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At least 1,459 records · Page 81

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NASA NTRS1973

Observations of zodiacal light from the Pioneer 10 Asteroid-Jupiter probe - Preliminary results.

The imaging photopolarimeter on the Pioneer 10 Asteroid-Jupiter probe is being used to measure starlight and zodiacal light during the cruise phase of the mission. The 2.5-cm-diam telescope is used with a 2.3 deg field of view to spin/scan the sky in the region between 10 and 151 deg from the direction of motion of the spacecraft. Orthogonal components of brightness are measured in two wavelength bands: blue (3900 to 5000 A) and red (5950 to 7200 A). The gegenschein was observed on Mar. 14, 1972, when the earth-sun-spacecraft angle was 3.4 deg and the spacecraft was 9.2 million km from the earth and 1.011 AU from the sun. The distribution of brightness relative to the antisolar point is similar to that obtained from ground-based observations. Taken together, these results show that the gegenschein is not associated with the earth.

Weinberg, J. L.↗

NASA NTRS1973

Determination of radii of satellites and asteroids from radiometry and photometry.

Visual photometry, which measures reflected solar radiation, can be combined with infrared radiometry, which measures absorbed and reradiated solar energy, to determine the albedo and hence the radius of small solar system objects. Equations and graphical solutions for radius and albedo are presented for cases where the object is at opposition, in equilibrium with the insolation, and has unit values for phase integral and infrared emissivities. Each of these assumptions is then discussed, and expressions are given for the dependence of the derived parameters on the assumptions. The Galilean satellites, whose radii are well known, provide a calibration of this technique. Applications are then discussed to Saturn's satellites Iapetus and Rhea and to asteroids (1) Ceres, (4) Vesta, and (324) Bamberga. It is shown that the technique is not subject to major systematic errors and that it is possible to derive radii, particularly for dark objects, with uncertainties of less than 10%.

Morrison, D.↗

NASA NTRS1973

Polarimetric observations of 9 Metis, 15 Eunomia, 89 Julia, and other asteroids.

White-light polarization curves for a number of asteorids are presented. All have well-developed negative branches, suggestive of relatively dark, texturally complex surfaces - possibly regoliths. Geometric albedos, estimated from the positive branch of the polarization curves, show that a significant dispersion in surface albedo exists within the asteroid belt.

Veverka, J.↗

NASA NTRS1973

Study of 1980 Comet Encke-asteroid missions using a spin-stabilized spacecraft. Volume 2: Technical Report

The data and analyses that support the conclusions and recommendations of a space mission to intercept comets and asteroids are presented. The scientific basis for the Encke missions is discussed and includes a summary of the physical characteristics of Encke, a statement of scientific objectives and priorities, a definition of the spacecraft payload, and a rationale for utilization and instrumentation of coma probes. Data are included for the reference mission selected as the baseline for science analysis and design studies. Alternate spacecraft configurations are compared in terms of their capability in meeting the science and mission requirements.

Bursnall, W. J.↗

NASA NTRS1974

Study of effects of uncertainties of comet and asteroid encounter and contact guidance requirements. Part 2: Tumbling problem studies

The problem of determining the rotational motion of a tumbling celestial body of the asteroid type using spacecraft-acquired data is addressed. The rotational motion of the body is modeled by free-Eulerian motion of a triaxial, rigid body and its translational motion with respect, to a nonrotating, observing spacecraft, which is not thrusting, is assumed to be uniform during the time observations are made. The mathematical details which form the basis for a digital simulation of the motion and observations are presented. Two algorithms for determining the motion from observations for the special case of uniform rotational motion are given.

Cochran, J. E., Jr.↗

NASA NTRS1974

Minor planets and related objects. XV - Asteroid /1620/ Geographos

Analysis of the light curves and color-phase and polarization-phase relations of Geographos according to observations over an eight-month period in 1969. The light curves are relatively smooth with two distinct maxima and minima. The largest amplitude of 2.03 mag is the largest observed on asteroids to date. The average B-V is 0.87 and the average U-B is 0.50. A laboratory model that best reproduced Geographos' light curves was a cylinder having hemispherical ends with a length to width ratio of 2.7; on the basis of this model and a relation of Bowell and Zellner (1973), Geographos was calculated to be approximately 1.50 km wide and 4.0 km long.

Dunlap, J. L.↗

NASA NTRS1974

Comet Encke flyby - Asteroid rendezvous mission

A multitarget mission mode is described which utilizes the solar electric propulsion (SEP) capability to rendezvous with an asteroid after the encounter with Encke. This mode can be defined as a 'no-risk' Encke flyby mission relative to SEP technology. Launched in mid-1980, the earth-Encke transfer is all-ballistic, and SEP operation begins after comet encounter and is relied upon only to accomplish the secondary target objectives. The discussion is based on an exploratory analysis and is therefore limited in scope to a description of trajectory profile and spacecraft mass characteristics.

Friedlander, A. L.↗

NASA NTRS1974

A study of unmanned mission opportunities to comets and asteroids

Several unmanned multiple-target mission opportunities to comets and asteroids were studied. The targets investigated include Grigg-Skjellerup, Giacobini-Zinner, Tuttle-Giacobini-Kresak, Borrelly, Halley, Schaumasse, Geographos, Eros, Icarus, and Toro, and the trajectories consist of purely ballistic flight, except that powered swingbys and deep space burns are employed when necessary. Optimum solar electric rendezvous trajectories to the comets Giacobini-Zinner/85, Borrelly/87, and Temple (2)/83 and /88 employing the 8.67 kw Sert III spacecraft modified for interplanetary flight were also investigated. The problem of optimizing electric propulsion heliocentric trajectories, including the effects of geocentric launch asymptote declination on launch vehicle performance capability, was formulated, and a solution developed using variational calculus techniques. Improvements were made to the HILTOP trajectory optimization computer program. An error analysis of high-thrust maneuvers involving spin-stabilized spacecraft was developed and applied to a synchronous meteorological satellite mission.

Mann, F. I.↗

NASA NTRS1974

Minor planets and related objects. XVII - Five-color photometry of four asteroids

Asteroids (6), (14), (39), and (51) were observed at the Leiden Southern Station. Only (39) Laetitia seems to show some variation in color (3% in visual-ultraviolet) correlated to the light curve. If a similar effect is present for (6) Hebe, it should only be present at small phase angles. The light curves of (14) Irene and (39) Laetitia show light variations of a few percent of the rotational period indicating probable surface macrostructure.

Wamsteker, W.↗

NASA NTRS1975

44 Nysa - An iron-depleted asteroid

The minor planet 44 Nysa has a unique combination of photopolarimetric parameters, with the most nearly neutral UBV colors, the shallowest negative polarization branch, and by far the highest polarimetric albedo yet obtained for any asteroid. Its surface apparently consists of a low-opacity, iron-free silicate strongly suggestive of enstatite achondrites.

Zellner, B.↗

NASA NTRS1975

Planets and satellites of the outer solar system, asteroids, and comets

The cosmogenic significance of outer solar system objects is evaluated with emphasis on planets and larger satellites of greatest biological interest; comets, asteroids, and the smaller satellites are also discussed. Principal physical and rudimentary photometric data for the five outer planets are presented in tables. Sufficient information on planetary motions is included for most calculations in physical planetology.

Newburn, R. L., Jr.↗

NASA NTRS1975

Asteroidal and planetary analysis

Photometric, spectrophotometric, and radiometric investigations of asteroids and planets are reported. Profiles of the planetary disk were used to study the physical structure of the Uranus atmosphere, and thermal and photographic properties of Saturn rings were theoretically modelled. Ground-based Mars observations were made for long-term comparison with Mariner 9 results.

Hartmann, W. K.↗

NASA NTRS1976

Radar observations at 3.5 and 12.6 cm wavelength of asteroid 433 Eros

A study of the asteroid 433 Eros using 3.5 and 12.6 cm radar waves indicates that the surface is very much rougher than any planetary or lunar surface observed by this method. A surface completely covered with sharp edges, pits, subsurface holes, or embedded chunks with scale sizes on the order of our wavelengths seems to be indicated. A model based on a rough rotating triaxial ellipsoid having radii in the rotation equator of 18.6 and 7.9 km agrees well with our data, although a strong wobble in the apparent center frequency of the spectra as rotation progresses indicates that one side may be more reflective than the other, or more likely, that the projected axis of rotation does not equally divide the projected area.

Jurgens, R. F.↗

NASA NTRS1976

The infrared spectrum of asteroid 433 Eros

The mineralogical composition of asteroid Eros has been determined from its infrared spectrum (0.9-2.7 micrometers; 28/cm resolution). Major minerals include metallic Ni-Fe and pyroxene; no spectroscopic evidence for olivine or plagioclase feldspar was found. The IR spectrum of Eros is most consistent with a stony-iron composition.

Larson, H. P.↗

NASA NTRS1976

Physical properties of asteroid 433 Eros

Newly available photometric, polarimetric, spectroscopic, thermal-radiometric, radar, and occultation results are synthesized in order to derive a coherent model for Eros. The geometric albedo is 0.19 plus or minus 0.01 at the visual wavelength, and the overall dimensions are approximately 13 by 15 by 36 km. The rotation is about the short axis, in the direct sense, with a sidereal period of 5 hr 16 min 13.4 sec. The pole of rotation lies within a few degrees of ecliptic coordinates 16 deg longitude and beta = +11 deg latitude. Eros is uniformly coated with a particulate surface layer several millimeters thick. It has an iron-bearing silicate composition, similar to that of a minority of main-belt asteroids, and probably identifiable with H-type ordinary chondrites.

Zellner, B.↗

NASA NTRS1976

Results of the asteroid-meteoroid particle experiment on Pioneer 11

The Asteroid-Meteoroid Detector, an electro-optical instrument that detects and measures particles in space by sensing the sunlight reflected from them, is part of the payload of both the Pioneer 10 and 11 spacecraft. The Pioneer 11 instrument is essentially identical to that on Pioneer 10, with some minor modifications. Reduction of the Pioneer 11 data was complicated by the failure of one of the four sensor channels at about 1.1 AU from the sun. Detailed analysis yielded 51 events between 1.0 and 3.5 AU, which were used to determine the particle concentration distribution in that region of space. Weighted curve-fitting of the results shows agreement with the concentrations derived from Pioneer 10 to within about a factor of two. As with the Pioneer 10 results, the exponent of the size dependency varies from about -1.7 for 100-micron radius particles to about -3.2 for 10-cm bodies. The instrument, its operation, and the method of data analysis are reviewed.

Soberman, R. K.↗

NASA NTRS1976

The lightcurve and rotation period of asteroid 139 Juewa

Results of broadband photoelectric photometry of 139 Juewa during five consecutive nights in March 1974 are presented. The synodic period found is 20.9 hr. A linear phase coefficient of about 0.080 is determined between phase angles of 0.9 to 1.5 deg. This value is similar to that for the lunar highlands and for three other asteroids (4 Vesta, 20 Massalia, 110 Lydia) at similar phase angles, indicating that these surfaces have comparable porosities. The composite light curve presented covers 80% of the rotational period; short-timescale features in the light curve are seen which correspond to topography a few kilometers in size.

Goguen, J.↗

NASA NTRS1977

UBV photometry of small and distant asteroids

Photoelectric magnitudes and colors on the ultraviolet (UBV) system are presented for 65 minor planets, including four Mars crossers, six Trojans, and main-belt objects down to 6 km in diameter. The Trojans all have very similar colors not characteristic of the main-belt population. A paucity of S-type asteroids at the smallest diameters, predicted from trends seen at larger sizes, is not observed. The newly available color data for small objects ranging from 1.0 to 5.2 astronomic units in heliocentric distance show the main belt to be a transition zone between predominantly silicate and carbonaceous compositions.

Zellner, B.↗