High-resolution Goldstone radar imaging of comet P/2016 BA14 (Pan-STARRS)

Comet P/2016 BA14 (Pan-STARRS) was discovered by Pan-STARRS on January 21, 2016 and approached Earth within 0.024 astronomical units (9.2 lunar distances) on March 22. It was originally classified as an asteroid but subsequent observations (Knight et al., CBET 4257, 2016) showed the presence of a faint, short tail suggesting that the object is a comet. The similarity of its orbit to that of comet 252P/LINEAR led to speculation of a common origin.We observed 2016 BA14 with radar using the 70-m DSS-14 (8560 MHz, 3.5 cm) and 34-m DSS-13 (7190 MHz, 4.2 cm) antennas at Goldstone as transmitters and the 100-m Green Bank Telescope in West Virginia as a receiver on four days spanning one week around close approach. The best images have range resolutions of 7.5 m/pixel and are the finest resolution comet images ever obtained at Goldstone. The maximum visible extent of the nucleus in the radar images is about 900 m, strongly implying that the diameter is more than 1 km. Its absolute magnitude of 19.5 and a diameter of at least 1 km imply an optical albedo of < 3%. The echo bandwidth is ~2.5 Hz, which suggests a slow rotation period of about 40 h that is consistent with the rotation evident in images obtained on each day. There are no obvious signatures of a coma in the radar data. The appearance of the leading edge of the nucleus varies significantly as it rotates: there are facets hundreds of meters in length, angular junctions between facets, depressions, and rounded regions. The radar images lack any prominent high-contrast surface features, but there are subtle signatures of linear ridges, concavities, and a raised region casting a radar shadow.