Apollo-Amor Objects Research Articles

End Products of Cometary Evolution: Cometary Origin of Earth-Crossing Bodies of Asteroidal Appearance

AbstractBecause there is no necessary connection between the time required to remove the volatile component of a cometary nucleus by solar heating (physical lifetime) and the dynamical lifetime of a comet, it is possible that a comet may evolve into an observable object of asteroidal appearance. Almost all comets have dynamical lifetimes much shorter than their physical lifetimes and in these cases complete loss of volatiles will not occur. Mechanisms do exist, however, whereby a small but significant fraction of comets will have longer dynamical lifetimes, permitting them to evolve first into Jupiter-family short period comets and then into comets with relatively safe decoupled orbits interior to Jupiter’s orbit. Observed Jupiter-family objects of asteroidal appearance (e.g., 1983SA) are much more likely to be of cometary rather than asteroidal origin. “Decoupling” is facilitated by several mechanisms: perturbations by the terrestrial planets, perturbations by Jupiter and the other giant planets (including resonant perturbations) and non-gravitational orbital changes caused by the loss of gas and dust from the comet. The dynamical time scale for decoupling is probably 105–106 years and almost all decoupled comets are likely to be of asteroidal appearance. Once decoupled, the orbits of the resulting Apollo-Amor objects will evolve on a longer (107–108 year) time scale, and the orbital evidence for these objects having originally been comets rather than asteroids will nearly disappear. Statistically, however, a large fraction of the bodies in deep Earth-crossing orbits with semi-major axes ≳ 2.2 AU are likely to be cometary objects in orbits that have not yet diffused into the steady state distribution. For plausible values of the relevant parameters, estimates can be made of the number of cometary Apollo-Amor “asteroids,” the observed number of Earthcrossing active and inactive short period comets, and the production rate of short period comets. These estimates are compatible with other theoretical and observational inferences that suggest the presence of a significant population of Apollo objects that formerly were active comets.

Where do the Apollo objects come from?

Monte Carlo calculations have been carried out to trace the orbital evolution of ∼1- to 45-km-diameter bodies produced by collisions in the asteroid belt interior to 2.6 AU. It is found that about 3% of the asteroidal fragments ejected at 100 m/sec in this size range are perturbed into Earth-crossing orbits as a consequence of the 3:1 Jovian commensurability resonance at 2.5 AU and the ν dot 6 secular resonance in the innermost asteroid belt. For ejection velocities of 100 m/sec the calculated steady-state number of Apollo objects (454) and Amor objects (1600) brighter than absolute V magnitude 18 is in the general range of the estimated number based on observations. Considering the uncertainties in both the theoretical results and the observational data, this should be considered to be good agreement. Even though the initial Earth-crossing orbits of these bodies are concentrated near 1 AU, their steady-state orbital distribution is widely dispersed. There is strongly suggestive, but not yet compelling, evidence for an additional component of Apollo-Amor objects that are inactive comets. It is found that a modest production rate (∼1 in 6 × 10 4 years) of extinct comets with V(1,0) < 18 in Encke-like orbits is required to maintain a steady-state population of cometary Apollos equal in number to the asteroidal Apollos.

Dynamical relations between asteroids, meteorites and Apollo-Amor objects

A Monte-Carlo technique has been used to investigate the orbital evolution of asteroidal collision debris produced interior to 2.6 AU. It is found that there are two regions primarily responsible for production of Earth-crossing meteoritic material and Apollo objects. The region adjacent to the 3:1 jovian commensurability resonance (2.5 AU) is unique in providing material in the required quantity and orbital distribution of the ordinary chondrites. This region should also supply a comparable preatmospheric flux of carbonaceous meteorites. The innermost asteroid belt (2.17-2.25 AU), via the v6secular resonance, provides a fluxca.9% that of the ordinary chondrites, and appears to be the strongest candidate for the basaltic achondrite source region. It is unlikely that a significant number of meteorites originate beyond 2.6 AU. It is speculated that enstatite achondrites are derived from the Hungaria region, interior to the main belt, and that iron and stony-iron meteorites originate from many main-belt sources interior to 2.6 AU.

Asteroidal catastrophic collisions simulated by hypervelocity impact experiments

We report the results of six impact fragmentation experiments carried out with free-falling macroscopic targets of different compositions and shapes, and with projectile velocities close to 9 km/sec, i.e., significantly higher than the sound velocity in the target materials. The data have been examined by deriving the mass and shape distributions of the fragments, by reconstructing two of the shattered targets in order to study the geometry of the fracture surfaces, and by analyzing the properties of the fine-grained high-velocity ejecta. The fragment mass distributions show clearly that the degree of target fragmentation depends strongly on the impact parameter. Apart from the few largest fragments, these distributions are well represented by two power laws with different exponents, connected at a size of about 1 cm. The fragment shapes are generally in good agreement with those observed in previous experiments, and no significant shape vs size dependence has been found down to sizes of the order of 0.1 mm. The fragments tend to become larger and possibly more irregular in shape when they are generated farther from the impact point. The fracture surfaces are oriented roughly along meridians and parallels (with the pole at the impact point) when the target is spherical, but are clustered around the symmetry planes when the target is ellipsoidal. Fine-grained particles, with typical sizes and velocities of 0.01 cm and 1 km/sec, respectively, are ejected at low-elevation angles and in a rather collimated way, starting both from the neighborhood of the impact point and from regions of incipient cracking. Particular attention has been paid to a comparison between these results and the observed properties of the outcomes of asteroidal catastrophic collisions, like the dynamical families and the small inner planet crossing objects. While the collisional theory for the origin of families is fully consistent with the experimental results (with some indication for a significant role of the parent asteroid's self-gravitation), the elongated shapes of several Apollo-Amor objects are much rarer among the laboratory fragments, and thus appear to require a different explanation.

On the rotation of cometary nuclei and small asteroids

AbstractThe spin rate distributions of comet nuclei and small asteroids are compared, and it is shown that Whipple’s (1982) finding of a faster average rotation for the asteroid sample was due to observational biases. In fact, the presently available rotational data do not exhibit any clear differentiation among comet nuclei and asteroids, except possibly for a higher abundance of short rotational periods among the Apollo-Amor objects.

Shapes of asteroids compared with fragments from hypervelocity impact experiments

Light-curve observations have shown that most asteroids are non-axially symmetrical in shape, probably as a result of fragmentation undergone by objects with negligible gravitational binding1. Some earlier laboratory simulations of catastrophic collisions at velocities not exceeding 4 km s−1 (refs 2–4) against cubic and parallelepipedal targets showed that the fragment shapes have a nearly gaussian distribution around the mean value of the axes' ratio 2:√2:1. We report here results from hypervelocity experiments performed at ∼10 km s−1 against free-falling bodies. We found that, regardless of the very different experimental conditions, the shape distribution obtained is closely similar to that obtained2–4 using targets of different shapes and materials and in good agreement with that of the main-belt asteroids of diameter smaller than ∼100 km. This distribution is not consistent with either the symmetrical shapes of the gravity-dominated asteroids, or the elongated Apollo–Amor objects.

Steady state populations of Apollo-Amor objects

The steady-state distribution of orbits of Apollo-Amor objects is calculated for a variety of possible sources. These include asteroids near the inner edge of the belt, cometary orbits similar to Encke, and hypothetical extinct cometary orbits with perihelia larger than that of Encke. In all but one case, the steady-state distributions are similar for all these sources, and predict Amor/Apollo ratios of 1.5 to 3. These ratios are lower than those predicted by work in which the effects of the ν 6 secular resonance were not considered. These results are in general agreement with observation, although the higher (∼3) Amor/Apollo ratios found for many of the sources may turn out to be unacceptably high. The absolute number of Apollo-Amors observed is found to require an injection rate of ∼15 objects/(10 6 years). This rate is easily achieved if the present existence of Encke is assumed to be a reasonably probable event, and if Encke becomes a ∼1-km-diameter Apollo object following exhaustion of its volatile material; best estimates of the injection rate from the asteroid belt [∼1.5/(10 6 years)] are too low. Hence a dominant cometary component is suggested. The predicted number of Apollo objects in small ( q < 1.0 AU, a < 1.4 AU orbits is in agreement with observation. Predicted lunar and terrestrial cratering rates agree approximately with observation. An unexplained difference between the lunar and terrestrial results is probably caused by uncertainties in the scaling laws or crater counts used. This discrepancy precludes an exact test of these calculations using cratering data.

Asteroid surface materials: Mineralogical characterizations from reflectance spectra

The interpretation of diagnostic parameters in the spectral reflectance data for asteroids provides a means of characterizing the mineralogy and petrology of asteroid surface materials. An interpretive technique based on a quantitative understanding of the functional relationship between the optical properties of a mineral assemblage and its mineralogy, petrology and chemistry can provide a considerably more sophisticated characterization of a surface material than any matching or classification technique for those objects bright enough to allow spectral reflectance measurements. Albedos derived from radiometry and polarization data for individual asteroids can be used with spectral data to establish the spectral albedo, to define the optical density of the surface material and, in general, to constrain mineralogical interpretations. Mineral assemblages analogous to most meteorite types, with the exception of ordinary chondritic assemblages, have been found as surface materials of Main Belt asteroids. C1- and C2-like assemblages (unleached, oxidized meteoritic clay minerals plus opaques such as carbon) dominate the population (∼80%) throughout the Belt, especially in the outer Belt. A smaller population of asteroids exhibit surface materials similar to C3 (CO, CV) meteoritic assemblages (olivine plus opaque, probably carbon) and are also distributed throughout the Belt. The relative size (diameter) distributions for these two populations of objects are consistent with an origin by sequential accretion from a cooling nebula (‘C2’ as surface layers, ‘C3’ as interior layers or cores). Based on information from meteoritic analogues and on qualitative models for the behavior of these materials during a heating episode, it seems unlikely that these ‘C2’- and ‘C3’-like asteroidal bodies have experienced any significant post-accretionary heating event either near surface or in the deep interior. The majority of remaining studied asteroids (20) of 65 asteroids exhibit spectral reflectance curves dominated by the presence of metallic nickel-iron in their surface materials. These objects are most probably the several end products of an intense thermal event leading to the melting and differentiating of their protobodies. These thermalized bodies are concentrated toward the inner part of the Asteroid Belt but exist throughout the Belt. The size of the proto-asteroid has apparently exercised control over the post-accretionary thermal history of these bodies. The available evidence indicates that all asteroids larger than about 450 km in (present) diameter have undergone a significant heating episode since their formation. The post-accretionary thermal history of the asteroidal parent bodies was apparently affected by both distance from the Sun and body size. The C2-like materials which dominate the main asteroid belt population appear to be relatively rare on earth-approaching asteroids. This suggests that most of these Apollo-Amor objects are not randomly derived from the main belt, but (a) may derive from a single event in recent time (∼107 yr), (b) may derive from a favorably situated source body, (c) may derive from a particular, compositionally anomalous region of the belt, or (d) may derive from an alternate source (e.g. comets).

Round-trip mission requirements for asteroids 1976 AA and 1973 EC

The recent discovery of 1976 AA has renewed interest in the possibility of modest asteroid sample-return missions. Such ventures may be logical precursors to more complex round-trip planetary missions. Both manned and unmanned mission requirements are assessed for two Apollo-Amor objects: 1976 AA and 1973 EC. It is shown that the propulsion requirements of 1-yr manned missions to either target are excessive, i.e., more than 20 Shuttle launches would be required. However, a low-energy 3-yr unmanned round-trip mission to 1973 EC has also been found, which requires only one Shuttle launch with a recoverable upperstage. It is apparent from these results that the discovery of a short-period low-obliquity object could have a profound impact on man's initial ventures beyond Earth-Moon space.

Photometry of the asteroid 1976 AA at 0.56 and 2.2 μm

We report observations at 0.56 and 2.2 μm of the Apollo asteroid 1976 AA made during its discovery apparition. We derive a 2.2-μm relative spectral reflectance (scaled to unity at 0.56 μm) of R(2.2 μm) = 1.5 ± 0.3. This 2.2-μm reflectance is not compatible with a carbonaceous surface composition. However, it is compatible with a wide variety of meteoritic types including ordinary chondrites, stony irons, and mesosiderites. Thus, 1976 AA may have a silicate surface similar to other Apollo-Amor objects.