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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Five-color photoelectric photometry of asteroid 433 Eros

Five-color (UVBGR) photoelectric light curves of Eros are presented which were obtained on nine nights during the asteroid's 1974/75 close apparition. Indications that three reversals occurred in the relative depths of the two minima between late December 1974 and late January 1975 are noted along with definite evidence of phase reddening. The maximum amplitude observed was 1.44 magnitudes, and the absolute visual magnitude at primary maximum (corrected to zero phase and to 1 AU from the earth and the sun) was about 10.8 magnitudes. Since the observations indicate that Eros has no significant rotational color variations, it is suggested that the light-curve amplitudes are due primarily to the asteroid's shape and to shadowing effects. Based on its colors relative to the sun, it is concluded that Eros appears to be an S-type asteroid.

Miner, E.↗
NASA NTRS1976

UBV photometry of asteroid 433 Eros

UBV observations of asteroid 433 Eros were conducted on 17 nights during the winter of 1974/75. The peak-to-peak amplitude of the lightcurve varied from about 0.3 mag to nearly 1.4 mag. The absolute V mag at maximum light, extrapolated to zero phase, is 10.85. Phase coefficients of 0.0233 mag/degree, 0.0009 mag/degree, and 0.0004 mag/degree were derived for V, B-V and U-B, respectively. The zero-phase color of Eros (B-V = 0.88, U-B = 0.50) is representative of an S (silicaceous) compositional type asteroid. The color does not vary with rotation. The photometric behavior of Eros can be modeled by a cylinder with rounded ends having an axial ratio of about 2.3:1. The asteroid is rotating about a short axis with the north pole at 15 deg ecliptic longitude and 9 deg ecliptic latitude.

Millis, R. L.↗
NASA NTRS1976

J, H, K photometry of 433 Eros and other asteroids

Results are reported for IR photometry of nine bright asteroids, including 433 Eros, which was conducted in the J, H, K, and (for Eros) L bands with an InSb photometer on the 1.3-m Kitt Peak telescope. It is found that the J-H and H-K color indices of all nine asteroids are slightly redder than those of the sun and that three C-type objects are somewhat redder in the same color indices than four S-types and an E-type. Eros is shown to have an unusually red J-K index; the shape of its IR spectral reflectance curve is attributed in part to the presence of the weak 2-micron pyroxene absorption band in the spectrum. The results show that JHK photometry is less diagnostic of asteroidal composition than higher-resolution IR spectrophotometry.

Chapman, C. R.↗
NASA NTRS1977

Radii and albedos of four Trojan asteroids and Jovian satellites 6 and 7

Results are reported for radiometric measurements of broadband 20-micron fluxes from the Trojan asteroids 617 Patroclus, 624 Hektor (for which the broadband 10-micron flux was also measured), 1172 Aeneas, and 1173 Anchises as well as from the outer Jovian satellites Himalia (J6) and Elara (J7). Geometric albedos and radii for the six objects are derived from the corrected monochromatic fluxes and visual magnitudes. It is found that all the objects have exceedingly low geometric albedos, indicating that the Trojans and possibly the outer Jovian satellites constitute a distinct class of small solar-system bodies. The composition of the Trojan asteroids is considered on the basis of available sizes, albedos, and shapes. Revised tables of the albedos and radii of all the Jovian satellites are presented. It is concluded that the Trojans are not composed primarily of ice and that an asteroidal origin for the comets of the Jupiter group is unlikely.

Cruikshank, D. P.↗
NASA NTRS1976

Asteroids and comparative planetology

The state of knowledge concerning the asteroids is reviewed and discussed in terms of the relationship of the asteroids with other solar system bodies. The data on spectral reflectances of asteroids are examined, and their implications for comparative planetology are discussed.

Matson, D. L.↗
NASA NTRS1977

Radar detectability of asteroids - A survey of opportunities for 1977 through 1987

The study of asteroids by earth-based radar during the period 1977 to 1987 is discussed. Detection modeling, including backscattering laws, limits of detectability, and in particular, measurement of center frequency, center-to-limb bandwidth, and areal spectral density, is reviewed. The number of detectable objects and detectable events falling into various 5-decibel radar target loss slots is estimated; a total of 60 different asteroids should be observable on about 130 occasions by either the Goldstone or Arecibo radar systems during the decade considered. Furthermore, the detectability margins for many objects should be large enough to permit more refined analysis of the radar spectrograms. Data acquired during the ten-year period may be used to find the radius, average surface roughness, rotation rate, direction of polar axis, and estimates of the Doppler frequency offsets for many of the asteroids. Also, information about radar albedos and orbital parameters may be obtainable in a number of cases.

Jurgens, R. F.↗
NASA NTRS1978

Most stony meteorites come from the asteroid belt

The presence of trapped solar gas in stony meteorites places their origin in the regoliths of asteroidal-type bodies. The most plausible sources are the C (carbonaceous) and S (siliceous) asteroids, in spite of the differences between the spectra of S asteroids and ordinary chondrites. This problem is a central one for the interpretation of both astronomical observations and dynamical theory.

Anders, E.↗
NASA NTRS1978

Geography of the asteroid belt

The CSM classification serves as the starting point on the geography of the asteroid belt. Raw data on asteroid types are corrected for observational biases (against dark objects, for instance) to derive the distribution of types throughout the belt. Recent work on family members indicates that dynamical families have a true physical relationship, presumably indicating common origin in the breakup of a parent asteroid.

Zellner, B. H.↗
NASA NTRS1978

Asteroid collisions, craters, regoliths, and lifetimes

Laboratory experiments and computer modeling are used to predict the development of regoliths on all asteroids more than a few tens of kilometers in diameter, allowing for a wide range in the intrinsic strength of asteroidal surface materials. The high frequency of interasteroid collisions requires nearly all asteroids to be fragments of precursors.

Chapman, C. R.↗
NASA NTRS1978

Asteroid mission alternatives

The options for asteroid missions are evaluated within the constraints of existing or planned launch vehicles and low-thrust propulsion systems. A wide variety of missions is possible, including flyby, rendezvous, and even sample return. The multi-asteroid rendezvous concept requires an ion drive low-thrust propulsion system of the type being developed for a comet rendezvous. It is indicated that there are plentiful opportunities for such missions to visit four asteroids with stay times of 60-90 days each and with transit times between rendezvous of the order of a year.

Niehoff, J. C.↗
NASA NTRS1978

Experiments on asteroids using hard landers

Hard lander missions to asteroids are examined using the Westphal penetrator study as a basis. Imagery and chemical information are considered to be the most significant science to be obtained. The latter, particularly a detailed chemical analysis performed on an uncontaminated sample, may answer questions about the relationships of asteroids to meteorites and the place of asteroids in theories of the formation of the solar system.

Turkevich, A.↗
NASA NTRS1977

Asteroid compositional types and their distributions

A table is presented listing the compositional types chosen for 336 main-belt asteroids, twelve Mars-crossers, two Hildas, and nine Trojans. There are about 100 asteroids with polarimetric data, 126 with reflection spectra, 165 with radiometric diameters, and 300 with UBV colors. Absolute visual magnitudes V(1,0 were computed from the blue magnitudes listed by Gehrels and Gehrels (1977) and B-V colors either known or estimated according to the type. The sample of main-belt asteroids classified in the table is complete to mean opposition apparent magnitude B(a,0) = 12. The frequencies of the compositional types are discussed. Attention is also given to the distributions over diameter and the distributions over heliocentric distances.

Zellner, B.↗
NASA NTRS1978

Infrared /JHK/ photometry of meteorites and asteroids

The paper reports JHK colors observed for ten asteroids and synthesized JHK colors for seven meteorite groups, samples of iron and nickel metal, pyroxene, olivine, feldspar, a lunar anorthite and some terrestrial mineral samples. Pronounced differences are apparent between the chondritic and achondritic meteorite classes; the chondritic classes show less subdued trends in J-H color which reflect their metamorphic grade. We find small but significant differences between the JHK colors of the predominant C and S classes of asteroids. All JHK colors of asteroids observed here fall within the limited domain defined by the various chondritic and iron-rich meteorites but are strikingly different from those of most achondritic meteorites.

Leake, M.↗
NASA NTRS1977

Lightcurve analyses for 170 small asteroids

Small asteroids, with sizes of about 1 km, appear to rotate faster than do the larger, about 200 km, asteroids. There is a lack of highly elongated objects, probably due to a collisional erosion process in the main asteroid belt.

Degewij, J.↗
NASA NTRS1977

The asteroid albedo scale. I - Laboratory polarimetry of meteorites

Laboratory reflection polarimetry is presented for crushed rocks, lunar soils, and 24 meteoritic samples. The polarization-phase curves found for the meteorites closely resemble telescopic results for minor planets, but only if the samples are crushed finely enough to coat the larger particles with dust. The results generally confirm the identifications previously made from reflection spectroscopy for the meteoritic analogues of various types of asteroids; the more metal-rich ordinary chondrites, for example, match telescopic observations of the S-type minor planets well enough, as do the basaltic achondrites for Vesta. No meteorite measured, however, has a geometric albedo less than 0.05 as required for many of the C asteroids. A calibration of the polarimetric slope-albedo law, now determined strictly for meteoritic samples, confirms the asteroid albedo and diameter scale derived by the independent method of thermal radiometry.

Zellner, B.↗
NASA NTRS1978

On the early history of the asteroids

Among the major features of the asteroids as a group is the fact that they are small and numerous rather than being a single planet, and that they have unusually high eccentricities and inclinations. Regarding the first, this paper presents two lines of argument concerning the concept that the asteroids formed with a mass-distribution similar to what we observe today. Considerations of planet accretion are used to clarify the role of Jupiter in preventing coalescence of asteroids into a single planet. Regarding the eccentricities and inclinations, it is proposed that they were excited by secular resonances associated with the presence of Jupiter within a dissipating solar nebula.

Heppenheimer, T. A.↗
NASA NTRS1978

Radio observations of asteroids: Results and prospects

Radio observations of the asteroids can provide information on the thermal and dielectric properties of the surface materials and because the radio emission arises somewhat below the surface, the data give some indication of layering. Observational difficulty has limited the investigations to only 6 asteroids: 1 Ceres and 324 Bamberga appear to have a layer of dust covering a more compacted material; the data on 4 Vesta cannot be matched by any current models for the surface; and the results for 18 Melpomene, 31 Euphrosyne and 433 Eros are too incomplete for firm conclusions. Future possibilities include more accurate radiometry of a few selected asteroids of different taxonomic classes and actual resolution of some of the larger objects by aperture synthesis techniques.

Dickel, J. R.↗