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NASA NTRS1990

The collisional evolution of the spin of a nonspherical body

The nonspherical shapes of asteroids are considered through a generalization of the Harris (1979) model of asteroid spin rates' collisional evolution. Since a body in spin equilibrium rotates about its axis of maximum moment of inertia, the computation of the mean-squared impact parameter through averaging over all directions of flux is not strictly valid; nevertheless, these complications have been found not to alter the conclusions reachable by means of the simpler model.

Harris, A. W.↗

NASA NTRS1985

The distribution of compositional classes in the asteroid belt: A cosmochemical fingerprint?

Studies of the physical properties of the asteroids show a nonrandom distribution of types across the belt for asteroid classes E, S, M, F, C, P, and D. The general trend is for asteroids in the inner belt to have higher albedos and stronger mafic silicate absorption features than those asteroids located further out in the belt. One interpretation of this trend is that the asteroids, which occupy the region between the silicate rich terrestrial planets and the volatile rich outer planets, have preserved in their heliocentric compositional distribution a cosmochemical fingerprint of the thermodynamic conditions present in the solar nebula at the time of their formation. This hypothesis predicts that the differences in the spectral properties among the low albedo classes (C, P, F, P, and D) are due to temperature controlled processes which formed carbonaceous opaques. If this is true then the exact composition of the opaque components could, in principle, be used to determine the thermodynamic conditions between the orbits of Mars and Jupiter during the formation of the asteroids.

Gradie, J.↗

NASA NTRS1998

Dynamics of the Trans-Neptune Region: Apsidal Waves in the Kuiper Belt

The role of apsidal density waves propagating in a primordial trans-Neptune disk (i.e., Kuiper belt) is investigated. It is shown that Neptune launches apsidal waves at its secular resonance near 40 AU that propagate radially outward, deeper into the particle disk. The wavelength of apsidal waves is considerably longer than waves that might be launched at Lindblad resonances, because the pattern speed, g(sub s), resulting from the apsis precession of Neptune is much slower than its mean motion, Omega(sub s). If the early Kuiper belt had a sufficient surface density, sigma, the disk's wave response to Neptune's secular perturbation would have spread the disturbing torque radially over a collective scale lambda(sub *) approx. = r(2(mu)(sub d)Omega/ absolute value of r dg/dr)(sup 1/2), where mu(sub d)equivalent pi(sigma)r(exp 2)/(1 solar mass) and Omega(r) and g(r) are respectively the mean motion and precession frequency of the disk particles. This results in considerably smaller eccentricities at resonance than had the disk particles been treated as noninteracting test particles. Consequently, particles are less apt to be excited into planet-crossing orbits, implying that the erosion timescales reported by earlier test-particle simulations of the Kuiper belt may be underestimated. It is also shown that the torque the disk exerts upon the planet (due to its gravitational attraction for the disk's spiral wave pattern) damps the planet's eccentricity and further inhibits the planet's ability to erode the disk. Key words: celestial mechanics, stellar dynamics - comets: general minor planets, asteroids

Ward, William R.↗

NASA NTRS1994

Environmental Perturbations Caused by the Impacts of Comets and Asteroids on Earth

The extinction mechanisms proposed at the Cretaceous-Tertiary geological boundary are reviewed and related to the impact of asteroids or comets in general. For impact energies below 10(exp 4) Megatons (less than 6 x 10(exp 4) years; asteroid diameter less than 650 m), blast, earthquake, and fire may destroy local areas up to 10(exp 5) square m. Tidal waves could flood a kilometer inland over entire ocean basins. The energy range from 105 to 106 Megatons (less than 2 x 10(exp 6) years; asteroid diameter less than 3 km) is transitional. Dust lifted, sulfur released from within impacting asteroids, and soot from fires started by comets can produce climatologically significant optical depths of 10. At energies beyond 10(exp 7) Megatons, blast and earthquake damage is regional (10(exp 6) square cm). Tsunami cresting to 100 m and flooding 20 km inland will sweep the coastal zones of the world's oceans. Fires will be set globally. Light levels may drop so low from the smoke, dust and sulfate that vision is not possible. At energies approaching 10(exp 9) Megatons the ocean surface waters may be acidified by sulfur. The combination of these effects would be devastating.

Toon, Owen B.↗

NASA NTRS2006

Asteroid-Meteorite Links: The Vesta Conundrum(s)

Although a direct link between the HED meteorites and the asteroid 4 Vesta is generally acknowledged, several issues continue to be actively examined that tie Vesta to early processes in the solar system. Vesta is no longer the only basaltic asteroid in the Main belt. In addition to the Vestoids of the Vesta family, the small asteroid Magnya is basaltic but appears to be unrelated to Vesta. Similarly, diversity now identified in the collection of basaltic meteorites requires more than one basaltic parent body, consistent with the abundance of differentiated parent bodies implied by iron meteorites. The timing of the formation of the Vestoids (and presumably the large crater at the south pole of Vesta) is unresolved. Peaks in Ar-Ar dates of eucrites suggest this impact event could be related to a possible late heavy bombardment at least 3.5 Gyr ago. On the other hand, the optically fresh appearance of both Vesta and the Vestoids requires either a relatively recent resurfacing event or that their surfaces do not weather in the same manner thought to occur on other asteroids such as the ordinary chondrite parent body. Diversity across the surface of Vesta has been observed with HST and there are hints of compositional variations (possibly involving minor olivine) in near-infrared spectra.

Pieters, C. M.↗

NASA NTRS1990

On the fragmentation of asteroids and planetary satellites

A general scaling model is defined which allows the extrapolation of small-scale collisional fragmentation experiment results, and existing collisional theories are considered within its framework. Scaling based exclusively upon the specific energy, Q, of the event (the ratio of projectile kinetic energy to the mass of the target body) is shown to hold when (1) the projectile and target material properties do not depend on size or time scales, and (2) the collision is governed by kinetic energy independently of impact velocity. Because neither of these conditions should hold, serious doubt is cast on the validity of Q's use as the sole scaling parameter.

Housen, Kevin R.↗

NASA NTRS1978

The nature of Trojan asteroid 624 Hektor

Near-simultaneous visual and thermal IR (20-micron) photometry of the Trojan asteroid 624 Hektor is reported which was performed when the asteroid was observed nearly along its rotation axis. The results confirm and refine the low albedo and large size of this asteroid and confirm the general rotational-pole position and aspect angle predicted by Dunlap and Gehrels (1969). Hektor is found to be a truly extraordinary object in that it is larger and far more irregular in shape than other measured Trojans and far more irregular than other belt asteroids of comparable size. It is proposed that Hektor could be a partially coalesced pair of Trojan asteroids which collided with energy too low to cause complete fragmentation, thus forming a dumbbell-shaped object. A possible scenario is outlined according to which the two pre-Hektor objects were neighboring relatively large primitive spheroidal planetesimals trapped in Jupiter's Lagrangian cloud. Observational and theoretical tests of this model are suggested.

Hartmann, W. K.↗

NASA NTRS1974

Small bodies and the outer planets and Appendices 1 and 2

Correlations of asteroid spectral reflectivity characteristics with orbital parameters have been sought. Asteroid proper elements and extreme heliocentric distance were examined. Only general trends were noted, primarily red asteroids and asteroids with IR (.95 micron) absorption bands are concentrated toward the inner part of the belt. Also, asteroids with the pyroxene band tend to have larger proper eccentricities relative to non-banded asteroids.

Davis, D. R.↗

NASA NTRS1989

On the shape and albedo variegation of asteroids - Results from Fourier analysis of synthetic and observed asteroid lightcurves

Fourier analyses have been conducted of those asteroids in the Lagerkvist et al. (1987) lightcurves with substantially complete phase coverage and point density, and for which a unique period has been identified. Fourier coefficients are thereby obtained for 188 lightcurves of 91 asteroids, as well as for 868 synthetic lightcurves from a set of models different in shape and albedo markings; by comparing model and asteroid lightcurve coefficients, the possible shape and albedo variations for 18 asteroids are derived. An attempt is made to define the general distribution of shapes of all asteroid samples analyzed.

Barucci, M. Antonietta↗

NASA NTRS1989

Distribution of taxonomic classes and the compositional structure of the asteroid belt

This paper reviews previous studies on the distribution of the asteroid taxonomic classes and analyzes the techniques, results, and interpretations of these studies, with special attention given to the strong and weak points of these studies and the important caveat for their interpretation. It is concluded that, in spite of the differences in class definition, the rather ordered heliocentric distribution of the composition is real and must be due to either primordial, evolutionary, or dynamical processes, or to a combination of all three. In general, the distribution of asteroid taxonomic classes is characterized by moderate-albedo asteroids dominant in the inner asteroid belt, and low-albedo asteroids prevalent in the outer belt and beyond.

Gradie, Jonathan C.↗

NASA NTRS1997

Environmental Perturbations Caused by the Impacts of Asteroids and Comets

We review the major mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary following an asteroid impact. We then discuss how the proposed extinction may relate to the impact of asteroids or comets in general. We discuss the limitations of these mechanisms in terms of the spatial scale that may be affected, and the time scale over which the effects may last. Our goal is to provide relatively simple prescriptions for evaluating the importance of colliding objects having a range of energies and compositions. We also identify the many uncertainties concerning the environmental effects of impacts. We conclude that, for impact energies below about 10(exp 4) Mts (megatons of TNT equivalent) - i.e., impact frequencies less than in 6 x 10(exp 4) yr, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively - blast damage, earthquakes, and fires should be important on a scale of 10(exp 4) or 10(exp 5) km (exp 2), which corresponds to the area damaged in many natural disasters of recent history. However, tsunami could be more damaging, flooding a kilometer of coastal plane over entire ocean basins. In the energy range of 10(exp 4) to 10 (exp 5) Mts (intervals up to 3 x 10(exp 5) yr; comets and asteroids with sizes up to 800 m and 1.5 km, respectively) water vapor injections and ozone loss become significant on the global scale. In the submicrometer dust injection fraction from the pulverized target material is much higher than is presently thought to be most likely, then dust injection could be important in this energy range.

Toon, Owen B.↗

NASA NTRS2012

In Situ Analysis of Orthopyroxene in Diogenites Using Laser Ablation ICP-MS

Howardites, eucrites and diogenites (HED) form a suit of igneous achondrite meteorites that are thought to have formed on a single asteroidal body. While there have been many different models proposed for the formation of the HED parent asteroid they can be generalized into two end member models. One is the magma ocean model (e.g. [1]) in which the entire HED parent body was continuously fractionated from a planet wide magma ocean with diogenites representing the lower crust and eucrites being upper crustal rocks. The second model hypothesizes that diogenites and eucrites were formed as a series of intrusions and/or extrusions of partial melts of a primitive proto-Vesta [2]. We use in situ trace element analysis together with major and minor element analysis to try and distinguish between these different hypotheses for the evolution of the HED parent body.

Elk, Mattias↗

NASA NTRS1992

The recovery of asteroids after two observations

It is shown that a generalization of the use of 'Vaisala orbits', briefly mentioned at the Asteroids 2 Conference, can be very conveniently accomplished by means of an inversion of the 'GEM' form of the Gauss method. The procedure can also be applied to Apollo objects and to indeterminate cases of normal three-observation orbit computation, and there is also a simple extension to situations involving four or more observations.

Marsden, Brian G.↗

NASA NTRS1990

A spherical harmonics method for asteroid pole determination

A new and general method is presented for determining pole orientations of unresolved solar system bodies that exhibit brightness variation with rotation. The accuracy of the method is limited only by the number of available apparitions, which imposes the degree where the Legendre expansion must be truncated. It is shown that power spectra of lightcurves provide enough information to extract pole coordinates from the data, independent of body shape and albedo variegation. The method has been tested using several computer-generated synthetic light-curves, and the accuracy is found to be about 5 deg in both the longitude and latitude of the pole, which is considerably better than that obtained from the standard triaxial ellipsoid model. Finally, the new spherical harmonics method has been applied to six well-observed asteroids: 16 Psyche, 22 Kalliope, 39 Laetitia, 44 Nysa, 45 Eugenia, and 349 Dembowska.

Lumme, K.↗

NASA NTRS1975

Asteroids - Spectral reflectance and color characteristics

We present visible and near-infrared spectral reflectance curves for 35 asteroids, making a total sample of 67 including our previous work. Several previously unknown spectral types have been discovered. Absorption bands and other features, from which information about the surface composition of the asteroids can be derived, continue to appear. Sampling statistics suggest we have observed about half of the significantly different mineralogical assemblages present in the asteroid belt. There is a general, but imperfect, correlation between spectral type and semimajor axis, which is consistent with the hypothesis that meteorites are derived from the asteroid belt. The color-frequency histogram for the 67 objects is bimodal, probably associated with 'stony' objects and 'carbonaceous' objects. Hirayama family pairs usually show disparate spectral types. 19 Fortuna may show strikingly different photometric properties on opposite sides.

Mccord, T. B.↗

NASA NTRS1987

The 1986 DA and 1986 EB: M-class asteroids in near-Earth orbits

The Earth-approaching asteroid population is composed of asteroids in orbits with short lifetimes compared with the age of the solar system. These objects which are comprised of Aten, Apollo, and Amor asteroids must be replenished from either cometary or mainbelt asteroid sources since lifetimes against collision with or ejection by a planet are on the order of 10 to 100 million years. The physical study of Earth-approaching asteroids is constrained by the generally long period between favorable apparitions and poorly known orbits. Broadband spectrophotometry on the Johnson UBVR system and the Eight-Color Asteroid Survey system were obtained at Kitt Peak National Observatory and on the Johnson JHK system and at 10 and 20 microns at the NASA Infrared Telescope Facility at Mauna Kea Observatory. These observations were used to determine the absolute visual magnitudes and to derive the visual geometric albedos and diameters on the IRAS system. The spectral reflectance properties and geometric albedos of the M-class asteroids are consistent compositions analogous to the iron nickel meteorites or the enstatite-metal assemblages of the enstatite chondrites. The issue of the source(s) of the near-Earth asteroids population was examined by comparing the classifications on the scheme employed by Gradie and Tedesco of 38 such asteroids. Most of the near-Earth objects is indeed the asteroid belt as the observations suggest, then a method for removing extinct nuclei of short period comets must be found since the rate of production of short period comets from the long period comets is relatively large.

Gradie, Jonathan↗

NASA NTRS1978

Asteroid surface materials - Mineralogical characterizations from reflectance spectra

The use of general and specific diagnostic spectral features and parameters to interpret most of the published high-quality reflectance spectra of asteroids is discussed. Such diagnostics are shown to provide the most complete and sophisticated mineralogically and petrologically based interpretation of the asteroid surface materials. Previous investigations of asteroid surface materials are exhaustively reviewed, emphasizing the general approaches employed, sources of information, previous characterizations of asteroid surface materials, and asteroid spectral groups. The interpretive methodology is then applied to spectral reflectance data for many individual members of the various spectral groups. A range of mineral assemblages similar to certain meteorite classes is identified, and evidence is presented for a selective high-temperature event that melted certain asteroid parent bodies and permitted their differentiation as some function of distance from the sun or the protosun and of the size of the protoasteroid. Possible candidates are proposed for the source bodies of different meteorite groups.

Gaffey, M. J.↗

NASA NTRS1987

Asteroid lightcurve inversion

One of the most fundamental physical properties of any asteroid is its shape. Lightcurves provide the only source of shape information for most asteroids. Unfortunately, the functional form of a lightcurve is determined by the viewing/illumination geometry and the asteroid's light scattering characteristics as well as its shape, and in general it is impossible to determine an asteroid's shape from lightcurves. A technique called convex-profile inversion (CPI) that obtains a convex profile, P, from any lightcurve is introduced. If certain ideal conditions are satisfied, then P is an estimator for the asteroid's mean cross section, C, a convex set defined as the average of all cross sections C(z) cut by planes a distance z above the asteroids's equatorial plane. C is therefore a 2-D average of the asteroid's 3-D shape.

Ostro, Steven J.↗