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NASA NTRS1991

The mineralogy of ordinary chondrites and implications for asteroid spectrophotometry

Published data from bulk chemical analyses of 94 ordinary chondrites are compiled in a table of normative mineralogy and discussed in detail. Significant variations in olivine, pyroxene, and metal abundance ratios are found within each chondrite class and attributed to redox processes superimposed on initial differences in metal/silicate ratios. The use of the diagrams constructed here to predict the mineralogic characteristics of asteroids on the basis of spectrophotometric observations is suggested.

Mcsween, Harry Y., Jr.↗

NASA NTRS1991

Asteroid families

More than 100 asteroid families are presented in Williams. Several examples of cratering events are known including family numbers 150, 162, 169, and 189. These are recognizable as many small fragments adjacent to and to one side (in three dimensions) of a much larger cratered body. Family numbers 138 and 140 are adjacent in proper element space. In population they are an intermediate step between the long recognizable families and the more frequent less populated families. Family number 164 is the fifth most populous family in the belt. All members are faint and nothing is known of the physical properties.

Williams, James G.↗

NASA NTRS1991

Observations of comets and asteroids

All available ground based observational techniques were used to study the chemical and physical properties of the small bodies of the solar system, primarily comets and next asteroids. The ultimate goal is to use these bodies to understand the formation and evolution of the solar system. Progress and accomplishments, and publications are given.

Ahearn, Michael F.↗

NASA NTRS1991

Visual and infrared studies of asteroids and the Pluto-Charon system

The strategy of the project was to: (1) analyze light curves of Pluto-Charon mutual eclipse event light curves to derive models of the Pluto-Charon system; (2) use these results in planning and reducing HST observations tentatively scheduled to be obtained in Aug. 1991 to determine Pluto-Charon mass ratio; and (3) obtain visual and IR photometry of selected asteroids to help determine their albedos, sizes, shapes, pole orientations, taxonomic classes, and phase functions.

Tedesco, Edward F.↗

NASA NTRS1991

Comet and asteroid dynamics

In order to provide the ground-based observing community and NASA flight projects with accurate comet and asteroids ephemerides, improvements are being made to the existing dynamic models and new data type are being investigated. For active comets, non-gravitational forces must be taken into account; these forces are assumed due to the rocket-like thrusting of outgassing cometary ices.

Yeomans, Donald K.↗

NASA NTRS1992

Near Earth asteroid orbit perturbation and fragmentation

Collisions by near earth asteroids or the nuclei of comets pose varying levels of threat to man. A relatively small object, approximately 100 meter diameter, which might be found on an impact trajectory with a populated region of the Earth, could potentially be diverted from an Earth impacting trajectory by mass driver rocket systems. For larger bodies, such systems would appear to be beyond current technology. For any size object, nuclear explosions appear to be more efficient, using either the prompt blow-off from neutron radiation, the impulse from ejecta of near-surface explosion for deflection, or as a fragmenting charge. Practical deflections of bodies with diameters of 0.1, 1, and 10 km require interception, years to decades prior to earth encounter, with explosions a few kilotons, megatons, or gigatons, respectively, of equivalent TNT energy to achieve orbital velocity changes or destruction to a level where fragments are dispersed to harmless spatial densities.

Ahrens, Thomas J.↗

NASA NTRS1992

Discovery: Near-Earth Asteroid Rendezvous (NEAR)

The work carried out under this grant consisted of two parallel studies aimed at defining candidate missions for the initiation of the Discovery Program being considered by NASA's Solar System Exploration Division. The main study considered a Discover-class mission to a Near Earth Asteroid (NEA); the companion study considered a small telescope in Earth-orbit dedicated to ultra violet studies of solar system bodies. The results of these studies are summarized in two reports which are attached (Appendix 1 and Appendix 2).

Veverka, Joseph↗

NASA NTRS1991

Angular momentum transfer in low velocity oblique impacts - Implications for asteroids

An experimental study has been conducted for the low-velocity oblique impact efficiency of angular momentum transfer, which is defined as that fraction of incident angular momentum that is transferred to the rotation of a target. The results obtained suggest that more energetic impacts are able to transfer angular momentum more efficiently. In the cases of ricochetted projectiles, the fraction of angular momentum carried off by the ejecta was noted to be less than 30 percent. It is suggested that, if asteroid spin rates are due to mutual noncatastrophic collisions and the taxonomic classes are indicative of bulk properties, the differences between corresponding spin rates will be smaller than expected from a consideration of relative strength and density alone.

Yanagisawa, Masahisa↗

NASA NTRS1991

Brine volcanism and the interior structures of asteroids and icy satellites

Brine vulcanism, in which MgSO4 and Na2SO4 are shown by carbonaceous chondrite mineralogies and elemental abundances to be the most important solutes, is presently suggested to account for the resurfacing observed on such icy satellites as Europa and Ganymede, and to have been a significant factor in the evolution of chondritic asteroids. Hypersaline brines have lower viscosities than familiar silicate lavas, although they remain more viscous than pure water; aqueous sulfate flows are seen as capable of constructing the vast, low-relief volcanic plains observed on Ganymede and Europa.

Kargel, Jeffrey S.↗

NASA NTRS1992

Asteroid lightcurve observations from 1981

Photoelectric lightcurve observations in the V band, together with estimates of the mean and maximum reduced magnitudes, are presented for 59 different asteroids; 15 new or significantly revised periods are reported. While fits are generally good for objects with moderate phase-relation slope, poor quality fits remain among the highest and lowest albedo objects studied in virtue of their respectively shallow and steep sloping-phase curves.

Harris, A. W.↗

NASA NTRS1992

Asteroid families - An initial search

A stereo examination was conducted for clusters in three-dimensional proper element space within a sample of both numbered and faint Palomar-Leiden Survey (PLS) asteroids. The clusters were then objectively filtered for small Poisson probability of chance occurrence; 104 were accepted as families with 4- to 12-member populations, and are interpreted as impact-generated. Structure is common in the well-populated families: the better-sampled families are accordingly discussed in terms of their geometry and taxonomy. Some families are very rich in faint PLS members.

Williams, James G.↗

NASA NTRS1992

Interplanetary mission design for the Comet Rendezvous Asteroid Flyby mission

As part of the CRAF/Cassini Project, the Comet Rendezvous Asteroid Flyby (CRAF) mission will explore primitive solar system bodies. CRAF is unique in that there are a number of scientifically acceptable targets from which to choose. In principle, each may be reached with several types of trajectories. This results in a large number of potential mission opportunities which must be considered. The various mission modes being considered will be discussed along with the criteria by which these missions are evaluated in order to reduce them to a smaller set. Also, the current primary and backup missions will be discussed.

Williams, S. N.↗

NASA NTRS1992

Orbital evolution of dust particles from comets and asteroids

Results are presented from a computer simulation in which dust grains of three different sizes were released at perihelion passage from each of (1) 15 main belt asteroids, (2) 15 short-period comets with perihelion greater than 1 AU, and (3) 5 such comets with perihelion less than 1 AU. The evolving-orbit calculations for each of the dust rains include the effects of solar and planetary gravity, radiation pressure, Poynting-Robertson drag, and solar wind drag. It is noted that when dust grains evolve to intersection with the earth's orbit, they retain orbital characteristics indicative of their origins.

Jackson, A. A.↗

NASA NTRS1992

Meteorite-asteroid spectral comparison - The effects of comminution, melting, and recrystallization

The present laboratory simulation of possible spectral-alteration effects on the optical surface of ordinary chondrite parent bodies duplicated regolith processes through comminution of the samples to finer rain sizes. After reflectance spectra characterization, the comminuted samples were melted, crystallized, recomminuted, and again characterized. While individual spectral characteristics could be significantly changed by these processes, no combination of the alteration procedures appeared capable of affecting all relevant parameters in a way that improved the match between chondritic meteorites and S-class asteroids.

Clark, Beth E.↗

NASA NTRS1992

Galileo orbit determination for the Gaspra asteroid encounter

This paper presents an overview of the orbit determination for Galileo's epochal encounter with the asteroid Gaspra on October 29, 1991. Topics discussed are a ground-based observation campaign to improve Gaspra's ephemeris before encounter, and the use of optical navigation together with Doppler and range data. The paper concludes by noting the steady improvement in the B-plane dispersions during the two months prior to encounter.

Kallemeyn, P. H.↗

NASA NTRS1992

Asteroids and Archaean crustal evolution: Tests of possible genetic links between major mantle/crust melting events and clustered extraterrestrial bombardments

Since the oldest intact terrestrial rocks of ca. 4.0 Ga and oldest zircon xenocrysts of ca. 4.3 Ga measured to date overlap with the lunar late heavy bombardment, the early Precambrian record requires close reexamination vis a vis the effects of megaimpacts. The identification of microtektite-bearing horizons containing spinals of chondritic chemistry and Ir anomalies in 3.5-3.4-Ga greenstone belts provides the first direct evidence for large-scale Archaean impacts. The Archaean crustal record contains evidence for several major greenstone-granite-forming episodes where deep upwelling and adiabatic fusion of the mantle was accompanied by contemporaneous crustal anatexis. Isotopic age studies suggest evidence for principal age clusters about 3.5, 3.0, and 2.7 (+/- 0.8) Ga, relics of a ca. 3.8-Ga event, and several less well defined episodes. These peak events were accompanied and followed by protracted thermal fluctuations in intracrustal high-grade metamorphic zones. Interpretations of these events in terms of internal dynamics of the Earth are difficult to reconcile with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by mantle rebound response to intermittent extraterrestrial asteroid impacts is supported by (1) identification of major Archaean impacts from microtektite and distal ejecta horizons marked by Ir anomalies; (2) geochemical and experimental evidence for mantle upwelling, possibly from levels as deep as the transition zone; and (3) catastrophic adiabatic melting required to generate peridotitic komatites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem that arises from comparisons between modern continental crust and the estimated sial produced by continuous two-stage mantle melting processes. The volume problem is exacerbated by projected high accretion rates under Archaean geotherms. It is suggested that impact shock effects have been largely obscured by (1) outpouring of voluminous basic/ultrabasic lavas, inundating shock-deformed crust and extending beyond the perimeters of impact excavated basins; (2) gravity subsidence and downfaulting of terrestrial maria, accounting for the burial and anatexis of subgreenstone basement; and (3) extensive shearing and recrystallization at elevated temperatures of impact structure, breccias, and mineral deformation features beneath impact-excavated basins, relics of which may be retained in structural windows in high-grade metamorphic terranes.

Glikson, A. Y.↗

NASA NTRS1992

Asteroids, Comets, Meteors 1991

Papers from the conference are presented and cover the following topics with respect to asteroids, comets, and/or meteors: interplanetary dust, cometary atmospheres, atmospheric composition, comet tails, astronomical photometry, chemical composition, meteoroid showers, cometary nuclei, orbital resonance, orbital mechanics, emission spectra, radio astronomy, astronomical spectroscopy, photodissociation, micrometeoroids, cosmochemistry, and interstellar chemistry.

Harris, Alan W.↗

NASA NTRS1992

Asteroids with unusual lightcurves: 14 Irene and 51 Nemausa

Some asteroids with peculiar light curves were chosen for more detail investigation. We present the first results of that study. Observations were obtained in 1989 and 1990 using the 70-cm telescope of Kharkov Observatory; the 40-cm telescope of Abastumani Observatory (Georgia), and the 60-cm telescope of the Main Astronomical Observatory of Ukrainian Academy of Sciences (Mt. Majdanak, Middle Asia). Observations and their reduction were carried out in a standard way. The aspect data, absolute magnitude of primary maximum, and lightcurve amplitude are given.

Belskaya, I. N.↗