M-class asteroids are a relatively rare type of asteroid in the main asteroid belt, located between the orbits of Mars and Jupiter. For many years, planetary scientists have thought that they were the remnants of small protoplanets that were shattered in the violent early days of the solar system, leaving only an exposed metal core behind. Unfortunately, determining whether an asteroid is mostly metallic is very difficult with traditional optical telescopes. For the past decade, we have used the Arecibo Planetary Radar to probe these objects, for only a radar telescope can give an unambiguous indication of a metallic composition.
Figure 1. The radar-derived shape model of asteroid 16 Psyche. It is roughly ellipsoidal with dimensions 279 x 232 x 189 km. Colors represent the deviation in topography from the surface mean. All sides are shown. The numbers in the top left corner of each image are the longitude and latitude, respectively, of the image center. Two crater-like depressions are evident in the southern hemisphere, shown in the lower right figure.
Asteroid 16 Psyche is the largest M-class asteroid in the main-belt and is of special interest because it is the target of a newly selected NASA Discovery Mission, also called Psyche. Until our study, scientists had only the most rudimentary knowledge of Psyche’s size and shape – information critical for mission planning purposes. In 2005, we obtained data that confirmed Psyche was dominantly metallic. In late 2015, we were able to obtain 18 new delay-Doppler radar images of Psyche with the Arecibo Planetary Radar. When combined with our previous radar observations, new optical observations from two adaptive optics telescopes, optical light curves, and three stellar occultations, we were able to generate a 3D shape model of Psyche with a resolution of about 15 km. Our model shows evidence for two crater-like depressions and suggests that there is significant variation in the metal content and color of the asteroid over the surface.
This work would have been impossible without the Arecibo facility. It contains the only radar telescope in the world capable of imaging main-belt asteroids.Title: Radar Observations and Shape Model of Asteroid 16 Psyche
Authors: Michael K. Shepard and 21 coauthors, including AO staff members Patrick A. Taylor and Linda A. Rodriguez-Ford.
Paper Reference: Icarus 281, p. 388-403. Published 2017.
ADS link: http://adsabs.harvard.edu/abs/2017Icar..281..388S