In 1985, this object was detected with radar from the Arecibo Observatory at a distance of 0.25 AU. The measured radar cross-section was 8 square kilometers. During the radar observations, a small minor-planet moon was detected around Sisyphus, although its existence was not reported until December 2007. Robert Stephens confirmed that it is a suspected binary, and Brian Warner added additional weight to this conclusion, giving hours as the satellite's orbital period, longer than the 25 hours previously reported by Stephens.
