NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

Near-Earth Object 

Survey and Deflection 

Analysis of Alternatives 

Report to Congress 

March 2007 
SUMMARY 

Section 321 of the NASA Authorization Act of 2005 (Public Law No. 109-155), also 

known as the George E. Brown, Jr. Near-Earth Object Survey Act, directs the NASA 

Administrator to transmit an initial report to Congress not later than one year after the 

date of enactment that provides: (1) an analysis of possible alternatives that NASA may 

employ to carry out the survey program of near-Earth Objects (NEO), including groundbased and space-based alternatives with technical descriptions; (2) a recommended option 

and proposed budget to carry out the survey program pursuant to the recommended 

option; and (3) an analysis of possible alternatives that NASA could employ to divert an 

object on a likely collision course with Earth. 

The objectives of the George E. Brown, Jr. NEO Survey Program are to detect, track, 

catalogue, and characterize the physical characteristics of NEOs equal to or larger than 

140 meters in diameter with a perihelion distance of less than 1.3 AU (Astronomical 

Units) from the Sun, achieving 90 percent completion of the survey within 15 years after 

enactment of the NASA Authorization Act of 2005. The Act was signed into law by 

President Bush on December 30, 2005. 

A study team, led by NASA’s Office of Program Analysis and Evaluation (PA&E), 

conducted the analysis of alternatives with inputs from several other U.S. government 

agencies, international organizations, and representatives of private organizations. The 

team developed a range of possible options from public and private sources and then 

analyzed their capabilities and levels of performance including development schedules 

and technical risks. 

Key Findings for the Survey Program: 

• The goal of the Survey Program should be modified to detect, track, catalogue, 

and characterize, by the end of 2020, 90 percent of all Potentially Hazardous 

Objects (PHOs) greater than 140 meters whose orbits pass within 0.05 AU of the 

Earth’s orbit (as opposed to surveying for all NEOs). 

• The Agency could achieve the specified goal of surveying for 90 percent of the 

potentially hazardous NEOs by the end of 2020 by partnering with other 

government agencies on potential future optical ground-based observatories and 

building a dedicated NEO survey asset assuming the partners’ potential ground 

assets come online by 2010 and 2014, and a dedicated asset by 2015. 

• Together, the two observatories potentially to be developed by other government 

agencies could complete 83 percent of the survey by 2020 if observing time at 

these observatories is shared with NASA’s NEO Survey Program.

• New space-based infrared systems, combined with shared ground-based assets, 

could reduce the overall time to reach the 90 percent goal by at least three years. 

Space systems have additional benefits as well as costs and risks compared to 

ground-based alternatives. 

• Radar systems cannot contribute to the search for potentially hazardous objects, 

but may be used to rapidly refine tracking and to determine object sizes for a few 

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NEOs of potentially high interest. Existing radar systems are currently 

oversubscribed by other missions. 

• Determining a NEO’s mass and orbit is required to determine whether it 

represents a potential threat and to provide required information for most 

alternatives to mitigate such a threat. Beyond these parameters, characterization 

requirements and capabilities are tied directly to the mitigation strategy selected. 

Key Findings for Diverting a Potentially Hazardous Object (PHO): 

The study team assessed a series of approaches that could be used to divert a NEO 

potentially on a collision course with Earth. Nuclear explosives, as well as non-nuclear 

options, were assessed. 

• Nuclear standoff explosions are assessed to be 10-100 times more effective than 

the non-nuclear alternatives analyzed in this study. Other techniques involving 

the surface or subsurface use of nuclear explosives may be more efficient, but 

they run an increased risk of fracturing the target NEO. They also carry higher 

development and operations risks. 

• Non-nuclear kinetic impactors are the most mature approach and could be used in 

some deflection/mitigation scenarios, especially for NEOs that consist of a single 

small, solid body. 

• “Slow push” mitigation techniques are the most expensive, have the lowest level 

of technical readiness, and their ability to both travel to and divert a threatening 

NEO would be limited unless mission durations of many years to decades are 

possible. 

• 30-80 percent of potentially hazardous NEOs are in orbits that are beyond the 

capability of current or planned launch systems. Therefore, planetary gravity 

assist swingby trajectories or on-orbit assembly of modular propulsion systems 

may be needed to augment launch vehicle performance, if these objects need to be 

deflected. 

Alternatives Considered to Detect, Track, Characterize, and Deflect/Mitigate NEOs 

The following tables provide a summary of the options considered. Technical 

descriptions of each option, as well as other combinations of alternatives, can be found in 

subsequent sections of this report. For each option, Table 1 shows the percentage of 

PHOs that would be found by the survey by the end of 2020 and the year each option 

would achieve 90 percent completion, starting with the option of sharing the use of 

potential ground-based observatories, which will be referred to as the “Reference” 

architecture through the rest of this document. Details regarding the availability of assets 

for each option are also found in subsequent sections. Table 1 shows that individually

each of the first three options fall short of meeting the Congressional goal. As shown in 

the last line of Table 1, the minimum survey architecture that achieves the goal would be 

a combination of the shared ground-based assets plus one of two dedicated asset options. 

2
Table 1. Detection and Tracking Capability Options & Summary Results 

Option* through 2020 Year 90%

Shared ground-based (Reference) 83% 2026 

Dedicated ground-based 85% 2024 

Dedicated Infrared sensor in Venus-like orbit 89% 2021 

Reference + One Dedicated Asset At least 90% Not Later than 2020 

* Details of each option are found in a subsequent section of this report.

Table 2. Characterization Options 

Option* Descriptions (O1 = Option 1) 

Option 1 Use Existing Assets + Detection and Tracking Systems 

Option 2 O1 + Dedicated Ground Systems 

Option 3 O1 + Dedicated Space-Based Remote Sensing (L1/L2) 

Option 4 O1 + Dedicated Space-Based Remote Sensing (Venus-Like 

Orbit) 

Option 5 O1+ O2+ O3 + 2 Flyby Missions to 8 Objects 

Option 6 O1 + O2 + O3 + 8 Orbiter Missions 

Option 7 O1 + O2 + O3 + Orbiters at a Fixed Threshold Probability 

of Impact 

* Details of each option are found in a subsequent section of this report.

Table 3. Impulsive Deflection/Mitigation Options 

Impulsive Technique* Description

Conventional Explosive 

(surface) 

Detonate on impact 

Conventional Explosive 

(subsurface) 

Drive explosive device into PHO, 

detonate 

Nuclear Explosive (standoff) Detonate on flyby via proximity 

fuse 

Nuclear Explosive (surface) Impact, detonate via contact fuse 

Nuclear Explosive (delayed) Land on surface, detonate at optimal 

time 

Nuclear Explosive (subsurface) Drive explosive device into PHO, 

detonate 

Kinetic Impact High velocity impact 

*A discussion of these techniques is found in a subsequent section of this report.

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Table 4. Slow Push Deflection/Mitigation Options 

Slow Push Technique* Description

Focused Solar Use large mirror to focus solar energy on a spot, heat 

surface, “boil off” material 

Pulsed Laser Rendezvous, position spacecraft near PHO, focus laser on 

surface, material “boiled off” surface provides small force 

Mass Driver Rendezvous, land, attach, mine material, eject material 

from PHO at high velocity 

Gravity Tractor Rendezvous with PHO, fly in close proximity for extended 

period, gravitational attraction provides small force 

Asteroid Tug Rendezvous with PHO, attach to PHO, push 

Enhanced Yarkovsky Change albedo of a rotating PHO; radiation from sunheated material will provide small force as body rotates 

* A discussion of these techniques is found in a subsequent section of this report.

Recommended Survey Program 

Currently, NASA carries out the “Spaceguard Survey” to find NEOs greater than 

1 kilometer in diameter, and this program is currently budgeted at $4.1 million per year 

for FY 2006 through FY 2012. We also have benefited from knowledge gained in our 

Discovery space mission series, such as the Near Earth Asteroid Rendezvous (NEAR), 

Deep Impact, and Stardust missions that have expanded our knowledge of near-Earth 

asteroids and comets. Participation by NASA in international collaborations such as 

Japan’s Hayabusa mission to the NEO “Itokawa” also greatly benefited our 

understanding of these objects. NASA’s Dawn mission, expected to launch in 

June 2007, will increase our understanding of the two largest known main belt asteroids, 

Ceres and Vesta, between the planets Mars and Jupiter. NASA conducts survey 

programs on many celestial objects— the existing Spaceguard program for NEOs, 

surveys for Kuiper Belt Objects, the search for extra-solar planets, and other objects of 

interest such as black holes to understand the origins of our universe. Our Discovery 

mission series in planetary science may offer additional opportunities in the future 

beyond our current survey efforts. 

NASA recommends that the program continue as currently planned, and we will also take 

advantage of opportunities using potential dual-use telescopes and spacecraft—and 

partner with other agencies as feasible—to attempt to achieve the legislated goal within 

15 years. However, due to current budget constraints, NASA cannot initiate a new 

program at this time. 

4
BACKGROUND 

Asteroids and comets are the two types of potentially hazardous objects (PHO) discussed 

in this study. For objects in the inner solar system, astronomers can distinguish these 

bodies on the basis of their appearance. Moving objects that appear as a star-like point of 

light are known as asteroids. Moving objects that appear diffuse or those that have 

visible tails are known as comets. People have known about comets since antiquity. The 

existence of asteroids, however, was not known until about 200 years ago when 

telescopes became powerful enough to detect the largest such objects. It has taken 

several generations of improvements in telescope design to detect and understand the 

small bodies that orbit near Earth. 

Differences in the appearance of comets and asteroids reflect in part a difference in their 

composition. Generally, asteroids are relatively rocky or metallic objects without 

atmospheres, while comets are composed in part of volatiles such as water ice that 

vaporizes when heated to produce a tenuous and transient atmosphere around the solid 

nucleus. Comets that are far from the Sun or those that have lost most of their volatiles 

can look like an asteroid. A volatile-rich object will develop an atmosphere only when 

heated sufficiently by a relatively close approach to the Sun. 

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

Objects (IEOs), depending on whether their orbits cross Earth’s orbit with a period of 

more than one year, cross Earth’s orbit with a period of less than one year, exist 

completely outside the Earth’s orbit, or exist completely within the Earth’s orbit, 

respectively. The distribution of these objects in the NEO population is shown in Figure 

1. 

Apollo

Semimajor Axis ≥ 1.0 AU

Perihelion ≤ 1.02 AU

Earth Crossing

Amor

1.02 AU < Perihelion ≤ 1.3 AU

Apollo 62% of known asteroids

Type Near-Earth Population

IEO 6 known asteroids

Amor 32% of known asteroids

Aten 6% of known asteroids

Apollo 62% of known asteroids

Type Near-Earth Population

IEO 6 known asteroids

Amor 32% of known asteroids

Aten 6% of known asteroids

Aten

Semimajor Axis < 1.0 AU

Aphelion ≤ 1.0167 AU

Earth Crossing

Inner Earth Objects (IEOs) 

Aphelion < 0.983 AU

Always inside Earth’s orbit 

(aka Apohele)

Figure 1. Near Earth Asteroid Orbit Types 

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Near-Earth Objects (NEOs) are asteroids and comets in orbits that allow them to enter 

Earth’s neighborhood, defined by astronomers as having a perihelion (closest approach to 

the Sun) of less than 1.3 AU (Astronomical Units, 1 AU is approximately 150 million 

km, the mean distance between the Sun and Earth). Extinct comets may make up 5-15 

percent of the NEO population, and some may retain volatiles. As of December 4, 2006, 

using the Safeguard Survey system described elsewhere in this report, NASA has 

identified 701 NEOs greater than 1 km in size and 3,656 NEOs smaller than 1 km in

size. Of the total number of NEOs surveyed, NASA has found only 63 comets. The 

estimated population of NEOs greater than 1 km in size is 1,100. The estimated 

population of NEOs greater than 140 meters in size is approximately 100,000 objects. 

A constant power law as shown in Figure 2 can be used to estimate the number of NEOs 

of a particular size based on our available observations. The figure shows a hundred-fold 

increase in the number of NEOs as the size decreases by an order of magnitude. In terms 

of the goals expressed by the George E. Brown, Jr. Near-Earth Object Survey Act, NASA 

estimates that the population of NEOs greater than 140 meters is approximately 100,000 

objects. Figure 2 also shows the approximate absolute magnitude (relative brightness) if 

the objects were placed at a standard distance of 1 AU from the observer, their average 

impact interval, and the approximate impact energy they would deliver in a collision with 

Earth. Given any size class of NEO, this estimate is probably accurate to within a factor 

of two or three, as there are not enough observations in some classes to form a 

statistically valid sample. 

Constant Power Law

Provides Good Fit to Data

~100k NEOs > 140m

~1k NEOs > 1km

Figure 2. Frequency of NEOs by Size, Impact Energy, and Magnitude 

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More relevant to this report is the definition of Potentially Hazardous Objects (PHOs),

asteroids and comets that have a potential to eventually impact the Earth. A PHO is an 

object in our solar system that passes within 0.05 AU (about 7.5 million km) of Earth’s 

orbit and is large enough to pass through Earth’s atmosphere and cause significant 

damage on impact; that is, about 50 meters and larger. In this report the term PHO will 

be used to indicate potential threats, with the understanding that those smaller than 1 km

are predominantly asteroids. Comets do not add substantially to the population below 1 

km. Approximately 21 percent of the NEOs of any given size class are expected to be 

potentially hazardous. 

In 2003, NASA chartered a Science Definition Team (SDT), which recommended an 

NEO survey program to produce a catalog that is 90 percent complete for PHOs larger

than 140 meters. The SDT determined that impacts from objects that are 140 meters in 

size would only produce regional effects, while larger objects would have corresponding 

wider effects such as large sub-global effects from impacts of a 300-meter object and 

global effects from 1-kilometer object impacts. Impact of objects 10 kilometers and 

larger are considered an extinction-class event for Earth. A survey program that 

completes a 90 percent survey of 140 meter or larger PHOs would also identify virtually 

all of the global risk from objects greater than 1 kilometer. A survey system could be 

constructed to catalog hazardous objects down to the air blast limit (about 50 meters 

in size). However, the Science Definition Team suggested that cataloging down to 140 

meters was the more prudent approach for the next-generation survey, a system which 

would also provide warning for 60-90 percent of objects capable of producing potentially 

dangerous air blasts. Essentially, the 140 meter object size is approximately where 

impacts transition from causing regional (e.g., a state or seaboard) to more localized (e.g. 

citywide) damage. 

Since by this definition, objects that do not pass within 0.05 AU of Earth’s orbit are not 

“potentially hazardous,” these objects are assessed (without necessity of discovery) to be 

no threat to Earth. Therefore, NASA recommends that the Survey’s goal be modified to 

detect, track, catalogue, and characterize 90 percent of all PHOs greater than 140 meters 

by the end of 2020 rather than 90 percent of all NEOs of greater than 140 meters that pass 

within 1.3 AU of the Sun as expressed in the George E. Brown, Jr. Near-Earth Object 

Survey Act. Limiting the objects to only PHOs will reduce the required population to be 

surveyed and the Survey program will have a more realistic goal. However, the Survey 

will still provide equally effective “warning and mitigation of the hazard” and 

corresponds with the recommendations of the 2003 SDT report.

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STUDY APPROACH FOR NEO SURVEY PROGRAM 

The PA&E study team expects that a survey system designed to detect NEOs down to 

140 meters in size would discover a total of 500,000 objects (of which 400,000 would not 

be NEOs), to include Main Belt Asteroids, during the first five-ten years of the Survey in 

the course of more than two million individual observations. About 20,000 (or 20 

percent) of the total estimated NEOs to be discovered will measure 140 meters or larger 

and be tracked as potentially hazardous. This volume of observations will require a dataprocessing capability that is 100 times more capable than the currently utilized cataloging 

system. After objects are detected, the system must be able to obtain follow-up 

observations, store and distribute collected data, and analyze these data for fortuitously 

observed but previously undetected objects. The current data management approach 

likely will not remain viable, and plans for the data-processing of this information must 

be carefully considered. 

Detect and Track 

Broadly, the options for the Survey Program to detect and track PHOs can be classified in 

three categories. Ground-based optical systems use large field of view sensors to scan 

the sky at night for PHOs. Space-based optical systems gather visible light from vantage 

points near the Earth or in Venus-like heliocentric orbits. Space-based infrared systems 

operate from similar vantage points, and use passively cooled infrared detectors to find 

and track objects. The advantages and drawbacks of each system are discussed in the 

following sections. 

Ground-Based Optical Systems 

Ground-based optical systems have several advantages over space-based systems. In 

general, ground-based systems are mostly based on mature technology and are relatively 

easy to maintain and upgrade because they are easily accessible. Consequently, these 

systems can be implemented using a phased approach and may take advantage of shared 

software. This typically means that ground systems cost less to build, verify, operate, 

maintain and upgrade than space-based counterparts. 

Because ground-based optical systems peer through the Earth’s atmosphere, drawbacks 

exist. Ground-based optical systems cannot operate during daylight or twilight and are 

subject to interference from weather, atmospheric turbulence, scattering from moonlight, 

and atmospheric attenuation. Significant atmospheric attenuation in the infrared-spectral 

region prevents these systems from determining accurate NEO sizes. These systems also 

will have difficulty finding objects in inner-Earth or Earth-like orbits. These objects will 

have fewer discovery opportunities from the ground because they are available only at 

the beginning and end of the night, when evening and morning twilight brightens the sky. 

Additionally, ground-based systems can have intangible programmatic issues related to 

access to the assets, as well as site and infrastructure maintenance. 

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Space-Based Optical Systems 

Space-based optical systems rely on mature technologies with a broad foundation of 

existing mission heritage. Like ground-based systems, their advantages are primarily 

based on operating location. Space-based systems can access almost the entire sky at any 

given time with no interference from weather, daylight, moonlight, or atmospheric 

attenuation. Also, they can observe objects in inner-Earth or Earth-like orbits more easily 

than ground-based systems, especially if the detectors are located at the Sun-Earth 

Lagrange 1 stability point or in a Venus-like orbit. 

Not only are space-based systems likely to be more expensive to develop than groundbased systems, space-based systems also have some additional risks. Getting a space-based 

system into place subjects it to possible launch and deployment failures and places it in an 

extreme environment that can result in a shorter lifetime (seven to ten years). This shorter 

lifetime is an important consideration if a NEO program is expected to continue to track 

objects for extended periods of time. In addition, they depend on spacecraft-to-ground data 

links and unique onboard software. 

With the exception of technology maturity, space-based infrared systems have the same 

advantages as space-based optical systems. For infrared systems this technology is 

maturing rapidly. Space-based, passively-cooled mid-infrared systems also have 

additional advantages as these systems require smaller apertures than optical systems of 

equal detection efficiency and provide more accurate estimates of object sizes. The 

object size uncertainties are less than 50 percent compared with 230 percent for visual 

detectors. A two-band infrared system could potentially lower the size uncertainties to 

about 20 percent. These space-based infrared systems also are much less affected by the 

problem of source interference. There are about 100 times fewer infrared sources per 

square degree at an infrared wavelength of 8 microns compared with the number of 

visible sources at 0.5 microns. In addition, space-based infrared systems have lower 

downlink data rate requirements than space-based visible detector systems because the 

mid-infrared sensors would have fewer pixels (by a factor of 15) with longer exposures 

(also a factor of 15) so there would be ~200 times less data to downlink. 

Characterization 

The study team recognized that characterization of PHOs to inform the options for 

mitigation or deflection depended on several factors. One factor is the possible 

relationship between detection, tracking, and characterization elements. For example, if a 

ground-based survey system were to be built, its concept of operations will likely limit its 

ability to do follow-up characterization unless search goals are refined. If a space-based 

survey system (such as an infrared system in a Venus-like heliocentric orbit) is built, 

options for characterization include upgrading the system with certain optical filters, 

building a second dedicated space-based characterization system based on the design of 

the first, or building a dedicated ground-based system. If detection and tracking assets 

are available after search goals are reached, some survey systems could transition to 

characterization functions after detection operations are less operationally intense. 

The most important factor in developing a characterization strategy is to tie its 

requirements to a specific mitigation strategy or selected deflection options. For 

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example, if only standoff nuclear explosive alternatives are considered, little 

characterization beyond orbit and approximate mass of the object would be required. If a 

deflection approach such as a space tug becomes the deflection alternative of choice, 

more information on the PHO structure, composition, and kinematics would be required 

and a higher investment in characterization is warranted. Due to the diversity and 

volume of information required to support all possible mitigation alternatives, a robust 

program would be required to enable all deflection systems. 

Two types of characterization information are necessary for mitigation, and that 

information is necessary for different phases of the mitigation problem. If developing a 

deflection option (or options) proceeds before an actual threat is identified, some

understanding of the general population of targets would be necessary. For example, if 

the distribution of PHO sizes and composition are not well understood, it would be 

difficult to know if a deflection concept would perform sufficiently to eliminate most 

likely threats. Additionally, if concepts, such as the space tug, are chosen for 

development, knowing the statistical distribution of rotation rates is a key development 

parameter. 

Second, some characterization is always needed on an actual threat once it is identified. 

For some alternatives, little characterization beyond mass and orbit will be required, 

although additional information may improve our ability to deflect such an object. 

Precursor missions, such as the recent Deep Impact mission or the proposed ESA Don 

Quixote mission, may be required to assure the successful design and implementation of 

a mission to divert a PHO. 

Remote characterization of a PHO is useful for mitigation in two instances. The first is to 

provide information after a threat has been identified, but before a decision for an in-situ 

characterization mission would be made. The second is to provide the only available 

information to inform a deflection mission, particularly when insufficient time is 

available to first visit the PHO. As the survey catalogue reaches 90 percent 

completeness, the likelihood of a short warning scenario should be reduced to less than 1 

percent of all warnings. 

In the case of a short warning scenario, remote means must be developed to provide the 

required information. Models can be developed from remote observations of a large 

number of NEOs or from in-situ data. These models then may be used to predict the 

characteristics of subsequent threats as they are detected. This information may allow 

planners to begin designing a deflection mission without first launching an immediate insitu mission. 

The study team found that it is premature to set specific characterization requirements to 

enable deflection until a mitigation strategy has been determined. Therefore, the study 

team developed characterization options that provide a range of capabilities. These 

options included the use of detection and tracking assets, dedicated ground and space 

systems for remote observation, and in-situ missions to inform mitigation options for 

threats with sufficiently high impact probabilities. 

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ANALYSIS OF ALTERNATIVES FOR NEO SURVEY PROGRAM 

Detection and tracking alternatives identified by the study team as viable included optical 

observing systems located on the ground and optical and infrared assets located in space. 

For ground-based alternatives, the study team considered shared use of possible future 

observatories to conduct NEO surveys, such as the PanSTARRS 4 (PS4), and the Large 

Synoptic Survey Telescope (LSST). The study team assumed that NASA would enter 

partnership agreements with other government agencies regarding the shared use of those 

potential ground-based observatories. 

The team also considered new NASA-funded facilities that would be dedicated to the 

search for potentially hazardous objects and would be based on the designs of these 

planned observatories. For this analysis, the study team assumed that any new NASAfunded ground-based observatory would be, in order to reduce development costs, a replica 

of either PS4 or LSST, and any new NASA-funded space-based asset would be an entirely 

new asset requiring full development. For the purposes of determining when the survey 

could be completed, acquisition of new systems was assumed to start October 1, 2007. 

Ground options included: 

• An architecture, which combines the sharing of the potential PS4 and LSST 

systems with a second dedicated NASA-funded LSST that potentially begins 

operations in 2015, was able to meet the goal of cataloging 90 percent of the 

potentially hazardous objects by 2020. 

• A shared PS4, a shared LSST, and a dedicated NASA-funded PS8 that potentially 

begins operations in 2014 were able to catalog 90 percent of potentially hazardous 

objects by 2024. (PS8 is a system of two PS4 telescopes.) 

• A dedicated, NASA-funded observatory based on LSST’s design that potentially 

begins operations in 2015 was also able to catalog 90 percent of potentially 

hazardous objects by 2024 without the need to share the assets of the other 

programs. 

Space-based search options included spacecraft located in low-Earth orbit, at Sun-Earth 

Lagrange stability points, and in heliocentric Venus-like orbits. Only an infrared system

operating in a Venus-like orbit was able meet the goal specified for the NEO survey 

program without the contribution of shared ground-based assets. All space-based 

alternatives were able to meet the goals when combined with a shared PS4 and a shared 

LSST. 

A space mission failure could delay achieving the 90 percent goal by six years (see Table 

10), after which the catalog could still be completed with the shared ground-based assets. 

Infrared systems operating in space could provide more accurate size estimates of up to 

80 percent of objects in the catalog. Observatories located in a Venus-like orbit are the 

most efficient at finding NEOs inside Earth’s orbit, a population which has the most 

uncertainty yet still poses a hazard due to gravitational orbit perturbations. Additionally, 

by the end of 2020, infrared systems in Venus-like orbits could find 90 percent of 

the objects measuring down to 80 meters, exceeding the 140-meter requirement. 

Selected space-based alternatives included: 

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• A 0.5-meter infrared system operating in a Venus-like heliocentric orbit that 

potentially begins operations in 2013 completes by itself 89 percent of the survey 

by 2020. 

• The same 0.5-meter infrared system in a Venus-like orbit working in concert with 

a shared PS4 and a shared LSST completes 90 percent of the survey by 2017. 

• A 0.5-meter infrared system operating at Sun-Earth L1 that potentially begins 

operations in 2013 working in concert with a shared PS4 and a shared LSST 

completes 91 percent of the survey by 2020. 

Infrared systems with a 1.0-meter aperture complete the survey about one year earlier 

than the 0.5-meter alternatives described above. Optical systems with 1.0-meter and 2.0-

meter apertures in Venus-like orbits, combined through partnerships with other 

government agencies on their potential PS4 and LSST ground-based systems, completed 

the survey by 2019 and 2018 respectively. 

Detection and Tracking Survey Alternatives Considered 

After an initial feasibility assessment of over three dozen concepts, Tables 5-7 list the 

detection, tracking, and data management alternatives that were considered viable for 

further analysis in this study. 

Table 5. Description of Ground-based Survey Alternatives 

Classification Concept Name Description

Visible - 

Ground 

Spaceguard Combined, existing ground-based detection efforts 

including LINEAR, NEAT, Catalina Sky, Spacewatch 

and LONEOS. 

Visible - 

Ground 

Shared LSST Large Synoptic Survey Telescope (LSST) is a 

potential 8m telescope that could be operational in 

2014 and would spend 75 percent of its time in survey 

mode. 

Visible - 

Ground 

Shared PS4 Potential four 1.8m Pan-STARRS telescopes 

searching same spot of the sky at a time with an 

effective aperture of 3.6m; they would spend 30

percent in survey mode. 

Visible - 

Ground 

Dedicated 

LSST 

Potential rebuild of Shared LSST, dedicated to NEO 

search. 

Visible - 

Ground 

Dedicated PS4 Rebuild of Shared PS4, dedicated to NEO search. 

Visible - 

Ground 

Dedicated PS8 Potential system of two PS4 telescopes searching same 

area of the sky at a time with an effective aperture of 

5.1m. 

Visible - 

Ground 

Dedicated PS16 Potential system of two PS8 telescopes (North and 

South of equator) searching differing sky regions at 

any given time, thus doubling the search area. 

Radar - Ground Arecibo Arecibo Radio Telescope is an operational groundbased radio telescope with a 305m fixed dish. 

12
Continued support for Arecibo operations is being 

decreased by other government agencies. 

Radar - Ground Goldstone Goldstone is an operational ground-based radio 

telescope with a 70m steerable dish. Similar or 

enhanced capability provided if Canberra 70m

upgraded for radar by the Deep Space Network. 

Radar - Ground Bistatic 100m Conceptual system composed of two 100m steerable 

radio antennas to be operated in a bistatic mode. 

Table 6. Description of Space-based Survey Alternatives 

Classification Concept Name Description 

Visible - Space 1m Vis Venuslike Orbit Concept for a space based 1m optical search telescope 

in Venus-like orbit (Heliocentric ~0.7AU from Sun). 

Visible - Space 2m Vis Venuslike Orbit Concept for a space based 2m optical search telescope 

in Venus-like orbit (Heliocentric ~0.7AU from Sun). 

Infrared - Space 0.5m IR L1/L2 Concept for a space based 0.5m IR search telescope at

Sun-Earth Lagrange point (L1/L2). 

Infrared - Space 1m IR L1/L2 Concept for a space based 1m IR search telescope at 

Sun-Earth Lagrange point (L1/L2). 

Infrared - Space 0.5m IR Venuslike Orbit Concept for a space based 0.5m IR search telescope in 

Venus-like orbit (Heliocentric ~0.7AU from Sun). 

Infrared - Space 1m IR Venuslike Orbit Concept for a space based 1m IR search telescope in 

Venus-like orbit (Heliocentric ~0.7AU from Sun). 

Table 7. Description of Data Management Alternatives 

Classification Concept Name Description 

Ops and Data

Management 

Scale Existing 

Systems 

Expand existing Minor Planet Center (MPC) capability 

to support expected increases in NEO detection rates.

Ops and Data

Management 

Adopt Other

Systems 

Adopt system based on Futron's Space Launch & 

Satellite Database, Aerospace Corp.'s Space Systems 

Engineering Database or Analytical Graphics Inc.'s

Satellite Database. 

Ops and Data

Management 

New Central

Repository 

Proposed framework used by the US National Virtual 

Observatory (NVO). 

Ops and Data

Management 

Back-up 

Facility 

Grow the MPC capability as the detection rate grows 

using the NVO as a backup archive. 

Survey Performance Simulation Results for Detection and Tracking 

The results presented in this section are based on analyses where absolute performance is 

expected to be within 5 percent of the results shown, and within 1-2 percent for the best 

performing concepts. Study results are expected to have an internal relative uncertainty 

of 1-2 percent. 

13
Table 8 shows the analysis results for the performance of various ground-based optical 

survey options acting alone and assuming no new discoveries until the beginning of the 

survey program. For example, the first line of Table 8 shows that the currently operating 

ground-based Spaceguard observing program will catalog 14 percent of all PHOs with 

diameters larger than 140 meters (D > 140 m) by the end of 2020. This system would 

take decades beyond 2030 to achieve the goal of 90 percent completeness. On the second 

line of Table 8, note that the shared PS4 will reach 72 percent survey completeness for

140-meter size objects by the end of 2020. It will reach 69 percent completeness after 

operating for 10 years and 90 percent completeness after 2030. 

Table 9 shows the analysis results for the performance of the space-based systems acting 

alone. The first line of Table 9 shows that a 0.5-meter IR telescope operating at the SunEarth L1 point could catalog 85 percent of the PHO population (D > 140 m) between 

2013 and 2020, 88 percent after 10 years (2023) and 90 percent shortly thereafter.

Table 8. Ground-based Survey Performance 

140 meter PHO Completion 

Survey Systems Start by end of 2020 10 years Year for 90% 

Spaceguard 1998 14% 8% >2030

PS4 (shared) 2010 72% 69% >2030 

PS4 (dedicated) 2013 72% 77% >2030 

PS8 (dedicated) 2014 74% 81% >2030 

PS16 (dedicated) 2014 77% 83% 2029 

LSST (shared) 2014 75% 81% >2030 

LSST (dedicated) 2015 85% 90% 2024 

 

Table 9. Space-based Survey Performance 

140 meter PHO Completion 

Survey Systems* Start by end of 2020* 10 years Year 90% 

0.5m IR @ L1 2013 85% 88% 2024 

1.0m IR @ L1 2014 86% 91% 2022 

0.5m IR in Venus-like 2013 89% 93% 2021 

1.0m IR in Venus-like 2014 92% 95% 2020 

1.0m VIS in Venus-like 2014 82% 88% 2025 

2.0m VIS in Venus-like 2016 87% 94% 2022 

Table 10 shows space-based alternatives used in conjunction with the reference 

architecture of shared ground-based systems. The fourth line of Table 10 shows that 

these systems could reach 91 percent by the end of 2020 provided the Spaceguard 

telescopes operate until the shared PS4 begins its potential operations in 2010, the shared 

LSST begins its potential operations in 2014, and the space-based 0.5m IR telescope at 

Sun-Earth L1 starts its potential operations in 2013.

Table 11 shows that the options that exceed the goals of the Survey program also provide 

other benefits. The middle column of the table shows that systems that operate in Venus14
like orbits are more efficient at finding Aten and Interior Earth Objects, a potentially 

underrepresented population of PHOs. The final column of the table shows that for 

combinations of space-based visual and IR detectors, some systems will be able to 

estimate object sizes to better than 50 percent for more than 70 percent of the catalog by 

the end of 2020. 

Table 10. Survey Performance of Combinations 

140 meter PHO Completion 

Survey Systems* 

by end of 

2020* 

Year 90%

complete 

Shared PS4 + Shared LSST 83% 2026 

Dedicated PS8 + Reference 85% 2024 

Dedicated LSST + Reference 90% 2020 

0.5m IR @ L1 + Reference 91% 2020 

1.0m IR @ L1 + Reference 91% 2020 

0.5m IR in Venus-like + Reference 97% 2017 

1.0m IR in Venus-like + Reference 97% 2017 

1.0m VIS in Venus-like + Reference 93% 2019 

2.0m VIS in Venus-like + Reference 95% 2018 

* Requirement is 90% by the end of 2020 

 +

 Shared PS4 + Shared LSST = Reference 

Table 11. Additional Benefits of Space Systems 

Survey Systems 

Diameter 

to 90%* Size

0.5m IR @ L1 + Reference 125 m 77% 

1.0m IR @ L1 + Reference 125 m 78% 

1.0m IR in Venus-like 125 m ** 

0.5m IR in Venus-like + Reference 90 m - 

1.0m IR in Venus-like + Reference 80 m 76% 

1.0m VIS in Venus-like + Reference 125 m 78% 

2.0m VIS in Venus-like + Reference 110 m - 

 * Diameter of PHOs catalogued to 90% complete by end of 2020 

** Not explicitly evaluated, likely 70-78% 

NASA uses the Spaceguard Survey system to find NEOs greater than 1 km in diameter. 

However, this system would not materially contribute to future searches because almost 

all objects it finds would be found much more quickly by one of the other nextgeneration space-based and/or ground-based systems. Assuming that a single system can 

cover the richest areas of the sky in one search period of approximately five days, the 

addition of another identical system generally adds very little benefit. Asteroids tend to 

cycle in and out of range of the detectors on timescales that would allow either of the two 

identical telescopes to “discover” a given NEO if it were observable at all. However, a 

second system may enable an improvement in the quality of the orbit prediction of a 

15
NEO because the two systems acting together would enable more observations. This

enhances the characterization of the NEOs. 

Figure 3 shows the survey completion as a function of time for several individual and 

combinations of systems. The results for the PHO completeness percentages are 

estimated to be accurate to ±2 percent for results near 90 percent. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

2008 2012 2016 2020 2024 2028

Calendar Year

Perce

nt D>140m PHO Pop

ulation C

atalogued

Existing Assets (in all options)

Shared LSST

Dedicated LSST

Shared PS4 + Shared LSST (Reference)

Reference + Dedicated LSST

Reference + 0.5m IR @ L1

Reference + 0.5m IR in Venus-like Orbit

GOAL

Figure 3. Survey Performance for Selected Alternative Systems 

Characterization Alternatives Considered 

The George E. Brown, Jr. Near-Earth Object Survey Act indicates two primary objectives 

for characterizing potentially hazardous objects. The first objective, to “assess the 

threat,” requires analysts to determine the orbit and approximate the mass of each PHO. 

Detection and tracking systems with judicious follow-up can warn of potential threats 

within the next 100 years, and some are able to provide very good size and mass 

estimates. Systems operating in the visible spectrum are limited by a factor of two for 

size estimates, resulting in a factor-of-eight uncertainty in mass. Infrared systems are 

much more accurate for determining size. 

Radar may quickly and precisely characterize and determine the orbit of about 10-25 

percent of the objects of interest within five years of their detection. While the number of 

objects observed by radar increases with time, the relative value of such observed objects 

dependent on radar to precisely determine the orbits of the full catalog declines over the 

same period. Orbits determined from optical data alone will nearly match the accuracy of 

radar-improved orbits after one to two decades of observation. Therefore, the utility of

16
radar is limited to a relatively few “short warning” cases that may be of very high interest 

during the survey, and in such cases radar would be utilized if available during the first 

decade of the Survey. 

The second objective of characterization is to “inform mitigation.” Depending on the 

mitigation strategy selected, this objective may require information beyond the size and 

orbit of PHOs. This information may include the structure, porosity, rotation rate, 

material composition, and surface features of the object. The deflection alternatives 

considered are sensitive to the maximum mass that needs to be deflected, but some

alternatives are orders of magnitude less sensitive than others.

Characterization by remote sensing provides some information about the diversity of 

objects in the population. From this information, analysts build models that can later be 

used to infer a limited number of characteristics of a particular object, but only in-situ 

encounters can provide the definitive observations necessary to calibrate the remote 

observations. More importantly, only in-situ visits can obtain the information needed by 

some of the deflection alternatives to mitigate a specific threat. For credible threats with 

sufficient warning, it is expected that in-situ characterization will always be performed to 

both confirm the probability of impact and to characterize the potential threat if 

deflection is necessary. 

Without the selection of specific search and mitigation strategies, a specific choice of a 

characterization program is premature. Therefore, this study analyzed a range of 

characterization capabilities listed in Table 12. For these options, Option 7 is similar to 

Option 6 in that it combines dedicated ground-based and space-based remote 

characterization with a number of in-situ orbiters. In Option 6, the chosen number of 

orbiters is eight, possibly one for each of the primary asteroid classes needed to calibrate 

remote characterization models. At least one representative of each asteroid type (which 

may actually number greater than eight) must be visited to contribute materially to the 

model verification purpose of in-situ visits. In Option 7, the strategy is to send 

characterization missions to only credible threats. 

For example, the Option 7 strategy may be to characterize the highest risk PHO 

discovered during each five-year interval. This approach, by sending a mission to a 

different PHO every five years, would over time visit enough representative asteroid 

types to validate characterization models. It would also provide in-situ orbital 

determination to verify or eliminate specific hazards at a routine and sustainable mission 

rate. However, the orbits of the targets for Option 7 are likely to be more difficult to 

reach than those that would likely be chosen for Option 6. 

Table 12. Characterization Capability Options

Option Descriptions (O1 = Option 1) 

Option 1 Use Existing Assets + Detection and Tracking Systems 

Option 2 O1 + Dedicated Ground Systems 

Option 3 O1 + Dedicated Space-Based Remote Sensing (L1/L2) 

Option 4 O1 + Dedicated Space-Based Remote Sensing (Venus-Like Orbit) 

Option 5 O1+ O2+ O3 + 2 Flyby Missions to 8 Objects 

Option 6 O1 + O2 + O3 + 8 Orbiter Missions 

Option 7 O1 + O2 + O3 + Orbiters for a Fixed Threshold Probability of Impact 

17
EXEMPLAR SURVEY PROGRAM 

An exemplar NEO Survey Program, that at a minimum meets the goals specified in the 

George E. Brown Jr. Near-Earth Survey Act, is shown in Table 13. This program could 

achieve the specified goal of surveying 90 percent of the PHOs by the end of 2020 

through NASA partnerships with other government agencies on potential future optical 

ground-based observatories: the Panoramic Survey Telescope and Rapid Response 

System (PanSTARRS-4 or PS4) and the LSST. Following at least one year of program 

formulation, NASA could build or fund the construction of a dedicated survey asset. For 

example, either an additional LSST, which may be operated by other organizations, or a 

0.5 meter infrared (IR) satellite in a Venus-like orbit. Other options for the dedicated 

Survey asset would be evaluated during program formulation. All costs shown are 

anticipated costs to NASA. 

Both the shared and dedicated assets would detect, track, and characterize NEOs. 

Analyses to date indicate that the estimated completion dates for development and 

estimated costs are sensitive to modeling errors that may vary up to three years. Note 

that these costs are rough ‘architecture costs’ that would need more rigorous analysis if a 

program were to be assessed for implementation. 

Currently, NASA carries out the “Spaceguard Survey” to find NEOs greater than 

1 kilometer in diameter, and this program is currently budgeted at $4.1 million per year 

for FY 2006 through FY 2012. We also have benefited from knowledge gained in our 

Discovery space mission series, such as the Near Earth Asteroid Rendezvous (NEAR), 

Deep Impact, and Stardust missions that have expanded our knowledge of near-Earth 

asteroids and comets. Participation by NASA in international collaborations such as 

Japan’s Hayabusa mission to the NEO “Itokawa” also greatly benefited our 

understanding of these objects. NASA’s Dawn mission, expected to launch in 

June 2007, will increase our understanding of the two largest known main belt asteroids, 

Ceres and Vesta, between the planets Mars and Jupiter. NASA conducts survey 

programs on many celestial objects— the existing Spaceguard program for NEOs, 

surveys for Kuiper Belt Objects, the search for extra-solar planets, and other objects of 

interest such as black holes to understand the origins of our universe. Our Discovery 

mission series in planetary science may offer additional opportunities in the future 

beyond our current survey efforts. 

NASA recommends that the program continue as currently planned, and we will also take 

advantage of opportunities using potential dual-use telescopes and spacecraft—and 

partner with other agencies as feasible—to attempt to achieve the legislated goal within 

15 years. However, due to current budget constraints, NASA cannot initiate a new 

program at this time. 

18
Table 1. Exemplar NEO Survey Program to Detect, Track, and Characterize

Detect, Track, & Characterize: 

≥140 meter PHOs

Total Architecture Costs* 

($M) 

Exemplar Survey Program (thru the year to reach 90%) Percent 

completed 

through 2020 

Year to reach 

90% $FY06 $RY 

Reference (Ground) Survey Assets 

(Shared PS-4 & Shared LSST) 

83% 2026 $469.0 (thru

2026) 

$693.5 (thru

2026) 

Reference plus a 

Dedicated LSST 90% 2020 $835.5 (thru

2020) 

$1076.2 

(thru 2020) 

Two of the 

Options for 

one 

additional, 

dedicated 

Survey Asset 

Reference plus a 

Dedicated 0.5-meter IR 

in Venus-like orbit 

97% 2017 $1005.9 (thru 

2017) 

$1239.9 

(thru 2017) 

* Total Architecture Costs include data management and program office costs. 

STUDY APPROACH AND ANALYSIS OF ALTERNATIVES FOR NEO 

DEFLECTION PROGRAM 

The study considered a wide range of techniques to divert a threatening object. These 

alternatives were broadly classified as “impulsive” if they acted nearly instantaneously or 

“slow push” if they acted over an extended period of time. Launch, orbit transfer, 

technology development, and object characterization requirements were assessed for each 

of these alternatives. They were applied to a set of five scenarios representing the likely 

range of threats. 

A representative set of potential PHO deflection approaches was presented during a 

public workshop NASA held in the course of this study. This study examined a number 

of techniques for deflecting a PHO, and the methods considered viable have been 

categorized as either impulsive or slow push techniques. Table 14 provides an overview 

of the impulsive methods. Likewise, Table 15 shows the slow push techniques, where 

the velocity change results from the continuous application of a small force. 

19
Table 14. Impulsive Mitigation Alternatives 

Impulsive Technique Description 

Conventional Explosive (surface) Detonate on impact 

Conventional Explosive 

(subsurface) 

Drive explosive device into PHO, detonate 

Nuclear Explosive (standoff) Detonate on flyby via proximity fuse 

Nuclear Explosive (surface) Impact, detonate via contact fuse 

Nuclear Explosive (delayed) Land on surface, detonate at optimal time 

Nuclear Explosive (subsurface) Drive explosive device into PHO, detonate 

Kinetic Impact High velocity impact 

Table 15. Slow Push Mitigation Alternatives 

Slow Push Technique Description 

Focused Solar Use large mirror to focus solar energy on a spot, heat 

surface, “boil off” material

Pulsed Laser

Rendezvous, position spacecraft near PHO and focus 

laser on surface, material “boiled off” surface 

provides small force 

Mass Driver Rendezvous, land, attach, mine material and eject 

material from PHO at high velocity

Gravity Tractor 

Rendezvous with PHO and fly in close proximity for

extended period, gravitational attraction provides 

small force 

Asteroid Tug Rendezvous with PHO, attach to PHO, push 

Enhanced Yarkovsky Effect 

Change albedo of a rotating PHO; radiation from sunheated material will provide small force as body

rotates 

In the impulsive category, the use of a nuclear device was found to be the most effective 

means to deflect a PHO. Because of the large amount of energy delivered, nuclear 

devices would require the least amount of detailed information about the threatening 

object, reducing the need for detailed characterization. While detonation of a nuclear 

device on or below the surface of a threatening object was found to be 10-100 times more 

efficient than detonating a nuclear device above the surface, the standoff detonation 

would be less likely to fragment the target. A nuclear standoff mission could be designed 

knowing only the orbit and approximate mass of the threat, and missions could be carried 

out incrementally to reach the required amount of deflection. Additional information 

about the object’s mass and physical properties would perhaps increase the effectiveness, 

but likely would not be required to accomplish the goal. It should be noted that because 

of restrictions found in Article IV of the Treaty on Principles Governing the Activities of 

States in the Exploration and Use of Outer Space, including the Moon and Other 

Celestial Bodies, use of a nuclear device would likely require prior international 

coordination. The study team also examined conventional explosives, but found they 

were ineffective against most threats.

20
Non-nuclear kinetic impact alternatives are the most effective non-nuclear option, 

transferring 10-100 times less momentum than nuclear options for a fixed launch mass. 

Impact velocities, varying from 10-50 km/s, produced a factor-of-three variation in 

deflection performance. In addition, kinetic impacts are also sensitive to the porosity,

elasticity, and composition of the target and may require large performance margins if 

these characteristics are not well determined. 

Slow push techniques analyzed in this study included a gravity tractor, which could alter 

the course of an object using the gravitational attraction of a massive spacecraft flying in 

close proximity, and a space tug, which could attach itself to a PHO and move it using 

high-efficiency propulsion systems. An attached space tug has generally 10-100 times 

more performance than the gravity tractor, but it requires more detailed characterization 

data and more robust guidance and control and surface attachment technologies. Slow

push techniques were determined to be useful in relatively rare cases (fewer than 1 

percent of expected threat scenarios). This technique could be effective in instances 

where small increments of velocity (less than 1 mm/s) could be applied to relatively small 

objects (less than 200 meters in diameter) over many decades. In general, the slow push 

systems were found to be at a very low technology readiness level and would require 

significant development efforts. 

Deflection Performance Analysis 

Figures 4 and 5 graphically represent deflection capabilities. The system performance 

required to deflect any object on a given trajectory may be described as the velocity 

change necessary to change its path multiplied by its mass. The “effective momentum 

change” performance parameter allows many different scenarios to be plotted 

simultaneously across a wide range of asteroid masses and required deflection velocities 

(∆V). It is displayed logarithmically on the Y-axis of these figures. The logarithmic Xaxis represents launch performance to place the deflection payload on an intercept 

trajectory. A key parameter that matches launch capability with a certain payload at a 

certain time (flight time) to intercept an asteroid is C3, which is equal to twice the 

specific (per unit mass) orbital energy and is represented in units of km2

/s2. The launch 

C3 corresponding to payload capabilities of the two launch systems considered (Delta IV 

Heavy and Ares V) are at the top of each figure. 

The lines to the right of each figure may be used to translate effective momentum change 

to the design parameters of PHO mass (and size) and deflection ∆V. Lines of constant 

object mass (and size) spaced logarithmically run diagonally across vertical lines 

representing a logarithmic range of deflection ∆V. As an example, following the 

diagonal line representing a mass of 1010 kg (approximately 200 m) to its extreme lower 

left at the vertical 1 cm/s ∆V line, this corresponds to an effective momentum change of 

108

 kg m/s on the far left. 

The lines plotted represent the performance of the deflection alternatives. If an 

alternative has a higher effective momentum change capability than is required, it is 

considered “feasible” for a single-launch deflection. Therefore, using the previous 

example of an effective momentum change of 108

 kg m/s and assuming that a Delta IV 

Heavy launch vehicle is used and that C3 = 25 km2

/s2 is required to intercept, all but the 

21
10 km/sec kinetic interceptor and the conventional explosives would meet performance 

requirements. None of the slow push techniques could meet this hypothetical scenario. 

Figure 4 shows that impulsive techniques using proximal nuclear explosives generally 

were found to provide greater potential for momentum transfer per kilogram of payload 

weight delivered to the threat than any other option considered. Standoff nuclear 

concepts, such as those producing highly concentrated and directionally focused x-rays or 

neutrons, were shown to present a generally lower risk of fragmenting a PHO than 

impulsive techniques involving direct contact, but also produce a lower effective 

momentum change than surface or subsurface nuclear explosives. Performance may vary 

significantly, depending on the type of nuclear device used and whether it is “off-theshelf” as opposed to optimized for the PHO deflection mission. Additionally, the 

performance of kinetic impactors was found to be somewhat less robust than any of the 

nuclear explosions. However, their effectiveness depends strongly on the structure of the 

PHO. Kinetic impactors may also be significantly less effective for objects which are 

essentially loose rubble piles. Conventional explosives were found to have the lowest 

performance among the impulsive techniques due to their relatively low-energy density. 

Figure 5 illustrates that slow push techniques may be useful for imparting momentum

changes smaller than 109

 kg m/s. The asteroid tug appears to have significantly greater 

performance than the gravity tractor for a given launch mass, even accounting for pulsed 

operation on a rotating PHO. The disadvantage of the asteroid tug is the additional 

complexity required to anchor the tug to the NEO, particularly if the PHO structure has 

not been well characterized or the target is rotating very rapidly. 

These figures show that nuclear explosives and kinetic impactors were generally found to 

provide greater potential for momentum transfer per kilogram of payload weight 

delivered to the NEO than other alternatives. Additionally, these figures illustrate how

the alternatives might be applied to hypothetical deflection scenarios. The inclusion of 

actual objects in these scenarios was chosen not because they represent actual impact 

threats, but because they are both publicly known and are representative of classes of 

potential threats. 

The hypothetical scenarios include missions to deflect: 

A. The 330-meter asteroid, Apophis, before its close approach to Earth in 2029. This 

scenario was divided into two design points: 

A1. For the first, knowing the asteroid’s orbit is assumed and a relatively large 

momentum change is required to deflect the object with the required certainty. 

Apophis must be deflected by at least one Earth radius or about 6,400 km to 

achieve a probability of collision of less than 10-6. 

A2. For the second, very accurate information about the object’s orbit is assumed 

and the impetus necessary to divert the asteroid with certainty is substantially 

reduced. Apophis must be deflected by at least five km to achieve a 

probability of collision of less than 10-6. 

B. Apophis after the close approach and before the 2036 Earth encounter, assuming a 

predicted collision. 

C. The 500-meter asteroid (VD17) that could be a threat in the year 2102. 

22
D. A hypothetical 200-meter asteroid, representative of 100-meter-class asteroids. 

E. A hypothetical asteroid larger than one km in diameter. 

F. A hypothetical long-period comet with a very short time (9-24 months) to impact. 

The approximate performance requirements for each of the scenarios are overlaid on 

Figure 4 for the impulsive techniques and Figure 5 for the slow push methods. 

Figure 4. Deflection Performance of Impulsive Alternatives 

23
Figure 5. Deflection Performance of Slow Push Alternatives 

POTENTIAL BENEFITS TO SCIENCE 

NEOs are primitive bodies, primarily asteroids that probably represent almost the full 

range of material contained in the main asteroid belt of our solar system. The population 

also contains the nuclei of extinct comets, which likely still reflect the composition of all 

but the most volatile species and still contain a significant inventory of organic 

substances. The most recent National Academy of Sciences’ Decadal Survey for solar 

system exploration summarizes the key science issues with respect to primitive bodies as 

follows: 

• Where in the solar system are the primitive bodies found, and what range of sizes, 

compositions, and other physical characteristics do they represent?

• What processes led to the formation of these objects?

• Since their formation, what processes have altered the primitive bodies? 

• How did primitive bodies make planets?

• How have they affected the planets since the epoch of formation? 

Characterization will certainly provide new information on the sizes, compositions, and 

other physical characteristics of asteroids and comet nuclei. Information on the material 

of these objects will also provide data to understand alteration processes. 

A wide area search, such as that being proposed for NEOs, will also substantially 

increase the identification of Kuiper Belt Objects (KBOs). For example, if 10 percent of 

24
the observing time on the proposed Dedicated LSST was spent in a KBO search mode, 

roughly 100,000 faint KBOs should be discovered. An expanded KBO database will 

allow the study of dynamical distributions, further resonances, the existence of a KBO 

demarcation beyond 50 AU, high-eccentricity/high-inclination orbits, size distributions, 

frequency of binary objects and collision rates, chemical compositions and the 

relationship of objects to dust disks around other stars. The survey will also provide a 

rich database of targets for future space missions. 

Detection surveys such as the proposed Pan-STARRS and LSST provide unique solarsystem science because they are designed to detect and perform follow-up studies of 

moving objects. Centaurs, Jupiter Family Comets, and certain extinct comets may be

related through a common origin in the Kuiper Belt. Dedicated assets will assure that 

appropriate follow-up is carried out over the annual timeframes that are required to 

produce orbits for the slower-moving objects found in the outer solar system. Thus, a

collateral result of the NEO survey program could be both the delineation of the structure 

of the Kuiper Belt and the discovery of many new minor planets. 

It also is important to understand what a vigorous characterization effort will not do. 

Characterization to inform deflection missions has not identified a need for sample return 

from either an asteroid or comet. Asteroid and comet sample-return missions are high 

priorities in the Decadal Survey, but they are not included in the trade space of this study. 

However, a vigorous survey program would identify likely candidates for scientific visits 

for the sample return missions identified as a priority in the Decadal Survey. Remote 

characterization will allow the most interesting objects to be selected for further scientific 

investigation and will allow the instruments and experiments of these missions to be 

tailored in ways that otherwise would not have been considered. NEOs are generally 

among the easiest asteroids to visit, and the design of a spacecraft to work in the 

relatively benign environment near one AU offers less cost and risk than a mission to the 

main belt. A sample return mission to a NEO characterized for a deflection mission will 

carry substantially lower risk than a mission to an object about which much less is 

known. 

POTENTIAL BENEFITS TO EXPLORATION 

There may be future longer-term options for system-level demonstrations that could 

contribute to PHO deflection demonstrations. These systems, currently in development, 

have considerable mass, precision rendezvous and docking capability, as well as 

considerable performance margin. 

Near-Earth Object Resources 

The study team noted the connection between the goals of the Vision for Space 

Exploration and a program to survey the population of NEOs. Discovering and exploring 

resources that exist on NEOs may lead to future utilization. 

Human Visits to Asteroids 

The Vision for Exploration directed NASA to extend human presence across the solar 

system, starting with a human return to the Moon by the year 2020, in preparation for 

human exploration of Mars and other destinations. NEOs are one of those potential 

25
"other destinations." NASA is currently developing a new launch system, the Ares I and 

V launch vehicles, and a new crew exploration vehicle, the Orion. It is possible that the 

systems used to return humans to the Moon could be used to also visit a NEO. 

26
Appendix: Acronyms and Definition of Terms 

Acronym Description

AU Astronomical Unit 

D Diameter 

ESA European Space Agency 

FY Fiscal Year 

HQ Headquarters 

IEO Interior Earth Object 

IOC Initial Operational Capability 

IR Infrared energy band 

IRTF InfraRed Telescope Facility 

Isp Specific Impulse 

JPL Jet Propulsion Laboratory 

KBO Kuiper Belt Objects 

L1 First Sun-Earth Lagrange Point 

LEO Low-Earth Orbit 

LINEAR Lincoln Near Earth Asteroid Research

LONEOS Lowell Observatory Near-Earth-Object Search 

LSST Large Synoptic Survey Telescope 

MOID Minimal Orbital Intersection Distance 

MPC Minor Planet Center 

NASA National Aeronautics and Space Administration

NEAT Near-Earth Asteroid Tracking 

NEO Near-Earth Object 

NVO National Virtual Observatory 

PA&E Office of Program Analysis and Evaluation 

Pan STARRS Panoramic Survey Telescope & Rapid Response System

PHO Potentially Hazardous Object 

PS Pan STARRS 

Vis Visible light band 

27