Presentation #514.02 in the session Observing Physical Properties of NEOs Part II.
Since January of 2021, Goldstone has observed ten binaries and one triple system (2001 SN263). The list below shows the absolute magnitudes and the minimum distance at which the objects were observed by radar. The satellites of 2008 UZ94 and 1989 JA were discovered by radar, although optical observers (P. Pravec pers. comm.) saw hints of the satellite for 1989 JA shortly before the start of radar observations. We were not able to detect the satellites of 1999 HF1, 2002 AW, 2003 QQ47, and 1994 AW1, and we detected only the outer satellite of 2001 SN263 due to low SNRs. 1999 RM45 and 1989 JA were strong radar targets due to their relatively close flybys. The delay-Doppler images of 1999 RM45 with 18.75 m range resolution revealed a symmetric primary ~320 m in diameter and a satellite 70-100 m in diameter. The images of 1989 JA with 18.75 m range resolution showed well-resolved, irregularly shaped primary with a diameter of ~700 m and the secondary in the realm of 100-200 m. We obtained thorough orbital phase coverage for the satellites of both systems, so these datasets are suitable for estimation of the system’s orbital parameters, mass, and bulk densities.
Since completion of the upgrade at Arecibo in 1999, Arecibo and Goldstone have observed 62 binaries and 4 triples out of 606 NEAs with H≤22. Radar discovered 52 out of 79 currently known multiple systems or ~66%. Margot et al. (2002) and Pravec et al. (2006) reported that at least 16% of NEAs larger than ~200 m in diameter are binary systems, although the latest radar statistics (which have not been de-biased) lowers this bound to ~11%. Radar SNRs are proportional to the transmitter power, P1/2and D3/2, where P is the rotation period and D is the diameter, so small, rapidly rotating satellites are more difficult to detect than larger, slowly-rotating ones in synchronous orbits about their primaries. The satellite can also escape notice if the Doppler resolution is too coarse or too fine with respect to the satellite’s bandwidth. Hence, the latest radar statistics are probably very conservative, and may reflect the fact that we have been observing more weak targets.
Government sponsorship acknowledged.