Orbits About Asteroid 4769 Castalia
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asteroid impact ejecta zero-velocity orbits↗
Engineering topics
Ostro, S. J.
Publications and source records attributed to Ostro, S. J..
Radar ranging to Ganymede and Callisto
Arecibo observations from 1992 February to March have yielded the first successful radar range measurements to the Galilean satellites. Round-up time delays were measured for Ganymede and Callisto with accuracies of 20 to 50 micrometer (3 to 7 km) and 90 micrometer (14 km), respectively. Both satellites showed round-trip delay residuals (relative to the E-3 ephemeris) of about a millisecond, most of which can be attributed to errors in the predicted along-track positions (orbital phases). Using a simple model that assumed that all of the ephemeris error was due to constant orbital phase and Jupiter range errors we estimate that Ganymede was leading its ephemeris by 122 +/- 4 km, Callisto was lagging its ephemeris by 307 +/- 14 km, and Jupiter was 11 +/- 4 km more distant than predicted by the PEP740 planetary ephemeris.
Harmon, J. K.↗
Radar Investigations of Europa, Ganymede, and Callisto
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Europa Ganymede Callisto↗
(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↗
Unusual radar echoes from the Greenland ice sheet
In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR) instrument collected the first calibrated data set of multifrequency, polarimetric, radar observations of the Greenland ice sheet. At the time of the AIRSAR overflight, ground teams recorded the snow and firn (old snow) stratigraphy, grain size, density, and temperature at ice camps in three of the four snow zones identified by glaciologists to characterize four different degrees of summer melting of the Greenland ice sheet. The four snow zones are: (1) the dry-snow zone, at high elevation, where melting rarely occurs; (2) the percolation zone, where summer melting generates water that percolates down through the cold, porous, dry snow and then refreezes in place to form massive layers and pipes of solid ice; (3) the soaked-snow zone where melting saturates the snow with liquid water and forms standing lakes; and (4) the ablation zone, at the lowest elevations, where melting is vigorous enough to remove the seasonal snow cover and ablate the glacier ice. There is interest in mapping the spatial extent and temporal variability of these different snow zones repeatedly by using remote sensing techniques. The objectives of the 1991 experiment were to study changes in radar scattering properties across the different melting zones of the Greenland ice sheet, and relate the radar properties of the ice sheet to the snow and firn physical properties via relevant scattering mechanisms. Here, we present an analysis of the unusual radar echoes measured from the percolation zone.
Rignot, E. J.↗
The Role of Groundbased Radar in Near-Earth-Object Hazard Identification and Mitigation
Groundbased radar is a key technique for the post-discovery reconnaissance of NEOs and is likely to play a central role in identification of possibly threatening objects during the foreseeable future.
Comets radar NEO imaging asteroids↗
Europa, Ganymede, and Callisto - New radar results from Arecibo and Goldstone
Observations of the icy Galilean satellites, performed during 1987-1991 with the Goldstone 3.5 cm system and the Arecibo 13 cm system, show significant enhancements in the knowledge of the satellite's radar properties. The most prominent radar features are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frequencies show Callisto to be lagging its ephemeris by 200 +/- 50 km.
Ostro, S. J.↗
Planetary Radar Astronomy
Planetary radar astronomy is the study of Solar System entities by transmitting a radio signal toward the target and then receiving and analyzing the echo. This field of research has primarily involved observations with Earth-based radar telescopes, but also includes certain experiments with a spaceborne transmitter and/or receiver.
radar↗
Asteroid and comet orbits using radar data
For the 30 asteroids and 4 comets for which radar astrometric data were given by Ostro (1991), orbits have been computed using both the radar and the existing optical measurements. The techniques required to process radar data in orbit determination solutions are outlined, and future radar observation opportunities for asteroids and comets are identified. For asteroids and comets that have only short intervals of optical astrometric data, the additional use of only a few radar observations allows a far more accurate extrapolation of their future motions. The use of radar data can often ensure an object's successful recovery at future earth returns and greatly assist efforts in monitoring the motions of the rapidly growing population of known near-earth objects, including their future close-earth approaches.
Yeomans, D. K.↗
Planetary Radar
Planetary radar astronomy is the study of solar system entities (the moon, asteroids, and comets, as well as the major planets and their satellites and ring systems) by transmitting radio signals towards a target and receiving and analyzing the echoes.
Aliasing antenna gain circular polarization ratio ↗
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.↗
Asteroid 1986 DA - Radar evidence for a metallic composition
Echoes from the near-earth object 1986 DA show it to be significantly more reflective than other radar-detected asteroids. This result supports the hypothesis that 1986 DA is a piece of NiFe metal derived from the interior of a much larger object that melted, differentiated, cooled, and subsequently was disrupted in a catastrophic collision. This 2-kilometer asteroid, which appears smooth at centimeter to meter scales but extremely irregular at 10- to 100-meter scales, might be (or have been a part of) the parent body of some iron meteorites.
Ostro, S. J.↗
Radar images of asteroid 1627 Ivar
Radar echoes from the near-earth asteroid 1627 Ivar, whose orbit crosses the earth's, reveal it to be about twice as long as it is wide, with a maximum dimension no less than 7 km and probably within 20 percent of 12 km. The surface is fairly smooth at centimeter-to-meter scales but appears irregular and nonconvex at kilometer scales.
Ostro, S. J.↗
The shape of Eros
Monte Carlo simulations are presently used to optimize estimation, ascertain associated errors, and guide bias-correction procedures, for the Eros polar silhouette convex hull that has been estimated from radar echo spectra. This hull is trapezoidal; this nonaxisymmetric shape may account for odd harmonics in Eros' echo spectral signature as a function of rotation phase. Additional constraints have been obtained for the figure of Eros through the inversion of the optical lightcurve to estimate the asteroid's two-dimensional average of the three-dimensional shape. This 'mean cross-section' and the polar silhouette exhibit similar elongations.
Ostro, S. J.↗
Radar observations of asteroid 1986 JK
The asteroid 1986 JK was observed with a 3.5 cm-wavelength radar in May and June, 1986, at less than 0.029 AU; its radar echo power circular polarization ratio indicates single backscattering from smooth surface elements. A working model constructed for the asteroid in light of these radar data postulates a 1-2 km object whose shape has little elongation and some polar flattening. Orbital and physical characteristics are rather cometlike. The radar astrometric data obtained are noted to be extremely powerful for orbit-improvement, so that a search ephemeris whose uncertainty is an order-of-magnitude smaller than that based on relevant optical data alone can be prepared by combining optical and radar data.
Ostro, S. J.↗
Radar detection of Phobos
Radar echoes from the martian satellite Phobos provide information about that object's surface properties at scales near the 3.5-cm observing wavelength. Phobos appears less rough than the moon at centimeter-to-decimeter scales. The uppermost few decimeters of the satellite's regolith have a mean bulk density within 20 percent of 2.0 g/cu cm. The radar signature of Phobos (albedo, polarization ratio, and echo spectral shape) differs from signatures measured for small, earth-approaching objects, but resembles those of large (greater than 100-km), C-class, mainbelt asteroids.
Ostro, S. J.↗
Radar astrometry of near-earth asteroids
An uncertainty analysis is conducted for four asteroids with different optical and radar observation histories in order to ascertain the degree to which near-earth asteroid ephemerides can be improved by means of radar observations. For any given set of optical and radar observations and their associated errors, the angular and earth-asteroid distance uncertainties are calculated as functions of time through 2001. The radar data furnished only a small absolute improvement in such cases as that of 1627 Ivar, where a long history of optical astrometric data exists, but dramatic reductions in ephemeris uncertainties are obtainable for asteroids having only short optical data histories.
Yeomans, D. K.↗
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.↗