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At least 217 records · Page 12
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NASA NTRS1985

Study and Estimation of Planet-crossing Asteroid Populations: Discoveries and Analysis

The populations of the various classes of planet-crossing asteroids, based on the results of the systematic Planet-Crossing Asteroid Survey (PCAS) search program are evaluated. Knowledge of the populations is crucial to the stimation of lunar and terrestrial planet cratering rates from asteroidal and cometary sources needed to establish the chronology of other surface features on those bodies. A secondary objective is to identify the sources of these crater-forming objects and their relative importance in producing the observed planet-crossing asteroid population. Some key PCAS asteroid discoveries relevant to the population problem and the progress in using the available search data to update and refine earlier estimates of the planet-crossing asteroid population are described.

Helin, E. F.↗
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 NTRS1985

Variation of the UBV colors of S-class asteroids with semimajor axis and diameter

A large fraction of the several thousand asteroids which exist mainly between the orbits of Mars and Jupiter have now been separated into a small number of more or less distinct classes. The most populous classes are S (moderate albedo and red UBV color) and C (low albedo and neutral UBV color), while other less populous classes include M, E, R, D, A, F, and P. Asteroids in the S and C classes are considered to be the most likely sources of the abundant stony-iron and carbonaceous chondritic meteorites. An identification of the source asteroid of a particular meteorite, could lead to the use of the meteorite as a sample of that asteroid. Such a sample could then be employed to determine the bulk composition and the evolution of the asteroid. There are, however, factors which may make the interpretation of the observed albedos, colors, and spectra difficult. In the present investigation, it is shown that the mean UBV color of S-class asteroids varies markedly with distance from the sun and may vary with diameter. Interpretations of these findings are discussed.

Dermott, S. F.↗
NASA NTRS1985

Near-earth asteroids - Possible sources from reflectance spectroscopy

The diversity of reflectance spectra noted among near-earth asteroids that were compared with selected asteroids, planets and satellites to determine possible source regions is indicative of different mineralogical composition and, accordingly, of more than one source region. Spectral signatures that are similar to those of main belt asteroids support models deriving some of these asteroids from the 5:2 Kirkwood gap and the Flora family, by way of gravitational perturbations. The differences in composition found between near-earth asteroids and planetary and satellite surfaces are in keeping with theoretical arguments that such bodies should not be sources. While some near-earth asteroids furnish portions of the earth's meteorite flux, other sources must also contribute.

Mcfadden, L. A.↗
NASA NTRS1986

Implications of the inferred compositions of asteroids for their collisional evolution

The inferred compositions of mainbelt asteroids set constraints on the nature and degree of collisional evolution of the asteroids. The existence of a basaltic crust on 4 Vesta seems to require that there has been very little collisional evolution since the presumably ancient solidification of its crust, which is consistent with most recent models of collisional evolution. But interpretations of the compositions of M- and S-type asteroids as collisionally stripped cores of differentiated parent bodies argue for much more extensive collisions. A major problem is presented by the extreme rarity of olivine-rich asteroids and olivine meteorites. Studies of the compositions of members of dynamical families suggest that most supposed families are either not real, or at least are not genetically related fragments of a cosmochemically reasonable precursor body. The mutual incompatibilities among various 'accepted' traits of the asteroids deserve further study in order to develop a self-consistent scenario for the chemical and physical evolution of the asteroids.

Chapman, C. R.↗
NASA NTRS1988

Radar investigation of asteroids and planetary satellites

The aim is to make radar reconnaissance of near-Earth asteroids, mainbelt ateroids, the Galilean satellites, the Martian satellites, and the largest Saturnian satellites, using the Arecibo 13-cm and the Goldstone 3.5-cm systems. Measurements of echo strength, polarization, and delay/Doppler distribution of echo power provide information about dimensions, spin vector, large-scale topography, cm-to-m-scale morphology, and surface bulk density. The observations also yield refined estimates of target orbital elements. Radar signatures were measured for 31 mainbelt asteroids and 16 near-Earth asteroids since this task began eight years ago. The dispersion in asteroid radar albedoes and circular polarization ratios is extreme, revealing huge differences in surface morphologies, bulk densities, and metal concentration. For the most part, correction between radar signature and VIS/IR class is not high. Many near-Earth asteroids have extremely irregular, nonconvex shapes, but some have polar silhouettes that appear only slightly noncircular. The signatures of 1627 Ivar, 1986 DA, and the approximately 180-km mainbelt asteroid 216 Kleopatra suggest bifurcated shapes. Observational milestones during 1987 and 1988 are noted.

Ostro, Steven J.↗
NASA NTRS1988

Computer modeling of large asteroid impacts into continental and oceanic sites: Atmospheric, cratering, and ejecta dynamics

Numerous impact cratering events have occurred on the Earth during the last several billion years that have seriously affected our planet and its atmosphere. The largest cratering events, which were caused by asteroids and comets with kinetic energies equivalent to tens of millions of megatons of TNT, have distributed substantial quantities of terrestrial and extraterrestrial material over much or all of the Earth. In order to study a large-scale impact event in detail, computer simulations were completed that model the passage of a 10 km-diameter asteroid through the Earth's atmosphere and the subsequent cratering and ejecta dynamics associated with impact of the asteroid into two different targets, i.e., an oceanic site and a continental site. The calcuations were designed to broadly represent giant impact events that have occurred on the Earth since its formation and specifically represent an impact cratering event proposed to have occurred at the end of Cretaceous time. Calculation of the passage of the asteroid through a U.S. Standard Atmosphere showed development of a strong bow shock that expanded radially outward. Behind the shock front was a region of highly shock compressed and intensely heated air. Behind the asteroid, rapid expansion of this shocked air created a large region of very low density that also expanded away from the impact area. Calculations of the cratering events in both the continental and oceanic targets were carried to 120 s. Despite geologic differences, impacts in both targets developed comparable dynamic flow fields, and by approx. 29 s similar-sized transient craters approx. 39 km deep and approx. 62 km across had formed. For all practical purposes, the atmosphere was nearly completely removed from the impact area for tens of seconds, i.e., air pressures were less than fractions of a bar out to ranges of over 50 km. Consequently, much of the asteroid and target materials were ejected upward into a near vacuum. Effects of secondary volcanism and return of the ocean over hot oceanic crater floor could also be expected to add substantial solid and vaporized material to the atmosphere, but these conditions were not studied.

Roddy, D. J.↗
NASA NTRS1988

Asteroid and comet flux in the neighborhood of the earth

Significant advances in the knowledge and understanding of the flux of large solid objects in the neighborhood of Earth have occurred. The best estimates of the collision rates with Earth of asteroids and comets and the corresponding production of impact craters are presented. Approximately 80 Earth-crossing asteroids were discovered through May 1988. Among 42 new Earth-crossing asteroids found in the last decade, two-thirds were discovered from observations at Palomar Observatory and 15 were discovered or independently detected in dedicated surveys with the Palomar Observatory and 15 were discovered or independently detected in dedicated surveys with the Palomar 46 cm Schmidt. Probabilities of collision with Earth have been calculated for about two-thirds of the known Earth-crossing asteroids. When multiplied by the estimated population of Earth-crossers, this yields an estimated present rate of collision about 65 pct higher than that previously reported. Spectrophotometric data obtained chiefly in the last decade show that the large majority of obvserved Earth-crossers are similar to asteroids found in the inner part of the main belt. The number of discovered Earth-crossing comets is more than 4 times greater than the number of known Earth-crossing asteroids, but reliable data on the sizes of comet nuclei are sparse. The flux of comets almost certainly was highly variable over late geologic time, owing to the random perturbation of the Oort comet cloud by stars in the solar neighborhood.

Shoemaker, Eugene M.↗
NASA NTRS1988

Asteroids and meteorites

The hypothesis that most meteorites are asteroid fragments generated by comparatively recent collisions in the asteroid belt runs up against spectrophotometric data indicating that asteroids with the composition of the most abundant meteorite class are either rare or absent in the asteroid belt. An understanding of the origin and evolution of the asteroid belt is essential to an adequate interpretation of meteoritic data; conversely, the detailed early asteroidal history preserved in the meteorites provides evidence otherwise unavailable for the earliest history of any other planetary bodies.

Wetherill, George W.↗
NASA NTRS1989

An overview of the asteroids

An introductory overview of the field of asteroid science is presented, with emphasis placed on the accomplishments of the 1980s. Following the survey of known asteroids, attention is given to the observations of asteroids with the IRAS. Particular consideration is given to the origin and evolution of asteroids and their interrelations. Possible future directions of asteroid research are discussed together with the potential of the Hubble Space Telescope for providing new data on asteroid surface chemistry, geology, structure, and morphology.

Binzel, Richard P.↗
NASA NTRS1989

Asteroid results from the IRAS survey

The IRAS Asteroid and Comet Survey yielded a data base of infrared flux densities for 1811 individual asteroids. Albedos and diameters for these have been derived via a standard thermal model. IRAS sampled a large number of small asteroids and detected many dark asteroids in the outer belt. High-albedo asteroids remain rare. Observations of the brighter asteroids at multiple wavelengths show the expected range of color temperatures through the main belt.

Veeder, Glenn J.↗
NASA NTRS1989

Physical characterization of asteroid surfaces from photometric analysis

The feasibility of using photometric models like Hapke's (1981, 1984, 1986) equation for deriving physical properties of asteroids from photometric observations is discussed. Using data for Ceres and Vesta, it is shown that the incomplete phase-angle coverage limits the reliable determination of Hapke's photometric parameters from asteroid disk-integrated phase curves (the second limitation is the nonsphericity of many asteroids). However, within this limitations, certain trends among asteroids can be compared. It is shown that there is a general similarity among the Hapke's parameters other than the h parameter, which characterizes the width of the opposition surge in terms of soil structure (porosity, particle-size distribution, and the rate of compaction with depth), derived for similar objects, e.g., average C asteroids and Ceres, and average S asteroids, 1982 Apollo, and Vesta.

Helfenstein, P.↗
NASA NTRS1989

Meteoritic parent bodies - Nature, number, size and relation to present-day asteroids

The relationship between meteoritic parent bodies and the present-day asteroids is discussed. Results on oxygen isotopic signatures and chemical distinctions among meteorite classes indicate that meteorites derive from a small number of parent bodies relative to the number of asteroids. The spectral properties of the ordinary chondrites and similar inclusions in meteoritic breccias differ from those of the abundant S asteroids (with no process known that can account for these differences); the closest spectral analogs of these chondrites are the rare near-earth Q-type asteroids. These facts lead to the question of why abundant meteorites have rare asteroidal analogs, while the abundant asteroids have rare meteoritic analogs. This question constitutes a prime topic for future studies.

Lipschutz, Michael E.↗
NASA NTRS1989

Asteroids - The big picture

An attempt is made to construct a simple model of the evolution of the asteroid belt from the data available about the individual asteroids. Data on the meaning of taxonomic types, the stratigraphy of the asteroid belt, the compositional meaning of Tholen space, collisional and dynamical history, and asteroid shapes are reviewed. Two main paradoxes concerning asteroids, called the ordinary chondrite mystery and the olivine problems, are briefly described. Finally, the tentative model is presented, showing how it takes into account condensation locations, heating, collisional evolution, and delivery of asteroids to earth.

Bell, Jeffrey F.↗
NASA NTRS1989

Asteroid observations with the Hubble Space Telescope and the Space Infrared Telescope Facility

The ways that the asteroids can be studied with the Hubble Space Telescope (HST) and the Space Infrared Telescope Facility (SIRTF) are examined. Spectrophotometry of asteroids and the study of asteroid surfaces, shape, spins, configuration, normal reflectance, and limb darkening of asteroids using the HST are addressed along with the detection of asteroid satellites and the discovery of small asteroids using the HST. The relation of the HST to its ground system is described, as are the spectrophotometric instruments of the HST. Imaging with the HST using the Faint Object Camera and the Wide Field and Planetary Camera is examined. Finally, the SIRTF observatory, instrumentation, and capabilities for solar system science are discussed.

Zellner, B.↗
NASA NTRS1989

Introduction to the Asteroids II data base

This paper describes the Asteroids II data base, which is a compilation of asteroid data published, or in press, as of March 1988 with some updates in early 1989. The Asteroids II machine-readable data base includes asteroid names and discovery circumstances; proper elements and family identifications; asteroid light-curve parameters; asteroid pole determinations; taxonomic classes; and absolute magnitudes and slope parameters, UBV colors, albedos, and diameters.

Tedesco, Edward F.↗
NASA NTRS1991

Near-infrared observations of primitive asteroids and a possible extinct comet

Photometry was obtained simultaneously in the near-infrared and in the visible, of several D-type asteroids, thought to be composed of the most primordial material in the asteroid belt. These objects have been previously classified as D-type based on their low albedo and broadband colors in the visible region of the spectrum. One of the objects observed is 944 Hidalgo, which is possibly an extinct comet, based on its orbital elements. The photometry further constrains the surface mineralogy and is a test of whether differences exist between these objects in different regions of the asteroid belt. Refining the knowledge of the surface composition of these objects will allow a better measure of the amount of thermal processing which has taken place in these so-called primitive asteroids. Spectral slope is assumed to be sensitive to surface composition. There is some variability in spectral slope among those objects classified as D-type, with indications of increasing slope into the near-infrared with heliocentric distance. The spectral features are quite subtle as expected for these low albedo objects, but there is possible evidence of subtle features in some of the D-type asteroids and possibly some active comets. Further analysis of these spectra and the implications for the outer region of the asteroid belt is presented.

Howell, E. S.↗
NASA NTRS1991

An ISU study of asteroid mining

During the 1990 summer session of the International Space University, 59 graduate students from 16 countries carried out a design project on using the resources of near-earth asteroids. The results of the project, whose full report is now available from ISU, are summarized. The student team included people in these fields: architecture, business and management, engineering, life sciences, physical sciences, policy and law, resources and manufacturing, and satellite applications. They designed a project for transporting equipment and personnel to a near-earth asteroid, setting up a mining base there, and hauling products back for use in cislunar space. In addition, they outlined the needed precursor steps, beginning with expansion of present ground-based programs for finding and characterizing near-earth asteroids and continuing with automated flight missions to candidate bodies. (To limit the summer project's scope the actual design of these flight-mission precursors was excluded.) The main conclusions were that asteroid mining may provide an important complement to the future use of lunar resources, with the potential to provide large amounts of water and carbonaceous materials for use off earth. However, the recovery of such materials from presently known asteroids did not show an economic gain under the study assumptions; therefore, asteroid mining cannot yet be considered a prospective business.

Burke, J. D.↗