range of values which brought to orbit within 0.05 AU of the Earth. In other

words, we required that each orbit in the sample be such that the NEOs can

currently pass within 0.05 AU of the Earth. This eliminates from the sample

objects which are currently (over a period of centuries) not in hazardous orbits,

even if they do cross a heliocentric distance of 1 AU (but far out of the plane of

the Earth's orbit).

Having now defined the element set, we generated "predictions" for each object

for each lunation for 10 years. This turns out to be 125 lunations, and of course

covers two or more complete orbits for all objects in the sample. Each

"prediction" includes position in the sky, rate of motion, and a magnitude factor

which includes inverse-square-law effect for earth and sun distances, and the

effect of solar phase angle. Thus in performing a survey simulation, one simply

adds the magnitude parameter to an assumed absolute magnitude (H)

corresponding to an object of a given diameter and albedo, and compares that to

the magnitude threshold of the system being evaluated, to determine if the object

is "detected", given that it is in the survey area of the sky. By selecting different

limiting magnitudes (which is equivalent to different H values of the asteroids),

one can "filter" the set of positions and magnitudes to obtain detection statistics

for a given system for a range of sizes and/or albedos of NEOs.

SURVEY COMPLETENESS VS. AREA

OF SKY COVERAGE

For our first simulations, we specified the area of sky covered per month as the

radius of a circle on the celestial sphere centered on the opposition point, which

is the most productive area to search. In this experiment we made no restrictions

for horizon or closeness to the galactic plane. In Figure 4, we plot the

completeness as a function of time for three areas of sky coverage for GEODSS.

The magnitude limit was chosen appropriate for 1 km objects of albedo 0.15

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (6 of 10) [12/5/2001 1:41:41 PM]
(typical S class albedo), or 2 km objects of albedo 0.04 (somewhat darker than

average C, D, etc. objects). The limiting magnitude was adjusted to allow for the

differences for each of the exposure times required to obtain the sky coverage

specified. In Fig. 4, the two curves for 30 and 100 second exposures are probably

realistic in that those amounts of sky coverage can easily be obtained within the

constraints of horizon limitations, etc. The third curve may be a bit unrealistic in

that it implies coverage of the entire celestial sphere to within 43 of the

direction of the sun. This might be achievable from space, but not from any one

ground site. It is clear from these results that the best strategy is to trade

magnitude threshold for sky area, all the way to an extreme not actually

achievable from the ground. The best strategy is always to maximize sky

coverage each month. We have carried out similar evaluations for the other

systems considered, with always the same result. This becomes especially clear

for the larger systems with fainter limiting magnitude. Such telescopes see

essentially everything >1 km in diameter, if it passes into the field of view. So

the completeness is mostly just a measure of the phase space covered by the

system.

SURVEYS ASSUMING MAXIMUM-AREA

COVERAGE

Having established that the best strategy is to cover as much sky area as possible

each month, we developed a simulation in which it is implicitly assumed that all

sky accessible from a given site is covered each month, to a specific limiting

magnitude (as a function of rate of motion). For each system, the magnitude limit

was selected appropriate for a rate of sky coverage that would result in covering

the whole sky each month from a given site. Figure 5 is a plot of completeness

vs. time for each of the three systems evaluated. For LONEOS, this plot is for 68

second exposures, with the magnitude and rate parameters shown in Table 1 and

Figure 1. For GEODSS, all-sky coverage corresponds to ~20 sec. exposures,

with limits which fall between those for 10 and 30 sec. exposures. For

Spacewatch, all-sky coverage cannot be achieved with the single-chip focal plane

presently being constructed. However, the telescope is optically and

mechanically capable of accommodating a 4-chip mosaic, which with an

improved read-out rate to allow efficient 10-sec exposures, could be upgraded to

have the capability to cover the whole available sky each month. Such an

upgraded system is entirely feasible and is comparable to the "full-up" systems

contemplated for the other telescopes. We can see in Figure 5 that all three of the

evaluated systems can detect most NEOs >1 km diameter (light) or >2 km

diameter (dark) in the ten year period of the survey simulation.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (7 of 10) [12/5/2001 1:41:41 PM]
COMPLETENESS AS A FUNCTION OF

SIZE OF NEO

We next adjusted the limiting magnitudes to evaluate the systems over a range of

sizes. The range covered was 0.125 km - 4 km for light objects (albedo 0.15), or

0.25 - 8 km for dark objects (albedo 0.04). Figure 6 is a plot of these results.

Note that completeness falls off seriously below 1 km (2 km for dark) for

LONEOS and GEODSS. SW-II maintains considerably better completeness

down to 0.5 km (light) or 1 km (dark).

We can average the results for light and dark objects, weighted by the relative

abundance of each type, to obtain an estimate of completeness for all objects as a

function of size. We note that there are ~10 times more high albedo objects than

dark ones among the known sample. But since dark objects must be about twice

the diameter to have the same intrinsic brightness and hence detectability as light

objects, and since objects of a given size are relatively ~5 times more abundant

than objects twice the diameter, in the same population, the bias-corrected

abundance of light and dark objects in the NEO population is probably about

50-50. Adopting this ratio, we plot in Figure 7 the completeness vs. size achieved

by each system, after 10 years of surveying.

As one final step, we note that in all that has gone before, we discuss only

completeness at a given size. We can integrate the completeness, weighting by

the expected relative abundance of objects as a function of size, to obtain a

completeness for all objects larger than a specified size. In Figure 8, we plot

completeness of all objects larger than a given size, as a function of the

lower-limit size, for the three systems evaluated. We have assumed a single

power-law population in the range 0.25 km - 4 km of:

where N(>D) is the number of bodies D. The power law index -2 is a

compromise average value in the range of interest.

Follow-up strategies

One can contemplate two strategies to catalog orbits rather than merely detect

objects. One way is to do targeted follow-up observations, either by assigning the

observations to a second telescope or by taking time from the discovery survey to

make these observations. A second mode is to cover the whole sky so often that

repeated detections of the same object are sufficient to yield orbit solutions from

the regular survey observations.

Presently, surveys are done in the first mode, of targeted follow-up. To make the

problem tractable, it is necessary to discriminate NEAs from the much more

abundant main-belt (MB) objects based on motion in the sky, before an orbit is

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (8 of 10) [12/5/2001 1:41:41 PM]
known. Thus there is a "blind spot" of slow sky motion where an NEA can

mimic a MB object and thus not be discriminated. As we go to surveys reaching

to fainter magnitude, discoveries will be made at greater distances, thus at slower

average motion, and the "blind spot" becomes a more significant loss factor. To

evaluate the two modes of obtaining orbits, we have assumed the use of a second

identical telescope. For purposes of this aspect of the study, we have used

GEODSS performance characteristics, but similar results could be derived for

other systems.

To explore the "targeted follow-up" mode, we ran a simulation of 5000 main belt

asteroids, predicted at 25 monthly positions, and tabulated the rate of motion

statistics as a function of position in the sky. From these data, we determined the

size of "box" in motion coordinates that is so heavily contaminated by main-belt

objects that discrimination on the basis of motion alone would not be possible.

We selected the box size such that we estimate that of any objects found with

motions lying outside the box, about half or more would be NEAs. Thus the

follow-up telescope would not be tasked with following up more than twice as

many objects as turned out to be NEAs. We then re-ran the simulation, with the

same limiting magnitude (20 sec. exposures) as before, but with objects scored as

"not detected" if their motion in the sky fell inside the "blind spot" for that

position in the sky.

We explored the alternate method of follow-up, that of simply covering the sky

more often, by reducing the limiting magnitude of the survey by 0.5 magnitude.

This is the reduction in threshold that results in reducing the exposure time by a

factor of ~2.5. In this scenario, both telescopes "plow the skies" at 2.5 times the

previous rate, hence the two telescopes cover the entire available sky about 5

times per month. We estimate that this should be enough to yield orbits on most

objects detected on most passes. Figure 9 is a summary of the completeness vs.

size of body for these two strategies. It should be noted that the curve for "no

follow-up is not directly comparable with the other two, as it assumes only one

telescope, while the other two assume two telescopes. So the "cost" of follow-up

is somewhat greater than one might infer from these plots. However, the other

two curves should be fairly valid measures of the two allocation strategies of

equal resources. Note that the "targeted follow-up" strategy is superior for

smaller size bodies, while the "multiple coverage" strategy is superior for the

larger bodies. Indeed, the curves "saturate" at the largest size range: essentially

everything that enters the field of view is detected. This is about 99% of the

entire sample, after 10 years. However, because of the "blind spot" that is a

necessary consequence of the "targeted follow-up" strategy, 7% of the objects are

"detected", nut never "recognized", because they never chance to exhibit

anomalous motion, no matter how bright they may be. Thus a 10-year search

using targeted follow-up can never be more than 92% complete, no matter how

faint the detection threshold may be.

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (9 of 10) [12/5/2001 1:41:41 PM]
CONCLUSIONS

If one asks the question, what is the likely largest size of any remaining

undetected object (that is, where completeness equals one over the number of

objects of that size expected), the answer is about 3 km for the evaluated

systems, after 10 years. Pushing this limit down to ~1 km would require a

Herculean effort. Thus we must accept some level of incompleteness. The

systems evaluated can yield completeness in the range 80-90% or better,

especially if all are used in concert. This level of completeness should reduce the

threat from collision by an undetected NEA to less than that posed by impacts

from LP comets, so in that context, we can declare these systems capable of

achieving the Spaceguard goal of reducing the hazard of asteroid collision by an

unknown object to below that from comets.

The most important lesson which emerges from this exercise is that the best

survey strategy is to cover the entire accessible sky every month, sacrificing

whatever magnitude limit is necessary to accomplish this. A very positive result

is that if that strategy is followed, adopting reasonable and even conservative

limits on sky observability, it is possible to obtain reasonable completeness in ten

years, including objects which never quite reach out to the orbit of the Earth and

hence never come to opposition. Thus the ability to observe closer to the sun or

to remove horizon limitations is not a sufficient justification in itself to move to a

space-based survey system.

ACKNOWLEDGEMENT

This work at the Jet Propulsion Laboratory, California Institute of Technology,

was supported under contract from NASA.

REFERENCES

D. Morrison, ed. The Spaceguard Survey: Report of the NASA

International Near-Earth-Object Detection Workshop. Jet Propulsion

Laboratory, Pasadena, 1992.

1. 

D. L. Rabinowitz, "Detection of Earth-Approaching Asteroids in near real

time," Astron. J. 101, 1518-1529, 1991.

2. 

P. D. Tennyson, "Applying Electro-Optical Space Surveillance

Technology to the Detection of Near Earth Asteroids," in Instrumentation

in Astronomy VIII, proc. of SPIE vol. 2198, 1286-1297, 1994.

3. 

Asteroid Comet Impact Hazards:NEO Survey Workgroup Report

http://impact.arc.nasa.gov/reports/neoreport/append3.html (10 of 10) [12/5/2001 1:41:41 PM]
Table 1. Parameter values used for system evaluations

Table 1

http://impact.arc.nasa.gov/reports/neoreport/figs/tableap_01.html [12/5/2001 1:41:42 PM]
Figure 1. Limiting visual magnitude vs. rate of motion in the sky for evaluated systems

Figure 1

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_01.html [12/5/2001 1:41:42 PM]
Figure 2. (a) Duration of darkness vs. month of the year; (b) net sky area visible all night

(less area near galactic plane, >25 above horizon) vs. month of the year; (c) rate of

sky coverage necessary to cover whole sky once per half-month, vs. month of the year.

Figure 2

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_02.html (1 of 2) [12/5/2001 1:41:43 PM]
Figure 2

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_02.html (2 of 2) [12/5/2001 1:41:43 PM]
Figure 3

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_03.html (1 of 2) [12/5/2001 1:41:44 PM]
Figure 3. Plots of a vs. e, i vs. e, and i vs. a for 1000 synthetically generated orbital

elements used for the survey simulations. e and i are distributed as two-dimensional

Maxwellian distributions with RMS dispersions chosen to match the observed

distribution of known NEAs: <e> = 0.54, <i> = 21 . To obtain values of a, we first made a

random (uniformly distributed) choice in the range from 0.5 AU to 2.9 AU. If the randomly

selected value of e brought the orbit inside the range 0.95 - 1.05 AU somewhere in the

orbit, the orbit was retained. If not, the orbit was rejected, as one which cannot get close

enough to the Earth to be a hazardous object. The upper limit of a was selected such

that objects are not both Earth and Jupiter approaching.

Figure 3

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_03.html (2 of 2) [12/5/2001 1:41:44 PM]
Figure 4. Rate of discoveries vs. time for one GEODSS telescope. Each curve represents a

different choice of exposure time, and consequently limiting magnitude (cf. Table 1), and

results in a different area of sky per month that can be covered. The curves represent the

discovery rate for ~1 km diameter objects of moderate albedo (0.15), or ~2 km diameter

objects of low albedo (0.04).

Figure 4

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_04.html [12/5/2001 1:41:44 PM]
Figure 5. Rate of discoveries vs. time for each of the three systems evaluated, assuming

that the rate of sky coverage is chosen such that all available sky area is covered each

month. The curves represent the discovery rate for ~1 km diameter objects of moderate

albedo (0.15), or ~2 km diameter objects of low albedo (0.04).

Figure 5

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_05.html [12/5/2001 1:41:45 PM]
Figure 6. Completeness of detections after 10 years for each of the evaluated systems, vs.

size of object. The scale for low albedo objects is given across the top of the plot, and

that for higher albedo objects is given across the bottom.

Figure 6

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_06.html [12/5/2001 1:41:46 PM]
Figure 7. Completeness of detections after 10 years for each of the evaluated systems,

vs. size of object, assuming a 50-50 mix of light and dark class objects at any given size.

Figure 7

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_07.html [12/5/2001 1:41:46 PM]
Figure 8. Completeness of detections after 10 years for each of the evaluated systems,

all objects larger than a given size. As with Fig. 7, a 50-50 mix of light and dark objects is

assumed.

Figure 8

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_08.html [12/5/2001 1:41:46 PM]
Figure 9. Completeness of survey vs. size of object (light and dark classes as in Fig. 6),

for different follow-up strategies (see text), using GEODSS as an example.

Figure 9

http://impact.arc.nasa.gov/reports/neoreport/figs/figureap_09.html [12/5/2001 1:41:47 PM]