K. D. Rosema

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.

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.

Recent radar observations of asteroid 1566 Icarus

Abstract We report Doppler-only radar observations of Icarus at Goldstone at a transmitter frequency of 8510 MHz (3.5 cm wavelength) during 8–10 June 1996, the first radar detection of the object since 1968. Optimally filtered and folded spectra achieve a maximum opposite-circular (OC) polarization signal-to-noise ratio of about 10 and help to constrain Icarus physical properties. We obtain an OC radar cross section of 0.05 km 2 (with a 35% uncertainty), which is less than values estimated by Goldstein, 1969 and by Pettengill et al., 1969 , and a circular polarization (SC⧸OC) ratio of 0.5±0.2. We analyze the echo power spectrum with a model incorporating the echo bandwidth B and a spectral shape parameter n , yielding a coupled constraint between B and n . We adopt 25 Hz as the lower bound on B , which gives a lower bound on the maximum pole-on breadth of about 0.6 km and upper bounds on the radar and optical albedos that are consistent with Icarus tentative QS classification. The observed circular polarization ratio indicates a very rough near-surface at spatial scales of the order of the radar wavelength.

INTERCONTINENTAL BISTATIC RADAR OBSERVATIONS OF 6489 GOLEVKA (1991 JX)

Abstract Unlike the major planets and main belt asteroids, some near-Earth objects (NEOs) make their closest approaches to Earth at high northern declinations and therefore are visible simultaneously from North America, Europe and/or Asia, providing occasional opportunities for intercontinental radar experiments. The first celestial target of intercontinental radar was the Earth-crossing asteroid 6489 Golevka (1991 JX), which in June 1995 passed 0.034AU from Earth at a declination of ∼40deg. High power (∼0.5MW), continuous-wave signals at 3.5cm wavelength were transmitted toward the asteroid from the 70m antenna at the JPL/NASA Goldstone Deep Space Communication Complex (DSCC) in California on 13, 14 and 15 June 1995 during several hours on each date. This illumination of the asteroid created an artificial radio source for astronomers anywhere on the asteroid-facing side of Earth. Five astronomical groups tried to detect the radar echoes and two succeeded. Detections were obtained on each of the three days by the 70m antenna at Evpatoria DSCC in Crimea and on June 15 by the 34m antenna at Kashima Space Research Center in Japan. The Goldstone 34m antenna monitored echoes throughout all the observations. From the results of the Goldstone-Evpatoria experiment it can be inferred that the asteroid is about 0.5km across, is not very elongated, possesses considerable surface irregularity and is very reflective presumably due to a large near-surface bulk density. This first intercontinental radar astronomy experiment can be considered as an initial step toward a global radar network for routine NEO investigations.