near Earth Objects - an overview | ScienceDirect Topics

1. Introduction
Near-earth objects (NEOs) reside in the vicinity of Earth near 1.0 AU (the mean distance between Earth and the Sun). Any object, such as an asteroid or comet, orbiting the Sun with a perihelion, q < 1.3 AU, well inside the orbit of Mars, is defined as an NEO. Aphelia, Q, of NEOs generally lie within a sphere of radius 5.2 AU, defined by Jupiter's orbit. Among this broad group are four subgroups: Amors, Apollos, Atens, and interior Earth objects (IEOs). Comets, releasing gas and dust with q < 1.3 would be referred to as near-Earth comets (NECs) if they posed an impact threat to Earth. Amors approach but don't cross the orbit of Earth. They have a semimajor axis, a > 1.0 AU, and perihelion 1.017 ≤ q = 1.3 AU, between the aphelion of Earth's orbit and inside the perihelion of Mars (Fig. 1a). Those that actually cross Earth's orbit, Apollos, have a > 1.0 AU and q = 1.017 AU, Earth's aphelion distance (Fig. 1b). Atens have a > 1.0 AU and q > 0.983 AU, Earth's perihelion distance. An object with both a and q < 0.983 AU, is an IEO.
FIGURE 1. (a) Amors approach Earth but do not cross its orbit. (b) Apollo orbits cross that of Earth.
The Amor asteroid, 433 Eros, was the first NEO discovered in 1898, by D. Witt of Berlin, Germany, using a photographic plate to record its position. It is also one of the largest NEOs, being 33 km in its longest dimension, with two other axes of 10.2 × 10.2 km diameter. 1862 Apollo, the first Earth-crossing asteroid, and 1221 Amor, the namesake of that group, were both discovered in 1932. It wasn't until 44 years later that 2062 Aten, the first of the group orbiting within Earth's orbit, was discovered by Eleanor Helin, still using photographic plates for the search. 1998 DK36 was the first IEO discovered in 1998.
As the dynamical evolution of asteroids and their role in probably causing biological extinction events on the Earth was recognized in the 1980s, dedicated searches for NEOs resulted in increased discovery rates. Due to both increased sky coverage and availability of sensitive digital detectors, the known NEOs number >4100 at this writing, compared to 85 known in early 1980 (Fig. 2). About 25 of the NEOs found since the 1990s are binary objects orbiting around a common center of mass; 15% of all NEOs are estimated to be binaries. 1862 Apollo, an asteroid between 1.2 and 1.5 km in diameter, was reported to be a binary in 2005.
FIGURE 2. Cumulative total of discovered near-Earth objects versus time. Large NEOs are defined as those with an absolute magnitude (H) of 18 or brighter.
(Data compiled by Alan Chamberlin, NASA/JPL.)
Most of the near-Earth objects originated in the Main Asteroid Belt, located between Mars and Jupiter, although some of them probably evolved into their current orbits from the reservoir of short-period comets extending beyond Jupiter and into the outer solar system. The range of composition and physical characteristics of asteroid-like near-Earth objects spans those found among the Main Belt, though 15% of them probably are derived from cometary reservoirs.
The Near-Earth Asteroid Rendezvous (NEAR) mission was the first designed to orbit an asteroid. 433 Eros was its target, and the spacecraft remained in orbit from 2000 to 2001, ending its mission with a controlled descent and successfully becoming the first spacecraft to land on an asteroid. NEAR accomplished the first detailed in situ measurements of an asteroid's surface morphology, mineralogy, chemistry, internal state, and magnetic properties. The Japanese-led Hayabusa mission was launched on May 9, 2003, on a 4 year mission to investigate asteroid 25143 Itokawa and to demonstrate the technology necessary to return samples to Earth. The spacecraft went into orbit around Itokawa in September 2005, performing remote sensing measurements for 3 months. The shape and surface morphology of this small near-Earth object is unlike any seen before. In November 2005, there were two scheduled touchdowns in which some surface material may have been collected. The return capsule is scheduled for a June 2010 return to Earth and will hopefully contain some surface material collected from Itokawa.
Case Study: Engagement With the Media
The community of NEO observers is well organized to report on so-called “close approaches”—predicted events in which asteroids pass our planet within a few lunar distances (1 LD=approximately 240,000 miles, or 384,000 kilometers) of Earth, but they are of particular interest when they pass within the orbit of Earth's Moon on their orbits around the Sun.25 While these close approaches tend to be measured in tens of thousands to hundreds of thousands or even millions of miles and do not pose any risk of impact with Earth, media reports tend to report these occurrences as “close shaves” or “near misses.” And media reports about asteroid impact hazards tend to emphasize the possibility of a catastrophic impact—“world-ending,” “civilization-destroying,” and so on. My NASA colleagues and I decided that we needed to engage directly with the media to inform them about NEO impact hazards. Our aim was to establish face-to-face contacts, and perhaps relationships, with journalists who are reporting on NEO impact risks.
Our first media engagement project was a seminar for science writers, held in conjunction with the annual meeting of the American Astronomical Society on January 9, 2014, in National Harbor, Maryland: “Everything you've always wanted to ask about near-Earth objects: what we know, what we don't know, what we need to know.” Our next media engagement project was a workshop for broadcast meteorologists held at NASA headquarters in Washington, DC, on March 16, 2018. We targeted broadcast meteorologists because they frequently report on NEO close approaches and other cosmic phenomena. We requested, and received, feedback from participants in this workshop, and we used that feedback to improve our next media engagement activity: a workshop for science writers held December 9, 2018, in Washington, DC, in conjunction with the annual fall meeting of the American Geophysical Union.
As participants in all three of these workshops indicated that they found them useful, the PDCO intends to conduct similar workshops in conjunction with other scientific conferences. PDCO staff are also considering the creation of a workshop “package” that members of the NEO science community can use for engagement with public audiences in schools, libraries, or other community centers. Again, it would be difficult to gauge the benefits of these activities, as it would require long-term follow-up with participants.
I continue to keep an eye on media coverage of NEO observations, NEO close approaches, NEO impact risks, and plans for planetary defense. Though I will not be engaging in any sort of formal analysis, I do believe that the more time and effort the PDCO team puts into propagating clear, concise, correct, and timely information about these topics, the better media coverage will be. A lesson learned here is that most journalists welcome access to reliable and authoritative information and that the burden is on us, the scientists, to reach out to them. We also have learned that, no matter what we do, we will not be able to dissuade the British tabloids from propagating melodramatic headlines, but we can counter their effects by cultivating trust-based relationships with other members of the media.
10.18.3.1.3 Sample and return from a low-gravity environment
Small solar system bodies, such as comets, asteroids, and TNOs, are believed to be relics of the formation of our planetary system. The near-Earth objects (NEOs) are very attractive targets for sample return missions, because the technological challenges are less demanding than for the high-gravity environment of a planet or the volatile composition of a comet. NEOs can be reached on much shorter timescales than Trojan asteroids, Centaurs, or TNOs. Dynamical calculations have shown that their orbital lifetime is small on cosmogonical timescales and that they are continuously being replenished from other sources, either from the main asteroids belt or from comets. A sample return mission needs to return a scientifically significant sample of the surface material of one or multiple NEOs. The amount of material will influence the research that can be performed. It would be advantageous to have enough material to apply all desired measurement techniques and tests. Most laboratory instruments require a very small amount of material (<<1 g) and only some investigations need a few grams. The sampling size required for one set of measurements is therefore of the order of several tens of grams. However, some redundancy is required and some additional material should be kept for future analysis. Furthermore, a larger amount would provide a better representation of the sampled area.
Retrieving a sample from a low-gravity body is significantly different from a sample return from a planet. The latter requires a lander, an entry and/or descent system, and a launcher to return to orbit. For a low-gravity environment, no dedicated launch vehicle is required after sample collection, and also the descent requirements are very different. A sample return from a NEO, a main belt asteroid or one of the Martian moons, can be achieved with a small spacecraft that can be launched on a ‘low cost’ launcher, if the relative velocity between the spacecraft and the target body is low enough to permit a rendezvous mission. Upon arrival the spacecraft will first go in orbit to perform a number of remote-sensing measurements of the surface and subsurface, as well as an accurate mapping of its gravitational properties, which will be a key prerequisite in the detailed determination of the navigation sequence for sampling maneuvers. A large number of potential sampling sites on the surface need to be examined in order to select the optimal locations for sampling maneuvers. For this high-resolution imaging in the visual and near infrared (IR) is the minimum requirement, however, depending on the available mass other instruments should be added to determine the global elemental and mineralogical surface composition of the target, as well as some information on the subsurface.
There are two main options to retrieve a sample from a low-gravity solar system body: (1) collecting the sample directly from the surface or (2) creating a debris cloud of asteroid material and collecting a sample from that cloud. The Hayabusa Mission to asteroid Itokawa has realized the first option whereby a touch-and-go sampling approach was used. Upon a brief contact with the surface, a projectile is fired and the debris is then funneled up a cone as the spacecraft retreats from the surface. The second option has not yet been space proven; however, the results of the Deep Impact Mission to comet Temple 1 can be used to roughly estimate the constraints for collecting a sample of some hundred grams with this method from a nonvolatile asteroid.
The samples have to be stored and sealed into a sample canister inside an Earth-entry vehicle. To reduce propellant requirements for the transfer back to Earth, the sampling mechanism and empty tanks can then be jettisoned. The spacecraft will return to Earth, where the Earth-entry capsule will perform a direct hyperbolic Earth entry.
VI.B Mitigation of Hazard
The mitigation process for an object found to be on an Earth collision trajectory requires confirmation of such an orbit many years ahead of time and characterization of the NEO in order to mount an international defense. Various techniques have been suggested to avert asteroidal or cometary impact. Most of these involve deflection of the threatening body, perhaps, by nuclear weapons (currently forbidden to be used in space by international treaty) set off some distance from the surface of the object to change the velocity by radiation without breaking it up but nudging it into a new orbit. If the object were broken up, there might be more pieces to encounter Earth and a new set of difficult problems. The fact that a comet or asteroid might be fragile and a collection of fragments to begin with compounds the danger of trying to move it. Other possibilities for changing the object's orbit to miss Earth include the use of a giant parabolic mirror to concentrate the Sun's rays and vaporize the surface rocks. Material flying off at high speed would cause a recoil action. The attachment of steerable solar sails to catch the pressure of sunlight is another concept. Methods might vary depending upon the rotation and tumbling of an asteroid. None of these techniques has ever been tried.
5. Sky Survey Telescopes
Although large telescope projects tend to get a lot of attention, recently there has been a corresponding quantum jump in the construction of visible and infrared survey telescopes. This has been made possible by the availability of large-format CCD and infrared arrays. In addition, the discovery of the Kuiper Belt has led to fundamental advances in our understanding of how our solar system formed. There is a great need to continue the survey of the Kuiper Belt because detailed knowledge of the size and orbit distributions of these objects will allow us to test theories of the orbital migration of the outer planets (Jupiter, Saturn, Uranus, Neptune), the origin of the short-period comets, and the cause of the late heavy bombardment of the inner solar system.
There is also an increased awareness that it is important to identify asteroids and comets that could collide with Earth (see Fig. 3). In 1998 the Congress of the United States directed NASA to identify within 10 years at least 90% of NEOs larger than 1 km that may collide with Earth. There are a number of scientific benefits that arise from the NEO surveys, including determining the origin of NEOs, identifying interesting NEOs that could be visited by spacecraft, improving our knowledge of the numbers and sizes of the asteroids in the main asteroid belt, and the discovery of new comets.
The reason that the discovery of all NEOS larger than 1 km is important is because if such an object collides with Earth the consequences will be catastrophic. If it is possible to predict that there will be a collision, it may be possible to divert the asteroid so that it misses Earth. The earlier such a prediction can be made, the more likely it is that the diversion is possible. This is a case in which there is a practical use for astronomy, and it is very fitting.
A number of programs are underway in the US and other countries that meet or exceed the requirements set by Congress. Table 2 shows a partial list of sky survey programs that are currently in progress or planned. Current productivity of various programs is shown in Figure 16, which shows all NEOs discovered irrespective of size. While the NASA directive is aimed at identifying NEOs larger than 1 km diameter, many NEOs smaller than 1 km are also discovered due to the sensitivity of the search programs and because small objects that come very close to Earth may be bright enough to be detected. A recent NEO, 2005 WX, approached to within 1.3 million km of the Earth and had an estimated diameter of only 10 m!
TABLE 2. Summary of Sky Survey Telescopes
Survey
Status
Aperture (m)
f/no
Field-of-view (degree2)
Magnitude limit
Speed (degree2 per hour)
Ref.
CSS – Mt. Lemmon
operational
1.5
2.0
1.3
21
20
1
CSS – Catalina Schmidt
operational
0.68
1.9
8
19.5
150
1
CSS – Siding Spring Uppsala Schmidt
operational
0.5
3.5
4.2
19.5
75
1
LINEAR
operational
2 × 1.0
2.2
2.0
19.4
1200
2
LONEOS (Schmidt)
operational
0.44
1.9
8.3
19.3
106
3
LONEOS (USNO)
in development
1.3
2.4
1.3
21.4
15
3
NEAT (Palomar)
operational
1.2
1.5
9.5
22.5
85
4
NEAT (MSSS)
operational
1.2
3.0
2.3
19.7
40.5
4
NEAT (Schmidt)
in development
1.2
2.5
9.4
−20.0
50
4
Spacewatch (Mosaic)
operational
0.93
3.0
2.9
21.5
160
5
Spacewatch (1.8 m)
operational
1.82
2.7
0.32
22.5
8.9
5
Pan-STARRS (Hawaii)
in development
4 × 1.8
4
3.0
24.0
700
6
Discovery Channel Telescope (Lowell)
in development
4.0
2.2
3.1
21.8
110
7
Large Synoptic Survey Telescope
proposed
6.9
1.25
7.0
24.0
2500
8
1. Field-of-view is the area of sky covered in a single exposure.
2. Magnitude limit is the faintest star recorded at visible wavelengths.
3. Speed is the rate at which observations can be carried out. One can see that of the operational facilities, LINEAR covers the most sky per hour (1200 degree2/hour) but the faintest stars it can observe at this speed is 19.4 mag. The Spacewatch (1.8 m) telescope can observe stars that are 3 magnitudes fainter but at a speed of only 8.9 degree2/hour).
FIGURE 16. Cumulative discoveries of near-Earth asteroids. The total number of large near-Earth asteroids (larger than 1 km) is increasing at a slower rate since most of the easy-to-detect NEOs have already been discovered. The remaining unknown NEOs are on orbits that are intrinsically more difficult to detect and therefore require a longer time to discover.
(From NEO.)(Courtesy of Alan Chamberlin.)
The number of known NEOS has been increasing due to the larger number of funded survey programs and advances in detector arrays that have allowed much larger areas of sky to be covered in a single exposure. The number of NEOs discovered as a function of time is shown in Figure 16. Note that while the total number of asteroids discovered is still increasing at a rapid rate, the number of new asteroids larger than 1 km discovered each year is decreasing. This is a result of the fact that the remaining unknown NEAs are intrinsically more difficult to detect. Their size and orbit distribution is different from the known population due to observational selection effects in the population of known objects. It is likely that existing survey programs (see Table 2) will just miss the goal of discovering at least 90% of all near-Earth asteroids larger than 1 km by 2008 as mandated by Congress. However, when the next generation surveys (see Table 2) come online within the next decade they will quickly complete the inventory of NEAs larger than 1 km.
There are three major ground-based sky surveys currently under development or study (see Table 2). The Discovery Channel Telescope is a 4.2-m telescope that is under construction near Flagstaff in Northern Arizona and should be operational by 2009. Another survey telescope that is under development is Pan-STARRS, which consists of four 1.8-m telescopes (with a combined aperture approximately equivalent to a 3.6-m telescope) to perform rapid wide-field surveying of the entire sky on a weekly basis. It is hoped that the full system will be operational by 2010, but a prototype single telescope unit will be operational on Haleakala on Maui by the end of 2007. The proposed Large Synoptic Survey Telescope is currently under engineering and design study and is envisioned to be a monolithic 8.4-m wide-field telescope (with a collecting area equal to a 6.7-m telescope). With its large diameter and fast focal ratio it should be capable of reaching 24th magnitude in single 10-s exposures. Due to their extreme depth and wide-field coverage each of these surveys should reach 99% completion for NEOs larger than 1 km diameter within two years of beginning operation.
2.1. Meteors
Looking up at the clear night sky, one can record about 10 faint meteors (or shooting stars in colloquial language) per hour. Once in a while, a brighter streak or trail of light or “fireball” will appear. Around the year 1800, the extraterrestrial nature of meteors was established when triangulation was used to deduce their height and speed. This technique is still used in modern meteor research by employing specifically equipped cameras and telescopes. About 50 years ago, radar techniques were also developed to observe faint meteor trains even during daylight.
Visible meteors result when centimeter-sized meteoroids enter the Earth's atmosphere at a speed greater than 10 km/s. At this speed, the energy of motion, which is converted to heat, is sufficient to totally vaporize the meteoroid. During the deceleration of the meteoroid in the atmosphere at about 100 km altitude, the meteoroid will heat up and atoms from its outer surface will be ablated until it is completely evaporated. A luminous train several kilometers in length follows the meteoroid. It is this ionized and luminous atmospheric gas and material from the meteoroid that is visible and that scatters radar signals. From triangulation of the meteor train by ground stations (several cameras or a radar station), the preatmospheric meteoroid orbit is obtained with high accuracy.
During the atmospheric entry of objects larger than several tens of kilograms or about 10 cm in diameter, a surface layer of several centimeters in thickness will burn away, and the object will be decelerated. That which reaches Earth's surface is called a meteorite. Meteorites of 1 kg to several tons are sufficiently decelerated and fall on Earth with the interior little altered by atmospheric entry. These meteorites are the source of our earliest knowledge about extraterrestrial material. [SeeMeteorites; Near-Earth Objects; Planetary Impacts.]
Much of the ablated material from a meteor will condense again into small droplets, which will cool down and form cosmic spherules that subsequently rain down to Earth. These cosmic spherules can be found and identified in abundance in deep-sea sediments and on the large ice masses of Greenland, the Arctic, and Antarctica. An average of 40 tons of extraterrestrial material per day in the form of fine dust falls onto the surface of Earth.
At certain times, meteor showers can be observed at a rate that is a hundred (and more) times higher than the average sporadic meteor rate (Table 1). Figure 4 shows several meteors in a photograph of the night sky taken on 17 November 1966. The visible rate was about one meteor per second. Because all of these meteoroids travel on parallel trajectories, to an observer they seem to arrive from a common point in the sky (the radiant), which in this case lies in the constellation Leo. Therefore, this meteor shower is called the Leonid shower.
FIGURE 4. An unusually strong meteor shower (Leonid) was observed on 17 November 1966. The meteor trails seem to radiate from the constellation Leo.
The explanation for the yearly occurrence of meteor showers is that all meteoroids in one stream closely follow a common elliptic orbit around the Sun but are spread out all along the orbit. Each year when the Earth crosses this orbit on the same day, some meteoroids of the stream hit the atmosphere and cause the shower.
Many meteor streams have orbits similar to those of known comets (cf. Table 1). It is a generally accepted view that meteor streams are derived from comets. Millimeter- to centimeter-sized particles that are emitted from comets at low speeds (m/s) are not visible in the normal comet tail but form so-called comet trails along a short segment of the comet's orbit. Their different speeds will slowly spread the particles out over the full orbit. Infrared observations by the Infrared Astronomical Satellite (IRAS) identified many such trails connected to short-period comets. Gravitational interactions with planets and collisions with other cosmic dust particles will scatter meteoroids out of the stream, and they will become part of the sporadic background cloud of meteoroids. The fact that some meteor showers display strong variations of their intensities indicates that they are young streams that are still concentrated in a small segment of the parent's orbit. The parent comet of the Leonids, the periodic Comet Tempel–Tuttle, has the same periodicity of 33.3 years. The parent object of one of the strongest yearly meteor showers, the Geminids, is 3200 Phaeton, which had been previously classified as an asteroid because it shows no cometary activity. However, its association with a meteor stream indicates that it is an inactive, dead comet that at some time in the past emitted large quantities of meteoroids. [SeePhysics and Chemistry of Comets; Cometary Dynamics; Near-Earth Objects.]
Fewer than one out of ten thousand radar meteors has been identified to be caused by interstellar meteoroids that pass through the solar system on a hyperbolic orbit. Their heliocentric speed is significantly higher than the solar system escape speed, confirming that they are of truly interstellar origin. The radius of these interstellar meteoroids is about 20 μm. These particles have been found to arrive generally from southern ecliptic latitudes with enhanced fluxes from discrete sources.
17.3 Big Photographic Data and Their Errors
It may seem that too much attention paid to such thorough exclusion of errors is superfluous. Big Data obtained in semiautomatic or automatic surveys or in space missions and transformed into catalogues of stellar characteristics using different computational algorithms always contain both systematic and random errors. Often this factor is crucial as it determines the time interval or sample characteristics when or where the data are reliable. For example, a dense network of reference stars with accurate positions is utterly required for newly discovered near-Earth objects. Their observations are conducted with CCD frames which have small field dimensions and a low number of reference stars. If there is any danger of a collision with the Earth, then a quickly determined accurate orbit is essential.
Independent sets of Big Data help to analyze errors and propose methods of their elimination.
In 2008–2010 at the Institute of Astronomy of the V. N. Karazin National University of Kharkiv (Ukraine) the XPM catalogue was created on the basis of combined data from the Two-Micron All Sky Survey (2MASS) and USNO-A2.0 catalogues. This catalogue was intended to derive the absolute proper motions of about 280 million stars distributed all over the sky, excluding a small region near the Galactic Center, in the magnitude range 12<B<19m. Proper motions were derived from the 2MASS Point Sources and USNO-A2.0 catalogue positions with a mean epoch difference of about 45 years for the Northern hemisphere and about 17 years for the Southern one. The zero point of the absolute proper motion frame (the “absolute calibration”) was specified with the use of about 1.45 million galaxies from 2MASS. The generated catalogue contains the International Celestial Reference System (ICRS) positions of stars for the J2000 epoch, original absolute proper motions, as well as B, R, J, H, and K magnitudes (Fedorov et al., 2009, 2010a). The XPM catalogue is one of the independent realizations of ICRS in the optical and near-infrared ranges. The comparison of the XPM with another ICRS realization, catalogue UCAC-3 (The Third US Naval Observatory CCD Astrograph Catalogue), showed that in UCAC-3 in the fields of 5∘×5∘ there have been significant systematic distortions in proper motions (Fig. 17.4) in the form of sharp discontinuities.
Fig. 17.4. The individual differences of proper motions of stars (XPM-UCAC3.0) in selected fields as a function of RA and Dec (Fedorov et al., 2010b).
This factor is crucial for astrometric tasks. For example, positional observations of artificial satellites, potentially dangerous objects, and other solar system bodies are carried out in consecutive series on apparent arcs 5 to 20 degrees long in CCD frames of small dimensions. The presence of systematic distortions of proper motions' field in the widely used reference catalogues (USNO, UCAC) leads to significant errors in positions of studied objects.
X Prospects
After the preliminary studies of comets Giacobini–Zinner and Halley via flyby reconnaissance, the future impetus of cometary research will be set by long-term investigations afforded only by rendezvous missions and, as a final goal, the return of samples of comet material in their pristine condition to Earth for laboratory analyses. Because of the potential threats of the catastrophic collisional effects of near-Earth objects (NEOs), there is also considerable interest in studying the physical properties of cometary nuclei. It is for these reasons that a number of comet missions have been planned and will be realized in the near future. These include NASA's Stardust mission for coma dust sample return, the CONTOUR mission for multiple comet flyby observations, and the Deep Impact project for active cratering experiment on a target comet. The Deep Space 1 spacecraft employing ion drive propulsion system will encounter Comet Borrelly in 2002. The title of the most daring and comprehensive comet mission belongs to the Rosetta mission of the European Space Agency. The Rosetta spacecraft will be launched in 2003 with Comet Wirtanen as its final target. After rendezvous, the spacecraft orbiting around the cometary nucleus will deploy a small spaceship for landing on the surface of the nucleus. The lander will provide in-situ measurements of the chemical and material properties of the cometary ice and dust mantle in great detail. Last but not least, the interior of the cometary nculeus will be probed by the radar tomography experiment on the mother spaceship.
The foregoing new initiatives, which will be among the major thrusts in planetary research, are expected to produce scientific data in the next decade. The outlook for cometary study is therefore extremely promising. Certainly even more ambitious projects might lie ahead at the beginning of the 21st century. At this point, we should mention the potential industrial and commercial interests in cometary materials as important resources in space.
The important contributions from the International Halley Watch network during the comet Halley epoch and more recently the worldwide observations of comet Hyakutake and comet Hale–Bopp in 1996 and 1997 have shown that coordinated ground-based observations are an indispensable part of cometary research. Besides optical observations, infrared and UV measurements using satellite-borne telescopes would be very informative in monitoring the coma activities, dust emission, and gas and dust compositions of a wide variety of comets at different stages of outgassing. For this, a dedicated planetary telescope capable of performing simultaneous imaging and spectroscopic observations in the wavelength range from ultraviolet to infrared would be most desirable. For example, there are many issues related to the thermodynamics and hydrodynamics of coma expansion that require IR observations at high spatial and spectral resolution. The extremely variable structures of cometary ion tails are closely related to momentum and energy-transfer from the solar wind to cometary ions; well-planned observations capable of measuring the ion densitis, plasma velocities, and spatial configurations such as ion rays and plasmoids are urgently needed. The results from comet Halley have shown that the effects involved are extremely complicated. The correlations of remote-sensing observations and in-situ measurements carried out in the missions just mentioned will be most useful from this point of view.
The number density and size distribution of comets in the outer solar system are critical to our understanding of the origin of the short-period comets and the distant Oort cloud. Surveys by a new generation of space telescopes in optical and IR wavelengths will possibly provide the definitive answer. The observations of extrasolar systems exemplified by Beta Pictoris will yield complementary knowledge on the formation of the solar nebula and its disk of planetesimal population.
Laboratory simulations of ice condensation and gas trapping at low temperatures have provided very interesting insights into the physical nature of the cometary ice and the surface sublimation processes. Further progress in this area is expected.
Theoretical studies, as always, are the basic tools in laying the foundation for understanding different physicochemical phenomena and their interrelations. The five areas of ongoing theoretical efforts (i.e., dust mantle modeling, gas thermodynamics and hydrodynamics, comet–solar-wind interaction, coma chemistry, and cometary orbital dynamics) are following a trend of increasing sophistication. In combination with the opportunities of new observations and space missions in the next decade, modern cometary physics has just reached a new era.
4. Puzzles and Promise
4.1 Telescopic Searches and Exploration
It is a rare but exciting event in science when a single idea by a small group of scientists ignites an entirely new field of study and redefines the scientific debate. That is exactly what happened to such diverse fields as impact physics, asteroid observations, and paleontology after Alvarez and colleagues hypothesized that the iridium anomaly found in Cretaceous–Tertiary (K/T) boundary sediments was the mark of an impact event that destroyed the dinosaurs. [SeePlanetary Impacts.]
Asteroid impacts are a consistent and steady-state fact in the solar system. One just has to look at the extensively cratered surface of any solid body to realize that impacts happen. To some extent, the fact that the Earth has active geological processes that erase the scars of impact craters rapidly and a thick atmosphere that filters out the smaller impactors has lulled us into a false sense of security.
The real question is not whether asteroids hit the Earth, but rather how often does it happen. Before they hit, these impactors are comets and asteroids with the same power law distribution of sizes that we see in the Asteroid Belt, so small impacts will be more frequent and large “species-killing” impacts will be much rarer. However, as those who live near dormant volcanoes should realize, rare events on human timescales can be common and frequent events on geologic timescales.
There is plenty of evidence in the geologic and fossil record for repeated major impacts, some of which are associated with mass extinctions. For instance, there were 5 mass extinctions during the last 600 million years, about what would be predicted by a purely impact-driven extinction model. The bottom line is that asteroid impacts should be treated as one of the steady-state processes that results from a dynamic solar system. Although the chances of a cratering event like the one that dug the almost 1 mile diameter Meteor Crater in Arizona happening on any random day are small, the probability is 100% that it will happen sometime. The only question is when? When faced with predictable dangers, it is sensible to take precautions. In the same way that people who live on the Gulf coast of North America track hurricanes and people who live in tornadoprone Oklahoma build houses with cellars, it seems a reasonable precaution to identify, track, and study the asteroids in near-Earth space. [SeeNear-Earth Objects.]
4.2 Origins of Asteroids
As pieces of a planet that was never formed, the asteroids represent important chemical and physical clues about the origin of the planets. But these clues can only be interpreted by having a reliable theory for how the asteroids themselves were formed. The key questions to be addressed for the asteroids include: How much material was originally in the region of the solar system where the asteroids were formed? What interrupted the formation of a planet here? Where did all the missing material go? What processes shaped both the structure of the individual asteroids and the characteristics of the Asteroid Belt as a whole?
We do have a reasonably complete census of asteroids in the main belt, down to a size of a few kilometers, and from that we can infer how the perturbing gravity of Jupiter and Saturn has shaped the distribution of asteroids today. We know some asteroids come in distinct spectral classes, and that there is a tendency for S-type asteroids to be found in the inner belt and C-types to be found in the outer belt. But while we can infer compositions for those types, based on the meteorite sample, we recognize that those inferences are very uncertain, and that there could well be material in the Asteroid Belt that is not sampled in our meteorite collections. Still, with the data in hand, we can sketch out a testable scenario for the formation and evolution of the Asteroid Belt, knowing that this is not a final answer but rather a best-guess, which we will continue to test and refine as we learn more about the asteroids.
Our first guess is to assume that the asteroids, like the planets, formed in a solar nebula of gas and dust that smoothly varied in density and temperature from the hot, dense center where the Sun was forming to the thin, cold outer edges where the nebula bordered interstellar space. [SeeThe Origin of the Solar System.] It is possible to calculate the rate at which dust in this cloud would encounter and stick to other bits of dust. These calculations indicate that it is possible in the early solar nebula for very loose balls of dust (more than 90% empty space) as large as a kilometer across to be formed. Relatively low-speed collisions between such dust balls would lead to further compression and accretion into objects big enough to not be carried away with the gas when the last of the solar nebula was pulled into the Sun or ejected in a massive early solar wind.
But it seems probable that these proto-asteroids looked very different from the asteroids we see today. When Jupiter and the other major planets formed, their concentrated gravity would have begun to stir up the asteroidal material. This stirring, and the absence of a nebula gas to damp down their motions, would have added enough energy to the orbits in the Asteroid Belt that further collisions between the asteroids would lead to asteroids breaking apart instead of sticking together. If one takes the present-day masses of the planets, adding a solar proportion of hydrogen and helium to the rocky planets’ compositions, and then imagines spreading this material in a disk around the Sun to simulate the smallest possible nebula capable of making planets, one can see that the amount of material in such a nebula varies smoothly from the center to the outer reaches of the solar system, with three notable exceptions. Inside Mercury and outside Neptune the nebula had distinct boundaries. And in the region of Mars and the Asteroid Belt, there appears to be a significant amount of mass missing today.
We saw in Section 2 how Jupiter and Saturn perturb asteroids out of the Asteroid Belt. But modeling the early solar nebula allows us to estimate just how much material was so perturbed. It suggests that Mars is made up of less than 10% of the material originally available in its region of the solar nebula, while the mass of the Asteroid Belt is less than 0.1% of the inferred original material present. The perturbations of asteroidal material by Jupiter and Saturn must have been extremely efficient, at least in the earliest stages of the solar system's history.
One inevitable result of having 99.9% of the mass of the Asteroid Belt excited into such orbits is that there must have been a very high collision rate among asteroids in the early solar system. These collisions would break larger asteroids into smaller pieces and destroy the smaller pieces entirely. But for the largest asteroids—many tens of kilometers in radius—impacts energetic enough to shatter them may not have enough energy to disperse the pieces completely.
Instead, the fragments were likely to reaccrete into piles of rubble, consistent with the structure that asteroids are inferred to have today.
As the Asteroid Belt is dissipated, the rate of collision likewise would have dropped. Given the present-day population of the Asteroid Belt, collisions that are capable of breaking pieces of an asteroid into earth-crossing orbits or creating families of asteroids where one asteroid once orbited still do occur. We do see young families of asteroids today. Likewise, by measuring short-lived radioactive isotopes formed in meteorites by cosmic rays, we can see peaks in the ages of certain meteorite classes that imply they were broken off a parent body at a specific moment some tens to hundreds of millions of years ago. But these events must be many, many times less frequent today than when the Asteroid Belt was much more heavily populated.
One result of this scattering of asteroids by Jupiter and Saturn may have been that a few rare bodies originally from the Asteroid Belt may have been captured into orbits around other planets. Among the moons suspected of being captured asteroids are the Martian moons Phobos and Deimos, and the irregular moons of the gas giant planets.
4.3 Spacecraft Missions to Asteroids
Although telescopic studies are by far the most prolific source of data on asteroids, critical science questions on asteroid composition, structure, and surface processes can only be addressed by spacecraft missions getting close to these objects. The range of spacecraft encounters includes flybys, rendezvous, and sample return missions, which provide information of ever-increasing detail and reliability. We have now seen the results of a number of flybys, starting with two by the Galileo spacecraft (243 Ida and 951 Gaspra) on its way to Jupiter.
The NEAR (Near Earth Asteroid Rendezvous) spacecraft, the first dedicated asteroid mission, flew past asteroid 253 Mathilde and arrived in orbit around 433 Eros in 2001. After orbiting Eros for one year and mapping its morphology, elemental abundances, and mineralogy with an X-ray/gamma ray spectrometer (XGRS), imaging camera, near-infrared reflectance spectrometer, laser rangefinder, and magnetometer, the spacecraft ended its mission by landing on the surface of Eros. [SeeNear-Earth Objects.]
The next mission to fly past an asteroid was Deep Space 1 (DS1). Primarily a technology demonstration to test the new solar-electric propulsion ion drive system, it flew past asteroid 9969 Braille on its way to comet Borrelly, but unfortunately a camera-pointing error during the Braille encounter limited the amount of useful data from that mission.
In late 2005, the Japanese space agency's ambitious Hayabusa asteroid sample return mission rendezvoused with asteroid 25143 Itokawa. This NEA turned out to have an extremely rough surface, as shown in Fig. 7. After several months of mapping and analysis, the spacecraft collected samples by shooting a small projectile into the surface and collecting some of the fragments splashed off. As of this writing, the spacecraft is on its way back to Earth and will parachute the sample pod onto the Australian desert in 2010.
Even though we have made great strides in exploring asteroids, they are still largely unexplored; indeed, in the case of the smaller NEOs, they are still largely undiscovered. They have great potential for science, for destruction, as resources in space, and for exploration. We are only just starting to understand these numerous objects that share our solar system.
5. Asteroid Physical Properties
Over a million asteroids with diameters greater than 1 km reside primarily in a belt between Mars and Jupiter. [SeeMain-Belt Asteroids.] The asteroid population extends interior to the orbit of the Earth and beyond Jupiter. These objects are the scattered and disrupted remains of an early population of protoplanets whose continued growth was interrupted early on by mutual gravitational stirring, the growth of a massive Jupiter, or both. The size distribution of asteroids provides insight into their origins and evolution through collisions and dynamical processes. Asteroid sizes are also important to determine the hazard of near-Earth objects. Asteroids are almost all unresolved to telescopes, and their brightnesses are insufficient for determining their sizes unless their albedos are known. By combining visible and thermal infrared observations of an asteroid, the diameter and albedo of an asteroid can be simultaneously determined. The difficulty of making radiometric (thermal) observations of asteroids from ground-based telescopes made for slow growth in the number for which physical properties could be determined. Space-based surveys in the thermal infrared greatly increased the numbers of asteroids for which albedos and diameters were available, providing new information about the composition of the Asteroid Belt.
Because most space-based infrared telescopes have operated in a pointed mode, targeting specific objects or locations for observations, and few have engaged in surveys, most asteroid albedos and diameters were derived from the IRAS survey in 1983. This survey resulted in 8210 observations of 2004 asteroids. A more limited survey by MSX in 1996 resulted in observations of 168 asteroids. By and large, such surveys rely upon the detection of asteroids in known orbits, since they are unable to provide sufficient astrometry to determine the orbits of newly discovered asteroids.
During the IRAS mission, there were tens of thousands of asteroids having known orbits. More than two decades later that number has increased by an order of magnitude. Coupled with greatly increased detector sensitivity, an all-sky survey is no longer necessary to produce an even larger catalog of asteroid diameters and albedos. Over the 5 year nominal mission of Spitzer, it is estimated that about 25,000 serendipitous asteroid measurements will be made.
Known asteroid diameters inferred from IRAS are shown in Fig. 17 as a function of heliocentric distance. The absence of small asteroids with increasing heliocentric distance is a consequence of the limits of detector sensitivity. The absence of larger asteroids beyond 4 AU, however, is real. This indicates that either the inner and outer belt/Jupiter Trojan populations had a very different collisional history, or that their origins are different.
FIGURE 17. Asteroid diameters versus heliocentric distance of detection. The lower limit of detected asteroid sizes reflects the sensitivity limits of the IRAS detectors. The outer asteroid belt is shown to have few large asteroids compared to the inner belt.
Albedo provides insight into composition. Meteorite studies show that very dark surfaces arise from largely carbonaceous materials, while high albedo surfaces are associated with silicic compositions lacking such carbonaceous material. IRAS confirmed that most C-type asteroids (thought to be carbonaceous) are indeed dark compared to the “stony” S-type asteroids, and that there is a trend toward darker asteroid surfaces with increasing heliocentric distance (Fig. 18). This is consistent with the view that there is not only a residual primordial composition gradient through the Asteroid Belt, but that inner belt asteroids (predominantly S-type) were significantly processed by heating in the early solar system, while the outermost asteroids have experienced little heating and have retained a more “primitive” mineralogy.
FIGURE 18. Geometric albedo versus heliocentric distance of known asteroids detected by IRAS. The high albedo asteroids are located almost exclusively in the inner portion of the main asteroid belt.
New observations often result in as many new questions as new answers, and the IRAS asteroid observations are no exception. Prior to IRAS, ground-based thermal observations had been preferentially made of the largest asteroids. It was noticed that there was a bimodal distribution in the inferred albedos, which was consistent with the main-belt asteroid population being dominated by dark C-type and bright S-type asteroids. IRAS added large numbers of observations of smaller asteroids and it was found that they had an albedo distribution quite different from the larger asteroids. Small asteroids have a unimodal distribution spanning the total range of albedos inferred for the large asteroids (Fig. 19). Since the small asteroids are fragments of larger asteroids, this might imply that surface minerologies are not representative of interior minerologies or that significant “space weathering” may affect the surface spectra of the larger bodies.
FIGURE 19. Large and small asteroids evidence different albedo distributions.
There has been some question as to whether all asteroid families mark the site of a past catastrophic disruption or whether in some cases asteroids might be clumped together due to dynamical forces such as gravitational perturbations on their orbits by Jupiter. Albedo distributions can also provide clues to the origin of some asteroid families. Assuming the parent to have been compositionally homogeneous, the fragments should exhibit similar spectral properties. On the other hand, members of purely dynamical clusters would not be expected to have similar compositions. IRAS scanned enough of the members of the largest families to show that family members had albedos more similar to each other than to the background asteroids nearby, giving support, in those cases, to the asteroid breakup hypothesis (Fig. 20).
FIGURE 20. A comparison of the albedo histograms of the Eos asteroid family and non-family members near the same location in the asteroid belt supports the hypothesis that the family members derived from a single parent body.
Thermal spectra can provide information including thermal inertia and composition. Spitzer observations of Jupiter Trojan asteroids have yielded results suggesting fine-grained silicates in a relatively low-density, perhaps “fairy-castle” matrix. Does this indicate a possible cometary origin? Evidence for complex carbon compounds on primitive asteroids has also been detected. A Spitzer survey of M-class asteroids, spectrally similar at visible wavelengths, indicates major differences in thermal inertia among them, which could indicate significant differences in surface densities (due to relative age and collisional processing) or conductivity (some M-types are metallic, some may be stony). Thermal infrared observations of asteroids from space-based facilities are revealing an increasingly diverse population of objects.