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At least 1,099 records · Page 61
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NASA NTRS1984

1984 AB - A unique Mars-crossing asteroid

Asteroid 1984 AB, discovered in January 1984, proved to be a unique object with a close dynamical relationship to Mars. A brief history of the discovery and subsequent 'evolution' of the orbit as it was refined is presented. The preliminary orbit of 1984 AB indicated that it might be a Mars Trojan, and an extended discussion of this interesting possibility is presented, but this hypothesis had to be dismissed after further observations had refined the orbit. The semimajor axis and orbital eccentricity are very similar to that of Mars. No other known Mars-crossing asteroid exists with an orbit as closely associated to Mars.

Helin, E. F.↗
NASA NTRS1985

Speckle interferometry of asteroids. I - 433 Eros

Analytical expressions are derived for the semimajor and semiminor axes and orientation angle of the ellipse projected by a triaxial asteroid, and the results are applied speckle-interferometry observations of the 433 Eros asteroid. The expressions were calculated as functions of the dimensions and pole of the body and of the asterocentric position of the earth and the sun. On the basis of the analytical expressions, the dimensions of 433 Eros are obtained. The light curve from December 18, 1981 is compared to the dimensions to obtain a geometric albedo of 0.156 (+ or - 0.010). A series of two-dimensional power spectra and autocorrelation functions for 433 Eros show that it is spinning in space.

Drummond, J. D.↗
NASA NTRS1985

Terrestrial impactors at geological boundary events - Comets or asteroids?

In the present evaluation of evidence presented to date for a 26-28 million year periodicity in the extinction record and the age of large, well dated terrestrial impact craters, it is noted that no simple, one-to-one relationship emerges between major asteroid and/or comet impacts, siderophile anomalies, and biological extinction events. While impacts may indeed be the major extinction-triggering event in some or even most cases, either other major events, or secondary effects of the impacts, may be the actual extinction-causing mechanism. Long term obscuration of insolation, planetary cooling, or lethal atmospheric pollution may vary among extinctions, depending on the actual state of the planet and its biota during the geological period in question. The source of 28 million year-period asteroidal impactors, moreover, remains unknown and thereby casts doubt on the entire periodicity scenario.

Weissman, P. R.↗
NASA NTRS1985

Asteroidal source of ordinary chondrites (Meteoritical Society Presidential Address 1984)

The orbital evolution of asteroidal fragments injected into the 3-1 Kirkwood gap resonance at 2.5 AU is investigated on the basis of a Monte Carlo simulation. The diameters of the fragments in the simulation were between 10 cm and 20 km, and it was assumed that the fragments cross the orbital path of the earth every one million years. The effects of close encounter planetary perturbations, the nu dot 6 secular resonance, and the ablative effects of the earth atmosphere were also taken into account. It is found that: (1) the predicted meteorite orbits closely matched the known orbits of ordinary chondrites; and (2) the total flux was in approximate agreement with the observed fall rate of ordinary chondrites. About 90 percent of the predicted impacting bodies were created by fragmentation of larger earth crossing asteroidal fragments, the largest of which were observed in the vicinity of the Apollo-Amor objects. The numerical results are presented in a series of graphs.

Wetherill, G. W.↗
NASA NTRS1985

Unmelted meteoritic debris in the Late Pliocene iridium anomaly - Evidence for the ocean impact of a nonchondritic asteroid

Ir-bearing particles have been recovered from two piston cores in the Antarctic Basin in the southeastern Pacific. In core E13-3, the particles closely correspond to the Late Pliocene Ir anomaly and have a fluence of about 100 mg/cm sq. In core E13-4, 120 km to the southwest, the particle fluence is about 4 mg/cm sq. Particles with diameters from 0.5 to 4 mm contain at least 35 percent of the Ir in this horizon. Three types of particles have been identified: (1) vesicular, (2) basaltic, and (3) metal. The vesicular particles appear to be shock-melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component. These particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of Na, K, Cl, and radiogenic Sr from the ocean water target. The basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact. Combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud. Estimates of the diameter of the projectile range from 100 to 500 m. By many orders of magnitude, this is the most massive achondrite sampled by a single meteorite fall.

Kyte, F. T.↗
NASA NTRS1985

Space debris, asteroids and satellite orbits; Proceedings of the Fourth and Thirteenth Workshops, Graz, Austria, June 25-July 7, 1984

The workshops covered a variety of topics relevant to the identification, characterization and monitoring of near-earth solar system debris. Attention was given to man-made and naturally occurring microparticles, their hazards to present and future spacecraft, and ground- and space-based techniques for tracking both large and small debris. The studies are extended to solid fuel particulates in circular space. Asteroid rendezvous missions are discussed, including propulsion and instrumentation options, the possibility of encountering asteroids during Hohman transfer flights to Venus and/or Mars, and the benefits of multiple encounters by one spacecraft. Finally, equipment and analytical models for generating precise satellite orbits are reviewed.

Kessler, D. J.↗
NASA NTRS1985

Is 1220 Crocus a precessing, binary asteroid?

Photoelectric data of the asteroid 1220 Crocus over a 13 night period in 1984 revealed the presence of two separate periods. The light curves were indicative of a precessing body, but not one in free precession due to motions induced by a collision. Closer examinations revealed periods of 30.7 and 7.9 hr with amplitudes of 0.87 and 0.15 mag, respectively. An analysis of the source of an external torque which could be causing a forced precession led to the hypothesis that 1220 Crocus has a satellite. Verification of the binary asteroid configuration will depend on more detailed light curves, the possible modulation of the shorter period by the longer, and possible use of the Space Telescope.

Binzel, R. P.↗
NASA NTRS1985

Submillimeter observations of the asteroid 10 Hygiea

The first successful ground-based observations of any asteroid at submillimeter wavelengths are reported. Observations of the asteroid 10 Hygiea at 370 and 770 micrometers are combined with observations in the near and thermal infrared (2.2, 10.6, and 21 micrometers) to study the thermal properties of Hygiea's regolith. The 'standard' (nonrotating) thermal model is consistent with the entire data set, although a rotating thermophysical model with a 'lunarlike' thermal inertia cannot be ruled out.

Lebofsky, L. A.↗
NASA NTRS1985

Ellipsoidal geometry in asteroid thermal models - The standard radiometric model

The major consequences of ellipsoidal geometry in an othewise standard radiometric model for asteroids are explored. It is shown that for small deviations from spherical shape a spherical model of the same projected area gives a reasonable aproximation to the thermal flux from an ellipsoidal body. It is suggested that large departures from spherical shape require that some correction be made for geometry. Systematic differences in the radii of asteroids derived radiometrically at 10 and 20 microns may result partly from nonspherical geometry. It is also suggested that extrapolations of the rotational variation of thermal flux from a nonspherical body based solely on the change in cross-sectional area are in error.

Brown, R. H.↗
NASA NTRS1985

Radar observations of asteroids and comets

Radar techniques for the observation of asteroids and comets are reviewed, emphasizing the logical basis for inferring physical properties from radar measurements. Results to date are reviewed, focusing on some recent highlights of the research to demonstrate the synergism between radar and other ground-based techniques. Particular attention is given to the asteroids 2 Pallas, 16 Psyche, 2101 Adonis, and the comet IRAS-Araki-Alcock.

Ostro, S. J.↗
NASA NTRS1986

Planet-Crossing Asteroid Search (PCAS)

511 independent fields were photographed with the 0.46m Schmidt during 15 observing runs along with a limited number of 1.2m Schmidt exposures. From these fields 122 new asteroids were reported. Special sets of field using the 1.2m Schmidt are continuing to yield numerous new faint asteroids. The combined result from both telescopes include 4 Mars-crossers, 3 Hungarias, 2 Phocaeas, a unique high inclination object (1985XB), 1 Apollo (1985PA) and Aten (3362) Khufu. Halley was regularly monitored (astrometry and large-scale phenomena) IHW participation. Scientifically value photographs were obtained from both 0.46m and 1.2m Schmidt Telescopes. Other accessible comets were observed including G-Z. Recently a new comet (1986d) was discovered.

Helin, E. F.↗
NASA NTRS1986

Passive microwave remote sensing of the asteroids using the VLA: Continuation

The structure and composition parameters for a selected set of the main belt asteroids were inferred by employing microwave remote sensing techniques developed for Earth observations. Precise flux density measurements made with the Very Large Array (VLA) and the 11 m mm-wave antenna of the National Radio Astronomy Observatory are used to define the microwave continuum spectra of these asteroids. These spectra were inverted in order to estimate the near-surface bulk properties (radii, roughness, composition) independent of previous optical or infrared spectroscopy.

Webster, W. J., Jr.↗
NASA NTRS1987

Computer simulations of 10-km-diameter asteroid impacts into oceanic and continental sites: Preliminary results on atmospheric passage, cratering and ejecta dynamics

A series of analytical calculations of large scale cratering events for both oceanic and continental sites were made in order to examine their effects on the target media and atmosphere. The first analytical studies that were completed consists of computer simulations of the dynamics of: (1) the passage of a 10 km diameter asteroid moving at 20 km/sec through the Earth's atmosphere, and (2) the impact cratering events in both oceanic and continental environments. Calculation of the dynamics associated with the passage of the asteroid through the atmosphere showed strong effects on the surrounding air mass. The calculations of the impact cratering events showed equally dramatic effects on the oceanic and continental environments. These effects are briefly discussed.

Roddy, D. J.↗
NASA NTRS1986

A comparison of the continuum colors of P/Halley, other comets, and asteroids

Early observations (1985 Feb.) of P/Halley were made with broadband BVJK filters when a substantial fraction of the light came from the nucleus; the resulting colors appear red, but with signal-to-noise ratios less than or = 5. More recent observations were made with the narrower International Halley Watch continuum filters when the light from the comet is dominated by the coma. The colors over the wavelength range 0.3 to 0.7 microns are more neutral. Continuum colors were also observed for other comets. All show moderately red to very red colors that can be best matched by asteroids of the P and D spectral classes, suggesting that the refractory component of comets is similar to that of the more distant asteroids.

Tholen, D. J.↗
NASA NTRS1986

Scaling laws for the catastrophic collisions of asteroids

Collisions of asteroids have traditionally been studied through laboratory experiments involving targets with masses some 15 to 20 orders of magnitude less than the bodies they are intended to simulate. Here the problem of extrapolation of experimental results up to the size regimes of interest is considered. Scaling relations are developed for the shattering threshold and the size and velocity distributions of collisional fragments. A methodology which has often been used assumes that collisional outcomes (e.g., the size of the largest remaining fragment) are completely characterized by Q, the kinetic energy of the impactor normalized by the mass of the target body. This scaling is shown to be an unlikely special case of a more general scaling theory which indicates that collisional outcomes should depend on target size and encounter velocity, even when Q is held constant. In particular, as target size increases, the critical value of Q required to shatter a body, and the characteristic fragment velocities should initially decrease (in qualitative agreement with the recent model of Farinella et al., 1982) up to an asteroid size of perhaps 40 to 50 km; then, as the gravitational forces start to dominate, the value of Q will again increase (in qualitative agreement with the recent model of Davis et al., 1983 and 1985).

Holsapple, K. A.↗
NASA NTRS1986

Catastrophic disruption in the solar system - Asteroid collisional history, origin of Hirayama families and disruption of small satellites

The process of collisional catastrophic disruption has played a significantly role in structuring the solar system. Diverse populations of bodies such as the asteroid belt, small satellites of Jupiter and Saturn and perhaps even the rings of Saturn have been created or substantially changed by catastrophic distruption. Understanding the outcome of large scale impacts is essential to learning about the early history of the solar system in the asteroid zone and the reason why a planet failed to form there.

Davis, D. R.↗
NASA NTRS1986

Asteroid lightcurve studies

The use of light curves to characterize the dynamics of asteroids is discussed in a review of recent statistical and theoretical investigations. The summary, detailed, and bibliographic lists comprising the IAU light-curve data set are briefly characterized, and studies of rotation rates, shapes and pole positions, and phase relations are surveyed. Rotation-rate differences among the taxonomic classes and with asteroid size are noted, along with a wide dispersion of rotation rates in objects smaller than about 100 km.

Harris, A. W.↗
NASA NTRS1986

Cometary nuclei and asteroids - Has a link been found?

Data from recent spectrophotometric observations of cometary dust are summarized and compared with similar data on asteroids. A number of systematic similarities are noted between comet materials and dark reddish-black asteroids such as the Trojans. It is suggested that Trojans and comet nuclei are members of a class of outer-solar-system planetesimals with colors due to opaque carbonaceous dust containing reddish low-temperature organic condensates.

Hartmann, W. K.↗