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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Asteroid radar astrometry

Measurements of time delay and Doppler frequency are reported for asteroid-radar echoes obtained at Arecibo and Goldstone during 1980-1990. Radar astrometry is presented for 23 near-earth asteroids and three mainbelt asteroids. These measurements, which are orthogonal to optical, angular-position measurements, and typically have a fractional precision between 10 to the -5th and 10 to the -8th, permit significant improvement in estimates of orbits and hence in the accuracy of prediction ephemerides. Estimates are also reported of radar cross-section and circular polarization ratio for all asteroids observed astrometrically during 1980-1990.

Ostro, S. J.↗
NASA NTRS1994

(abstract) Asteroid Radar Astronomy at Goldstone in the 1990s

Since 1990, much effort has been devoted to optimizing Goldstone's capability to study near-Earth objects (NEOs). Three different configurations are now available. The nominal system on the 70-m antenna (DSS 14) uses separate feed horns for transmitting and receiving, and the 15-plus seconds required for transmit-to-receive switching renders the system useless for targets much closer than about 0.025 AU (about 10 lunar distances, about 25 seconds of echo time delay) and unwieldy even for targets twice that far. However, an additional, recently installed single horn system shrinks switching time to about 5 seconds, letting DSS 14 observe NEOs as close as about 5 lunar distances. Closer targets will require two-station observations, with DSS 14 transmitting continuously while DSS 13, a 34-m antenna 22 km away, receives. That configuration, first used for Toutatis, will also be required for delay-Doppler imaging that places thousands of pixels on any NEO whose rotation period P(h) and echo roundtrip time delay RTT (s) satisfy P > RTT. Between January 1990 and June 1994, the Goldstone radar has detected 11 asteroids. During the next 2 years, seven asteroids are scheduled for Goldstane observations. After completion of the Arecibo upgrade, Goldstone will play an important complementary role to that much more sensitive instrument.

asteroids radar Goldstone delay-Doppler imaging Ar↗
NASA NTRS2002

Asteroid Radar Astronomy at the Dawn of the New Millennium

This talk will use images, movies, and radar-derived three-dimensional models to present recent highlights of asteroid radar research, concentrating on low-V mission candidates. Additional information is contained in the original extended abstract.

Ostro, S. J.↗
NASA NTRS1979

Radar observations of asteroids

Radar observations of asteroids use simple CW waveforms, the transmissions lasting for the duration of the round-trip delay. The echo is received, spectrum-analyzed, and integrated for a similar period. Since radar coherently illuminates the target, the surface scattering properties at radio wavelengths are directly determined as a function of angle and polarization. The distance and radial velocity obtained with radar complement the angular position of the object as determined from telescope measurements. Radar cross sections, scattering law, and radius are given for radar-observed asteroids.

Pettengill, G. H.↗
NASA NTRS1984

Radar Investigations of Asteroids

Radar investigations of asteroids, including observations during 1984 to 1985 of at least 8 potential targets and continued analyses of radar data obtained during 1980 to 1984 for 30 other asteroids is proposed. The primary scientific objectives include estimation of echo strength, polarization, spectral shape, spectral bandwidth, and Doppler shift. These measurements yield estimates of target size, shape, and spin vector; place constraints on topography, morphology, density, and composition of the planetary surface; yield refined estimates of target orbital parameters; and reveals the presence of asteroidal satellites.

Ostro, S. J.↗
NASA NTRS1981

Radar investigation of asteroids

Radar investigations were conducted of selected minor planets, including: (1) observations during 1981-82 of 10 potential targets (2 Pallas, 8 Flora, 12 Victoria, 15 Eunomia, 19 Fortuna, 22 Kalliope, 132 Aethra, 219 Thusnelda, 433 Eros, and 2100 Ra-Shalom); and (2) continued analyses of observational data obtained during 1980-81 for 10 other asteroids (4 Vesta, 7 Iris, 16 Psyche, 75 Eurydike, 97 Klotho, 216 Kleopatra, 1685 Toro, 1862 Apollo, 1865 Cerberus, and 1915 Quetzalcoatl). Scientific objectives include estimation of echo strength, polarization, spectral shape, spectral bandwidth, and Doppler shift. These measurements: (1) yield estimates of target size, shape, and spin vector; (2) place constraints on topography, morphology, and composition of the planetary surface; (3) yield refined estimates of target orbital parameters; (4) reveal the presence of asteroidal satellites.

Ostro, S. J.↗
NASA NTRS1998

Icarus Revisted: Three Decades of Radar Observations of Asteroids

Three decades after the close encounter of 1566 Icarus, which marked the beginning of asteroid radar astronomy, we had a chance to observe the asteroid afresh by using a more developed radar system, and here we present the results of the study.

Icarus asteroid radar astronomy radar cross sectio↗
NASA NTRS1981

Radar investigation of asteroids

Software to support all stages of asteroid radar observation and data analysis is developed. First-order analysis of all data in hand is complete. Estimates of radar cross sections, circular polarization ratios, and limb-to-limb echo spectral bandwidths for asteroids 7 Iris, 16 Psyche, 97 Klotho, 1862 Apollo, and 1915 Quetzalcoatl are reported. Radar observations of two previously unobserved asteroids were conducted. An Aten asteroid, 2100 Ra-Shalom, with the smallest known semimajor axis (0.83) was detected. Preliminary data reduction indicates a circular polarization ratio comparable to those of Apollo, Quetzalcoatl, and Toro.

Ostro, S. J.↗
NASA NTRS1995

Radar Observations of Near-Earth Asteroids

Radar echoes can furnish high-resolution images of near-Earth asteroids, reveal their rotation states and metal abundances, and improve the accuracy of trajectory predictions a thousandfold, a capability that is important for asteroids making very close Earth approaches and that can dramatically reduce the cost and risk of spacecraft missions.

Near-Earth↗
NASA NTRS1987

Physical properties of asteroids from radar observations

The basic experimental strategies of asteroid radar astronomy are summarized and new information based on recent radar observations about asteroid surface properties is presented. Particular attention is given to the use of radar techniques for constraining asteroid shapes and pole directions. The geometric relationship between a rotating asteroid and its radar echo power spectrum is analyzed.

Ostro, S. J.↗
NASA NTRS1982

Radar backscattering from a rough rotating triaxial ellipsoid with applications to the geodesy of small asteroids

Radar geodesy of the asteroid 433 Eros was reported by Jurgens and Goldstein (1976). Their measurements were based on the spectral properties of a rough rotating triaxial ellipsoid. This paper presents the theory by which theoretical spectra based on a backscattering model of the form of the nth power of the cosine of theta, or any reasonable backscattering model, can be computed. Some general properties of these spectra are demonstrated, and simple measurements of the apparent radar cross section, center frequency, and effective bandwidth as a function of time are shown to be useful in determining the sizes of the semi-major axes, the exponent n in the scattering model, the backscatter efficiency of the surface material, the rotation period, and the declination of the observer above or below the rotation equator. Some of these parameters are correlated, and multiple estimation may be difficult unless the observations span a period sufficiently long so as to present a wide range of declinations of the observer. However, the departure from a spherical shape aids in separating the scattering properties from the target dimensions and facilitates the simultaneous estimation of all free parameters.

Jurgens, R. F.↗
NASA NTRS1993

Radar images of asteroid Toutatis

Radar images of a near-Earth asteroid, Toutatis, were obtained from earth in December 1992. The asteroid was found to be a compact binary consisting of two irregularly shaped cratered objects and rotating with a period of 10 and 11 days. These results carry significant implications for understanding the origin and evolution of near-earth asteroids.

Mclaughlin, William I.↗
NASA NTRS1986

Radar investigation of asteroids

The number of radar detected asteroids has climbed from 6 to 40 (27 mainbelt plus 13 near-Earth). The dual-circular-polarization radar sample now comprises more than 1% of the numbered asteroids. Radar results for mainbelt asteroids furnish the first available information on the nature of these objects at macroscopic scales. At least one object (2 Pallas) and probably many others are extraordinarily smooth at centimeter-to-meter scales but are extremely rough at some scale between several meters and many kilometers. Pallas has essentially no small-scale structure within the uppermost several meters of the regolith, but the rms slope of this regolith exceeds 20 deg., much larger than typical lunar values (approx. 7 deg.). The origin of these slopes could be the hypervelocity impact cratering process, whose manifestations are likely to be different on low-gravity, low-radius-of-curvature objects from those on the terrestrial planets. The range of mainbelt asteroid radar albedoes is very broad and implies big variations in regolith porosity or metal concentration, or both. The highest albedo estimate, for 16 Psyche, is consistent with a surface having porosities typical of lunar soil and a composition nearly completely metallic. Therefore, Psyche might be the collisionally stripped core of a differentiated small plant, and might resemble mineralogically the parent bodies of iron meteorites.

Ostro, S. J.↗
NASA NTRS1979

Radar observations of asteroid 1 Ceres

Radar observations of asteroid 1 Ceres were made at a 12.6-cm wavelength from the Arecibo Observatory in March/April 1977. The measurements, made with a received circular polarization orthogonal to that transmitted, yield a radar cross section of (0.04 + or- 0.01) piR-squared, for R = 510 km. The corresponding radar reflectivity is less than that measured for any other celestial body. Within the accuracy of measurement, no significant variation of cross section with rotational phase is apparent. The shape of the power spectrum suggests that Ceres is rougher at the scale of the observing wavelength than the moon and inner planets, but smoother than the outer three Galilean satellites.

Ostro, S. J.↗
NASA NTRS1979

Radar observations of asteroid 1580 Betulia

Radar observations of the asteroid 1580 Betulia, made at a wavelength of 12.6 cm, show a mean radar cross section of 2.2 + or- 0.8 sq km and a total spectral bandwidth of 26.5 + or- 1.5 Hz. Combining bandwidth measurements with the optically determined rotation period sets a lower limit to the asteroid's radius of 2.9 + or- 0.2 km.

Pettengill, G. H.↗