Status of Current Research Programs
Measuring The Physical Characteristics of Near-Earth Objects
By virtue of their close approaches to Earth, NEOs are the smallest observable solar system bodies (Box 2.1) for which ground-based physical and spectroscopic studies can be conducted. In the past, low discovery rates, lack of rapid access to large-aperture telescopes, and the state of astronomical detector technology made it a challenge to measure physical characteristics for any but the brightest NEOs. Prior to the 1990s, only about 20 NEOs had been measured for their compositional, rotational, or thermal properties.1
With the widespread application of sensitive CCD detectors in the 1990s, telescopic measurements of physical or mineralogical characteristics are currently available for about 80 NEOs, where the most common data are measurements of spectral properties. Time series measurements of the brightness variations of NEOs produce light curves revealing rotation rates in the range of several hours to days, similar to known rotations for main-belt asteroids. More limited data on the rotation rates of comet nuclei suggest that comets rotate more slowly than most asteroids, making rotation a possible discriminator between asteroidal and cometary sources for NEOs. Light curve data for NEOs suggest that they generally rotate in the plane of their principal axes. An exception is (4179) Toutatis, which is a non-principal axis rotator with rotation and precession periods of 5.41 and 7.35 days, respectively.2 Thermal properties have been measured for a few NEOs; the data suggest that many such small objects do not have substantial regoliths.
These measurements3 reveal that most short-period comets have diameters in the range of 1 to 10 km. Their shapes are inferred to be irregular ellipsoids, with the ratio between the two largest principal axes falling in the range of 1.1 to 2.6. Their surfaces have low albedos, with most albedo estimates being <4%. Typically, only a small fraction (<10%) of the surface is active during perihelion passage, which suggests that most of the surface may consist of a mantle of nonvolatile material. Although the internal properties of comets are unknown, low densities are inferred from their slow rotations (5 to 70 hours), the frequent occurrence of splitting during perihelion passage, and the tidal breakup of Shoemaker-Levy 9. These results suggest that the physical structure of comets may be described as strengthless agglomerations of gravitationally bound planetesimals with a bulk density between 0.5 and 1.0 g/cm3. Most of this decade's physical measurements of NEOs have been made as target of opportunity observations obtained shortly after an object's discovery. Figure 2.1 shows an example of the circumstances that favor observations shortly after discovery. Historically, discoveries most often have occurred when an object made a close approach to Earth. In many cases, these apparitions are followed by a
BOX 2.1 Asteroid Magnitudes and Sizes
The absolute magnitude H of an asteroid is a constant that represents its intrinsic brightness due to reflected sunlight in the V spectral band (the yellow-green region centered near the peak of the solar energy spectrum). The observed magnitude V of an asteroid depends on H and on the asteroid's distance from Earth (Δ), its distance from the Sun (r), and the angle (α), between the lines of sight to Earth and the Sun as seen from the asteroid:
H is determined from observations of V at times corresponding to specific values of Δ* and r (in astronomical units); f(α) is the phase function (a function of α that can be estimated in various ways).
The absolute magnitude depends on the size of the asteroid and its albedo (its reflectivity in a given spectral band). Thus, the asteroid's size can be estimated from H if the albedo can be measured or inferred from other observations (e.g., its spectral type). At absolute magnitude 18, an average S-type (relatively bright) asteroid is about 0.8 km in diameter, and an average C-type (dark) asteroid is about 1.6 km in diameter. There is, however, a factor-of-three uncertainty in the measured albedos of S-type asteroids and a similar uncertainty for C-type asteroids. Thus, the 0.8-km object cited above could reasonably be anywhere between 0.5 and 1 km in diameter. This factor-of-three uncertainly corresponds to a factor-of-eight uncertainty in volume.
decades-long interval during which no additional close approach will occur. Fainter-magnitude limits being achieved by CCD surveys conducted to discover NEOs are reducing the bias toward discovering objects only when they are in the near vicinity of Earth.
Prospects for progress in measuring NEO physical characteristics, especially their spectral properties, are illustrated in Figure 2.2, which shows the apparition circumstances for known near-Earth asteroids through the end of the twentieth century. With the commissioning of the Keck II 10-m telescope and NASA's participation in the project, there is the potential for pre-mission physical measurements at visible and near-infrared wavelengths of specific NEOs of high scientific interest. A dedicated 2-m-class telescope would permit most discovered NEOs to become accessible for physical characterization.
Perhaps the greatest progress and potential for direct physical measurements of NEOs will occur through radar observations that complement spectral studies.4 Of the 37 near-Earth objects detected as of 1997 by radar, 4 were first observed before 1980, 20 were first observed during the 1980s, and 13 were first observed during the 1990s. The Goldstone antenna of NASA's Deep Space Network is responsible for the sole detections of nine NEOs and the most informative observations of three others. Images with several tens of meters of resolution have been obtained for three objects—(1620) Geographos, (4179) Toutatis, and (6489) Golevka—and shape models have been constructed for those objects and (4769) Castalia.5 At least 100 of the currently known NEOs are expected to be detectable by the Arecibo telescope during its first decade of operation following the completion of a major upgrade in 1997. Given current NEO population estimates, a thorough survey could reveal a sufficient number of close Earth approaches to allow Arecibo to construct 1000-pixel images of about one object per month. Thus, radar offers tremendous potential for achieving detailed shape models for a large number of NEOs.
Understanding the Mineralogical and Chemical Compositions of Asteroids
Visible and near-infrared reflectance spectroscopy provides the most sensitive and broadly applied remote sensing techniques for characterizing the major mineral phases present within asteroids.6 At visible and near-
infrared wavelengths, recognizable spectral absorptions arise from the presence of the silicate minerals pyroxene, olivine, and sometimes feldspar, as well as nickel-iron metal, spinel, primitive carbonaceous assemblages, and organic tholins. Water-bearing minerals such as phyllosilicates also exhibit distinct absorption features at near-infrared wavelengths. Seemingly dormant comets can have their activity revealed through the detection of fluorescence emission bands. Complementing these optical techniques are radar albedo measurements, which give primarily diagnostic information on the presence and abundance of metal phases.
At visible wavelengths, multiple-filter photometry measurements in the 1980s have given way to CCD spectrographs in the 1990s. The transition to CCD spectrographs is occurring more slowly at near-infrared wavelengths, where new instruments with capabilities for mineral spectroscopy are only now becoming available. Currently, visible-spectrum observations are the most common physical measurements being made of near-Earth objects. Access to larger telescopes, which improve the observational limits for visible-spectrum physical measurements, will correspondingly provide the opportunity for compiling a substantial sample of measurements at wavelengths other than visible.
Although the most reliable mineralogical interpretations require measurements extending into the near infrared, measurements in the visible wavelengths allow preliminary characterization according to the taxonomic groups established for main-belt asteroids. Many near-Earth asteroids fall into taxonomic categories over the same range as asteroids in the inner main belt. Most common in the inner asteroid belt and among NEOs are objects
having S-type asteroid spectra, which are interpreted to be olivine-pyroxene assemblages with a wide range of olivine-to-pyroxene abundance ratios. Spectral types corresponding to C-type asteroids are also seen within the NEO population. At least one metal-rich NEO is inferred from a very high radar reflectivity and supported by spectroscopic and radiometric observations.
Short-period comets are composed of roughly equal mixtures (by mass) of ice and dust, with the ice component consisting primarily of water (H2O), carbon monoxide (CO), and carbon dioxide (CO2). Although the ice component is inferred from daughter products measured in cometary comae, the chemical composition of the dust has been directly measured through mass spectrometers on board the Giotto and VEGA spacecraft sent to comet Halley and interplanetary dust particles (IDPs) collected in Earth's upper atmosphere.7 The dust for comet Halley consists of two major components: a refractory organic phase composed of carbon, hydrogen, oxygen, and nitrogen (CHON) and a magnesium-rich silicate phase. Within CHON particles, carbon and oxygen approach cosmic abundances, whereas nitrogen is intermediate between cosmic abundances and those found in C1-chondrites. The abundance of hydrogen is more like that in C1-chondrites. For rock-forming elements, the abundances are found to be within a factor of two with respect to cosmic abundances.
Deciphering the Relationships Among Asteroids, Comets, and Meteorites
Many meteorites are thought to be samples of NEOs, most of which are, in turn, derived from main-belt asteroids. The difficulty lies in determining exactly what kinds of meteorites are related to which asteroids or asteroid classes (Box 2.2). The few advances in unraveling asteroid-meteorite connections relate to main-belt rather than near-Earth asteroids, since the latter are generally faint and must be observed within a rather narrow window of opportunity. Because of the spectral similarities and apparent relationships between many main-belt and near-Earth asteroids, however, these discoveries also serve as linkages between NEOs and meteorites. However, it should be noted that some recent research suggests that a large fraction of meteorites may be derived directly from the main asteroid belt, without near-Earth intermediaries.8
The main-belt asteroid (4) Vesta is now usually acknowledged as the parent body for HED (howardite-eucrite-diogenite) meteorites. Rotational reflectance spectra for Vesta indicate a surface dominated by eucrite basalts, excavated locally in large craters to reveal plutonic rocks similar to diogenites. Howardites are regolith breccias composed of mixed eucrite and diogenite. New CCD spectra of small bodies with orbits between Vesta and the adjacent 3:1 mean orbital resonance with Jupiter (known to be a dynamical “escape hatch” that allows asteroid fragments to arrive in Earth-crossing orbits) indicate that they also have compositions similar to HED meteorites.9 Thus, these igneous meteorites can be assigned with some confidence as samples of Vesta, the third-largest asteroid (500 km in diameter), although other parent bodies are possible. Several NEOs have the same taxonomic classification as (4) Vesta.
In contrast, spectral analogs for the parent asteroids of ordinary chondrites (the most common types of meteorite falls) have proved elusive. A possible relationship between these meteorites and S-type asteroids has been debated for decades. Part of the problem in tying S asteroids to meteorites lies in the fact that S-type asteroids exhibit a wide range of properties, probably reflecting a correspondingly wide range of compositions. Present evidence suggests that space weathering (a kind of optical alteration due to exposure to the space environment and shock) has modified the spectral properties of asteroid surfaces, thus masking their true compositions.10 Shock due to impacts has been shown to lower albedo and modify spectral character, and shock-blackened chondrites are relatively common. Spacecraft rendezvous and sampling missions to S asteroids should resolve the issue of whether spectral masking occurs. It is likely, however, that the S asteroid class also contains nonchondritic objects.11 Spectral variations observed during asteroid rotation indicate that the surfaces of some S asteroids are compositionally heterogeneous (Figure 2.3), implying either that they may be differentiated or that they may have accreted from compositionally diverse chondritic materials. Both of the main-belt asteroids encountered during Galileo spacecraft flybys—(951) Gaspra and (243) Ida—are of the S class, as is the NEAR mission target (433) Eros.
The C-type asteroids have low albedos and relatively featureless spectra and are conventionally thought to be spectral analogs for carbonaceous chondrite parent bodies.12 Some carbonaceous meteorites have suffered aque-
BOX 2.2 Asteroid Taxonomy and Meteorite Classification
The asteroid taxonomic groups are derived from studies of a great variety of visible and near-infrared spectra and albedos of main-belt asteroids, which are relatively large and easy to observe.* The earliest recognized and largest groups are S (siliceous), C (carbonaceous), and M (metallic). Each of these taxonomic groups consists of a diverse collection of objects; for example, the S group has now been subdivided into numerous subgroups. Asteroid (4) Vesta and a large number of small, nearby asteroids compose the V group. About a dozen other letters of the alphabet have been used to categorize main-belt asteroids. This same system is utilized for near-Earth objects (NEOs) when sufficient spectral information is available. However, classification into one of the taxonomic groups is only a first stage, and compositional interpretation commonly requires more detailed spectral information.
Meteorites are more accessible, and laboratory investigations of their chemistry and petrology have resulted in an even more detailed classification system. Most meteorites are chondrites, stony objects of roughly solar chemical composition (minus the most volatile elements). Of these, nearly all fall in the broad ordinary chondrite class, which has been further subdivided. Other, highly reduced chondrites fall in the enstatite chondrite group, whereas those containing appreciable organic matter are the carbonaceous chondrites. Most other stony meteorites are achondrites, of which the HED (howardite-eucrite-diogenite) group is prominent. The achondrites are igneous rocks or, in the case of the so-called primitive achondrites, residues after the extraction of small quantities of melt. Iron meteorites are samples of differentiated asteroid cores or segregated small pods of metal. They exist in many forms, distinguished by their compositions and cooling histories. Pallasites and other stony-iron meteorites are metal-silicate mixtures that also formed in other differentiated bodies.
ous alteration at low temperatures, resulting in the formation of complex assemblages of hydrous clay minerals, carbonates, sulfates, and organic molecules. The spectral signature for water of hydration in phyllosilicates is particularly diagnostic for asteroids that have suffered aqueous alteration. A few carbonaceous chondrites have been dehydrated at high temperatures, and controversy exists concerning whether meteorites of this type are commonly represented among the main-belt C asteroid population. The NEAR spacecraft imaged C-type asteroid (253) Mathilde during a flyby in June 1997. C-type asteroids are also recognized among the NEOs.
The terminal stages of a comet's life are not well understood, but it is likely that progressive loss of volatiles during repeated passages close to the Sun causes depletion of surface volatiles, creating an inert mantle that seals off the interior. Without the presence of a coma or tail, such a body will resemble an asteroid. At least one NEO, (4015) Wilson-Harrington, has exhibited one episode of cometary behavior,13 and the orbital similarity between (3200) Phaeton and the associated Geminid meteor stream suggests that this body may also have been a comet. Although it is generally thought that cometary materials are sampled as interplanetary dust particles but are not represented in the world's meteorite collections, this view may reflect ignorance about the nature of the nonvolatile (rocky) components of cometary nuclei. Comets, as well as dark D- and P-type asteroids located in the outer main belt, may have been sampled as interplanetary dust particles, or micrometeorites.14
Some main-belt and Earth-approaching asteroids have nondiagnostic (relatively featureless) spectra. Included in this category are E and M types, which spectrally resemble enstatite achondrite and iron meteorites, respectively. A spectral and dynamical link has been made between NEO (3103) Egar and enstatite achondrites.15 Although such connections are implied in most asteroid classification schemes, they have not been rigorously demonstrated, and several of these asteroids exhibit absorption bands due to water of hydration which appear to make these particular objects incompatible with nominal analogs. There are also asteroid classes (e.g., T- and F-
type asteroids) for which no analogous meteorite types are recognized, as well as meteorite types (e.g., ureilites) for which no asteroidal parents have been suggested.
Further information on linkages between various kinds of meteorites and their asteroidal parents may be provided by the identification and study of asteroid families, which are probably disrupted fragments of larger bodies. Families thought to represent chondritic asteroids tend to be homogeneous in composition, whereas those from differentiated asteroids are not. Precise connections between families and known meteorite types, however, remain problematical.
Understanding the Formation and Geologic Histories of Near-Earth Objects
A wealth of information regarding the geology, age, and evolution of asteroidal bodies can be obtained from spacecraft, as clearly illustrated by Galileo observations of the main-belt asteroids (951) Gaspra16 and (243) Ida17 (see Figure 2.3). The Galileo mission provided the first observations of the populations of small impact craters on asteroids, evidence for the probable presence of large spall surfaces, qualitative information on the thickness and development of asteroidal regoliths, support for the idea of optical maturation of surface materials, hints of possible internal structure, and the discovery of a co-orbiting moonlet around Ida. Gaspra was found to have a low crater density suggestive of a young surface age on the order of a few hundred million years. The inferred young
age of Gaspra is consistent with a short collisional lifetime as expected for asteroids of this size (Gaspra has a mean diameter of approximately 14 km). Ida was found to have a crater density five times higher than that found on Gaspra, suggestive of an age near 1 billion years. Presumably this age reflects the time of collisional disruption of the parent body of the Koronis asteroid family, of which Ida is a member. Both Ida and Gaspra are elongate irregular bodies that have been interpreted as individual collisional fragments formed by the catastrophic disruption of larger objects, although other interpretations have been offered.
Craters smaller than 1 km on Ida exhibit a complete range of erosional form and a size-frequency distribution indicative of a steady-state or equilibrium population. This, together with the downslope orientation of chutes and bright stripes and the presence of dark-floored craters and fresh craters with bright rims, points to the existence of a regolith. From the depths of flat-floored craters, the regolith is inferred to be a few hundred meters thick in places. On the younger surface of Gaspra, on the other hand, there is little direct evidence of a regolith. However, bright materials associated with fresh craters along ridges on Gaspra have a stronger 1-micron absorption band than darker materials found in interridge areas. Observations suggest that both Ida and, to a lesser extent, Gaspra have undergone moderate optical maturation.
The geologic information obtained by spacecraft observations of main-belt asteroids is complemented by radar observations of a few NEOs. Among the three NEOs best observed by radar, (1620) Geographos is a single, coherent splinter, (4179) Toutatis is an aggregate of debris, and (4769) Castalia consists of two objects in contact.18,19 Radar data strongly suggest the presence of craters on Toutatis (Figure 2.4), but no NEO has yet been studied with high enough spatial resolution to determine details of its geology such as the presence of a regolith. Because of the expected young ages and very low surface gravity of NEOs, some researchers suspect that regoliths on these objects may be generally thin, patchy, or in some cases, absent. Diverse thermal properties deduced from infrared observations of a number of these objects may be consistent with these expectations.
To the extent that asteroid-meteorite connections can be considered firm, studies of meteorites provide quantitative information on asteroid formation and geologic evolution. From HED meteorites,20 the mineralogy and chemistry of (4) Vesta—the probable HED parent body—are now reasonably understood; the timing of its
melting has been constrained by radiometric dating; geochemical evidence for core formation has been discovered; and models for its thermal evolution are being formulated. The thermal histories of ordinary chondrite parent bodies are also interesting. Most ordinary chondrites have been metamorphosed, and detailed thermal models for their parent asteroids have been constructed based on decay of short-lived radionuclides.21 Chondrite cooling histories, determined from nickel diffusion profiles in metal grains, suggest that many bodies were disrupted by impacts and subsequently reaccreted into “rubble piles.”22 Earth-approaching S asteroids, possibly representing fragments of bodies heated to varying degrees, may be especially instructive for understanding asteroid thermal evolution and accretionary structure. Many carbonaceous chondrites have suffered aqueous alteration, and the source of the fluids that caused alteration in C-type asteroids was probably ice, originally accreted along with rocky material and later melted by decay of short-lived radionuclides, electromagnetic induction heating, or impacts.23 Information on the maturity of asteroid regoliths and the duration of their exposure has also been gained from studies of meteorite regolith breccias.
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