Joseph S. Jao

Radar imaging and characterization of the binary near-Earth asteroid (185851) 2000 DP107

Potentially hazardous asteroid (185851) 2000 DP107 was the first binary near-Earth asteroid to be imaged. Radar observations in 2000 provided images at 75 m resolution that revealed the shape, orbit, and spin-up formation mechanism of the binary. The asteroid made a more favorable flyby of the Earth in 2008, yielding images at 30 m resolution. We used these data to obtain shape models for the two components and to improve the estimates of the mutual orbit, component masses, and spin periods. The primary has a sidereal spin period of 2.7745 +/- 0.0007 h and is roughly spheroidal with an equivalent diameter of 863 m +/- 5 %. It has a mass of 4.656 +/- 0.43 x 10^11 kg and a density of 1381 +/- 244 kg m^{-3}. It exhibits an equatorial ridge similar to the (66391) 1999 KW4 primary, however the equatorial ridge in this case is not as regular and has a ~300 m diameter concavity on one side. The secondary has a sidereal spin period of 1.77 +/- 0.02 days commensurate with the orbital period. The secondary is slightly elongated and has overall dimensions of 377 x 314 x 268 m (6 % uncertainties). Its mass is 0.178 +/- 0.021 x 10^{11} kg and its density is 1047 +/- 230 kg m^{-3}. The mutual orbit has a semi-major axis of 2.659 +/- 0.08 km, an eccentricity of 0.019 +/- 0.01, and a period of 1.7556 +/- 0.0015 days. The normalized total angular momentum of this system exceeds the amount required for the expected spin-up formation mechanism. An increase of angular momentum from non-gravitational forces after binary formation is a possible explanation. The two components have similar radar reflectivity, suggesting a similar composition consistent with formation by spin-up. The secondary appears to exhibit a larger circular polarization ratio than the primary, suggesting a rougher surface or subsurface at radar wavelength scales.

Radar generates high-resolution topographic map of the Moon

The National Aeronautics and Space Administration’s (NASA’s) long-term exploration goals include resuming manned missions to the Moon that will culminate in a permanent manned lunar station. Before embarking on such a mission, a series of unmanned robotic missions are required to ascertain the best locations for manned exploration or a permanent lunar base. The south polar region of the Moon has attracted much recent attention because significant amounts of frozen water may be trapped in permanently shadowed regions in craters there.1, 2 Knowledge concerning the lunar topography is of particular importance in planning survivable landings and locating regions accessible to exploration. Synthetic aperture radar exploits the motion of a radar relative to a surface to create fine-resolution images essentially independent of the distance from the radar to the surface. By creating images from two spatially separated radars, an interferometric image can be derived containing topographic information. JeanLuc Margot and co-authors used the Goldstone Solar System Radar (GSSR) in this configuration to derive topographic maps of the lunar surface at 150m spatial resolution and 50m vertical accuracy.3–5 Subsequently, in 2006, the GSSR system was upgraded to support the collection of finer-resolution imagery of the lunar surface. The topographic map described here was derived from this data.

Lunar topographic mapping using a new high resolution mode for the GSSR radar

Mapping the Moons topography using Earth based radar interferometric measurements by the Goldstone Solar System Radar (GSSR) has been done several times since the mid 1990s. In 2008 we reported at this conference the generation of lunar topographic maps having approximately 4 m height accuracy at a horizontal posting of 40 m. Since then GSSR radar has been improved to allow 40 MHz bandwidth imaging and consequently obtained images and interferograms with a resolution of about 4 m in range by 5 m in azimuth. The long synthetic aperture times of approximately 90 minutes in duration necessitated a migration from range/Doppler image formation techniques to spotlight mode processing and autofocusing methods. The improved resolution imagery should permit the generation of topographic maps with a factor of two better spatial resolution with about same height accuracy. Coupled the with the recent availability of new lidar topography maps of the lunar surface made by orbiting satellites of Japan and the United States the geodetic control of the radar generated maps products can be improved dramatically. This paper will discuss the hardware and software improvements made to the GSSR and present some of the new high resolution products.