[Previous] | [Session 43] | [Next]
M. C. Nolan, E. S. Howell, J. K. Harmon (Arecibo Obs./NAIC), D. B. Campbell (Cornell U./NAIC), J.-L. Margot (Caltech), J. D. Giorgini (JPL/Caltech)
On 2001 July 7-9 we observed comet C/2001A2(B) (LINEAR) with the Arecibo Planetary radar. We obtained radar echoes with a velocity dispersion of about +40 to -80 m/s (+300 to -600 Hz Doppler shift at at 2380 MHz), presumably from a particle cloud surrounding the nucleus. The total radar cross section was 4±2 km2 in the expected (opposite-circular, or "OC") polarization, indicating that a substantial particle cloud was responsible for the echo. The echo was visible in the unexpected (SC) polarization as well at about 20% of the OC strength, indicating that the particle size was not small compared to the 12.6 cm wavelength of the radar.
The comet's nucleus would be expected to be visible as a narrow feature centered on the ephemeris velocity of the comet. All of the spectra would be consistent with a 10 Hz wide, 5-sigma peak at the ephemeris velocity, which would correspond to a 2 km object with a 6 hour rotation period, assuming a reasonable cometary radar albedo of 4%. However that feature is not clearly distinguishable from the much broader coma echo.
In part of the July 7 data, a relatively narrow (~150 Hz) peak is seen near the ephemeris velocity. If this peak were the nucleus, and assuming a fairly rapid (3 h) rotation rate, the nucleus would have to be 16 km in diameter to have that bandwidth. If the nucleus were that large, it should have presented a much stronger echo, unless its radar albedo was 0.01 or less. Thus, either the nucleus was small (D < 3 km), it was masked by the particle cloud, or the radar albedo is extremely low (less than 0.01). The latter two possibilities seem less likely than simply a small nucleus. The data seem to rule out anything larger than a 20-Hz wide, 6-sigma peak, so we give an upper limit of 3 km diameter for the nucleus.
Imaging was attempted on 2001 July 10, but as yet no echoes have been seen in the data.