Radar Echoes from Near-Earth Asteroid 1866 Sisyphus

L. A. M. Benner, S. J. Ostro, D. B. Campbell, I. I. Shapiro, J. S. Chandler


Arecibo echo power spectra from 1866 Sisyphus obtained in December, 1985. This is a weighted sum of 41 runs obtained on three tracks during December 8, 9, and 10 when the narrow spike on the right side of the echo was in nearly the same location in Doppler frequency. Inspection of spectra during each day showed no apparent change in the location of the spike during each track. CW data obtained on December 6/7 (during a track that crossed the date boundary) show a spike at negative frequencies.

The rotation period of Sisyphus is 2.4 hours; during each track, there was considerable rotation, yet the spike did not move, but its position clearly changed between some of the days.

The resolution is 3 Hz at the top and 1 Hz at the bottom.

If we use the 1-sigma levels to estimate the positive and negative edges of the echo, then the bandwidth of the broadband component is about 93 Hz. Given the rotation period of 2.4 hours, and assuming that the bandwidth doesn't vary significantly as the asteroid spins (which is true to the extent that we can tell), this places a lower bound bound on the pole-on breadth of 8.0 km/cos(delta), where delta is the unknown subradar latitude. The polar axis isn't well constrained, so if Sisyphus has a "muffin" shape similar to those of the primaries of other NEA binaries, then its effective diameter could be somewhat less than 8.0 km.

Echo power spectrum from the night of Dec. 6-7, 1985, shown at a resolution of 2 Hz. This is a weighted sum of 15 OC runs. This is the night that Steve Ostro discovered the narrow spike, and unlike data obtained on the next three nights, the spike is at negative frequencies. The bandwidth of the broadband echo is close to 100 Hz and is consistent with results obtained from the weighted sum in the top figure.

The appearance of the CW echo power spectra is the same as those for numerous binary near-Earth asteroids seen starting in September, 2000 with 2000 DP107. In addition, in Dec. 1985, Steve Ostro scoured everything at Arecibo trying to see if he made a cabling or setup mistake. He also checked for other equipment problems but found nothing, so he concluded that the spike was real. As a result, and due to the fact that the spike did not move in Doppler frequency with the asteroid's well-established period of 2.4 hours, we have classified Sisyphus as a binary system.

Attempts to detect Sisyphus with ranging observations in 1985 were unsuccessful.

Last update: 2013 July 30