Engineering topics

Lebofsky, L. A.

Publications and source records attributed to Lebofsky, L. A..

At least 37 records · Page 2

NASA NTRS1986

IRAS survey of asteroids

The Infrared Astronomical Satellite (IRAS) scanned the celestial sphere during 1983 through bandpasses centered at wavelengths of 12, 25, 60 and 100 microns. In addition to sources fixed on the sky, many moving objects were observed. This data set is the largest and least biased survey of asteroids and comets ever conducted. Included are main belt asteroids, Mars-crossers, earth-approachers, and Trojans. Measurements for the known asteroids have been reduced and organized into a number of data products which are available to researchers. The data for 'new' asteroids are being processed.

Matson, D. L.↗

NASA NTRS1985

A search for nitrogen on Triton

Improved spectrophotometry of Triton at 2.16 microns was obtained with the MMT infrared photometer at a resolution of 1.0 percent. The observations do not reveal an absorption feature due to liquid nitrogen. Three explanations are consistent with this finding: (1) there is liquid nitrogen on the surface but it is not distributed uniformly; (2) the feature varies with time, e.g., it is sometimes obscured by cloud cover; and (3) the feature might be relatively broad.

Rieke, G. H.↗

NASA NTRS1985

The nature of C-class asteroids from 3-micron spectrophotometry

Narrowband spectrophotometry between 2.3 and 3.5 micrometers is presented for 14 main-belt C asteroids greater than 100 km in diameter. Absorption features at 3 micrometers due to water of hydration are present in the spectra of nine of the asteroids, with intensities ranging from 6 to 23 percent. The other five asteroids have no such absorption greater than 2 percent in intensity. The present C-asteroid population may be fragments of larger parent bodies with anhydrous C3-like cores and hydrated C1I- or C2M-like mantles.

Feierberg, M. A.↗

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

2.7- to 4.1-micron spectrophotometry of icy satellites of Saturn and Jupiter

Spectrophotometry is presented in the 2.7-4.1 micrometer spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus and Hyperion) and Jupiter (Europa, Ganymede and Callisto). The 3.6-micrometer reflectance peak characteristic of fine-grained water ice is observed prominently on the satellites of Saturn, faintly on the leading side of Europa, and not at all on Ganymede, Callisto or the dark side of Iapetus. The spectral reflectances of these icy satellites may be affected by their equilibrium surface temperatures and magnetospheric effects.

Lebofsky, L. A.↗

NASA NTRS1985

Results from observations of the 15 June 1983 occultation by the Neptune system

Observations of eight Neptune occultations from six sites in the southwestern Pacific on June 15, 1983. The data were used to search for evidence of rings around Neptune down to a distance of 0.03 Neptune radii from the planetary surface, but the results were negative. An astrometric analysis of the timings yielded solution for the equatorial radius a(0) of Neptune at 1 microbar pressure and the oblatness e at this level. The results are: a(0) = 25,295 + or - 50 km; e = 0.022 + or - 0.004, from which a value for a(1) (equatorial radius at 1 bar pressure) of 24,830 + or - 100 km; and a rotation period of P = 15 h (+3, -2 h) is derived. These results are based on a recent determination of Neptune's pole position and mass quadruple moment, and are consistent with the hypothesis that Neptune and Uranus have homologous mass distributions, although the constraint on interior structure is a weak one. If the probable uncertainty of the pole position is taken into account, error bars on e and derived quantities should be increased by about 50 percent. The present determinations of Neptune atmospheric temperatures at 1 microbar are consistent with earlier results and when combined with all previous data give an average value of 156 + or - 10 K. There is only slight evidence for any latitude dependence in the temperatures. Profiles with a high signal-to-noise ratio suggest the possible presence of an absorbing layer at altitudes higher than the 1-microbar level.

Hubbard, W. B.↗

NASA NTRS1984

2060 Chiron - Visual and thermal infrared observations

Five-color (wavelength = 0.36-0.85 microns) and thermal infrared (wavelength = 22.5 microns) photometric observations of the unusual asteroid 2060 Chiron were made. Between 0.36 and 0.85 microns, Chiron's reflectance spectrum is similar to those of C-class asteroids as well as Saturn's satellite Phoebe. However, the thermal IR measurements imply an albedo greater than 0.05 (i.e., a diameter of less than 250 km at the 2-sigma level) that is probably higher than those of C-class asteroids or Phoebe.

Lebofsky, L. A.↗

NASA NTRS1984

Satellites of Saturn - Optical properties

Spacecraft and ground-based optical, radio and spectrophotometric data on the Saturn satellites are reviewed for clues as to the composition and microstructures of the objects. Minas, Enceladus, Tethys, Dione and Rhea have similar photometric phase integrals, with some diversion exhibited by Rhea. Hyperion is nearly the same color as Iapetus, which displays an overall hemispherical optical asymmetry. Water ice has been observed on many of the satellites and on small, cratered objects within the rings. The frost layers are more extensive than those detected on Jovian satellites. Titan has a methane atmosphere. Finally, Phoebe is suspected of being a captured, asteroidal body.

Cruikshank, D. P.↗

NASA NTRS1983

Detection of silicate emission features in the 8- to 13-micron spectra of main belt asteroids

A presentation is given of 8.0-13.0 micron spectra (Delta lambda/lambda = 0.02-0.03) for six main belt asteroids, which range from 58 to 220 km in diameter and sample the five principal taxonomic classes (C, S, M, R and E). Narrow, well-defined silicate emission features are present on two of the asteroids, the C-type 19 Fortuna and the M-type 21 Lutetia. No comparable emission features are observed on the S-types 11 Parthenope and 14 Irene, the R-type 349 Dembowska or the E-type 64 Angelina.

Feierberg, M. A.↗

NASA NTRS1983

The composition of asteroid 2 Pallas and its relation to primitive meteorites

High resolution spectroscopic observations of asteroid 2 Pallas from 1.7-3.5 microns are reported. These data are combined with previous measurements from 0.4-1.7 microns to interpret Pallas' surface mineralogy. Evidence is found for low-Fe(2+) hydrated silicates, opaque components, and low Fe(2+) anhydrous silicates. This assemblage is very similar to carbonaceous chondrite matrix material such as is found in type CI and CM meteorites, but it has been subjected to substantial aqueous alteration and there is a major extraneous anhydrous silicate component. This composition is compared to that of asteroid 1 Ceres. Although there are substantial differences in their broad band spectral reflectances it appears that both asteroids are genetically related to know carbonaceous chondrites.

Larson, H. P.↗

NASA NTRS1982

A search for frosts in Comet Bowell /1980b/

Infrared observations of Comet Bowell represent the first search for frost signatures in a comet beyond 2 AU from the sun. Broad- and narrowband photometry has been obtained as well as CVF spectrophotometry of this comet and there is no evidence for absorption features in the spectral area between 1.25 and 2.3 microns. Models of the coma have been generated which constrain the volatile content of the grains an; are in agreement with the observed albedo. The darkness of the coma particles at large heliocentric distances indicates a low albedo nucleus as well. Brightness variations during the observing period seem to indicate an active nucleus at 4.5 AU from the sun.

Campins, H.↗

NASA NTRS1982

Infrared observations of the dark side of Iapetus

J, H, K, L and L-prime broadband and 2.8-3.8 micron narrowband photometric observations have been made of the dark side of Iapetus with circularly variable filter techniques. A 3-micron absorption feature is found which is too deep to be a residual water frost feature from the bright material around the poles. In light of laboratory simulations showing that the dark material must have a strong 3-cm absorption, it is suggested that the material may be a hydrated silicate mixed with a small amount of water frost.

Lebofsky, L. A.↗

NASA NTRS1982

Occultation by a possible third satellite of Neptune

The 24 May 1981 close approach of Neptune to an uncataloged star was photoelectrically monitored from two observatories separated by 6 kilometers parallel to the occultation track. An 8.1-second drop in signal, recorded simultaneously at both sites, is interpreted as resulting from the passage of a third satellite of Neptune in front of the star. From the duration of the event, the derived minimum diameter for an object sharing Neptune's motion is 180 kilometers. If the object was in Neptune's equatorial plane and there are no significant errors in the prediction ephemeris, the object was located at a distance of 3 Neptune radii from Neptune's center.

Reitsema, H. J.↗

NASA NTRS1981

The 1.7- to 4.2-micron spectrum of asteroid 1 Ceres - Evidence for structural water in clay minerals

A high-resolution Fourier spectrum (1.7-3.5 microns) and medium-resolution spectrophotometry (2.7-4.2 microns) were obtained for Asteroid 1 Ceres. The presence of the 3-micron absorption feature due to water of hydration was confirmed. The 3-micron feature is compared with the 3-micron bands due to water of hydration in clays and salts. It is concluded that the spectrum of Ceres shows a strong absorption at 2.7-2.8 microns due to structural OH groups in clay minerals. The dominant minerals on the surface of Ceres are therefore hydrated clay minerals structurally similar to terrestrial montmorillonites. There is also a narrow absorption feature at 3.1 microns which is attributable to a very small amount of water ice on Ceres. This is the first evidence for ice on the surface of an asteroid.

Lebofsky, L. A.↗

NASA NTRS1981

Unidentified features in the spectrum of Triton

Photometry at 3.5 microns was taken with the Infrared Telescope Facility of the Multiple Mirror Telescope to obtain an improved IR spectrum of Triton. The observations show features that are not in detailed agreeement with previous identifications of methane or methane frost, although the general spectral behavior leading to such identifications has been confirmed. It is believed that a combination of surface and atmospheric absorptions may resolve some discrepancies; for instance, many hydrated minerals and frosts which are dark between 3.0 and 4.0 microns may combine with a carbon monoxide atmosphere to yield the data in question.

Rieke, G. H.↗

NASA NTRS1981

The albedo and diameter of 1862 Apollo

Infrared thermal emission measurements are reported of 1862 Apollo, which is the type example of an earth-crossing asteroid. A geometric albedo of 0.21 plus or minus 0.02 is derived which is within the albedo range of the S class of asteroids. The effective diameter was observed to vary with rotation from 1.2 plus or minus 0.1 to 1.5 plus or minus 0.1 km.

Lebofsky, L. A.↗

NASA NTRS1981

Spectroscopic evidence for aqueous alteration products on the surfaces of low-albedo asteroids

The spectral reflectances of primary components of the carbonaceous chondrite matrix materials (CCMM) and their secondary alteration products are compared with new observations of low-albedo asteroids. It is shown that the spectral differences between C-type asteroids and carbonaceous chondrites are consistent with differing degrees of aqueous alteration of CCMM.

Feierberg, M. A.↗

NASA NTRS1981

Infrared reflectance spectra of Hyperion, Titania, and Triton

Medium-resolution infrared (1-2.5 microns; Delta-lambda/lambda = 0.05) photometry of Triton, Titania, and Hyperion and medium-resolution (1.5-2.4 microns; Delta-lambda/lambda not greater than 0.01) spectroscopy of Triton are presented. Hyperion and Titania have spectra roughly similar to the laboratory spectrum of water frost, while the spectrum of Triton is inconsistent with the spectra of frosts likely to be major surface constituents.

Lebofsky, L. A.↗