J. F. Chandler

Radar Images of Asteroid 4179 Toutatis

Delay-Doppler images of the Earth-crossing asteroid 4179 Toutatis achieve resolutions as fine as 125 nanoseconds (19 meters in range) and 8.3 millihertz (0.15 millimeter per second in radial velocity) and place hundreds to thousands of pixels on the asteroid, which appears to be several kilometers long, topographically bifurcated, and heavily cratered. The image sequence reveals Toutatis to be in an extremely slow, non-principal axis rotation state.

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.

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.

Focused 70-cm Wavelength Radar Mapping of the Moon

We describe new 70-cm wavelength radar images of the lunar near-side and limb regions obtained via a synthetic-aperture-radar patch-focusing reduction technique. The data are obtained by transmitting a circularly polarized pulsed waveform from the Arecibo telescope in Puerto Rico and receiving the echo in both senses of circular polarization with the Robert C. Byrd Green Bank Telescope in West Virginia. The resultant images in both polarizations have a spatial resolution as fine as 320 m 450 m near the lunar limb. The patch-focusing technique is a computationally efficient method for compensating for range migration and Doppler (azimuth) smearing over long coherence times, i.e., 983 s, which is needed to achieve the required Doppler resolution. Three to nine looks are averaged for speckle reduction and to improve the signal-to-noise ratio. At this long wavelength, the radar signal penetrates up to several tens of meters into the dry lunar surface materials, thus revealing details of the bulk loss properties and decimeter-scale rock abundance not evident in multispectral and other remote-sensing data. Application of the new radar images to the analysis of basalt flow complexes in Mare Serenitatis shows that the long-wavelength radar data are sensitive to differences in both flow age and composition, and may be particularly useful for studies of smaller deposits that do not have robust crater statistics. The new 70-cm lunar radar data are archived at the National Aeronautics and Space Administration Planetary Data System.

Testing for Lorentz Violation : Constraints on Standard-Model-Extension Parameters via Lunar Laser Ranging

We present constraints on violations of Lorentz invariance based on archival lunar laser-ranging (LLR) data. LLR measures the Earth-Moon separation by timing the round-trip travel of light between the two bodies and is currently accurate to the equivalent of a few centimeters (parts in 10(11) of the total distance). By analyzing this LLR data under the standard-model extension (SME) framework, we derived six observational constraints on dimensionless SME parameters that describe potential Lorentz violation. We found no evidence for Lorentz violation at the 10(-6) to 10(-11) level in these parameters. This work constitutes the first LLR constraints on SME parameters.

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 objects 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.

Radar imaging of the lunar poles.

We have used a radio telescope at Arecibo Observatory, Puerto Rico, to map features of the lunar poles — some as small as 300 metres across — by collecting long-wavelength radar images that can penetrate several metres of lunar dust. We find that areas of the crater floors at the poles that are in permanent shadow from the Sun, which are potential cold traps for water or other volatiles, do not give rise to strong radar echoes like those associated with thick ice deposits in the polar craters on Mercury. Any lunar ice present within regions visible to the Arecibo radar must therefore be in the form of distributed grains or thin layers.

Mars radar mapping: Strong backscatter from the Elysium basin and outflow channel

Abstract Radar reflectivity maps of Mars, obtained at Arecibo in 1990 using a modified delay-Doppler technique, have revealed strong depolarized echoes coming from the Elysium flood basin and outflow channel as well as from Elysium Mons. This result is consistent with recent studies which show that much of the basin/channel floor is covered with lava flows.

Radar images of asteroid 1627 Ivar

Radar echoes from the near-earth asteroid 1627 Ivar, whose orbit crosses the earths, 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.

The spin vector of Venus

Analysis of radar observations from 1964 through 1983 yields the following values for the components of the spin vector of Venus: P = 243.026 + or - 0.006 d (retrograde); alpha = 272.75 + or - 0.09 deg; and delta = 67.10 + or - 0.09 deg, where the standard errors quoted are three- to five-fold larger than the statistical standard errors and encompass the changes in results obtained by various tests designed to expose possible systematic errors. These values demonstrate conclusively that the spin state of Venus is not in resonance with the relative orbital motions of Venus and earth.