Radar Observations of Near-Earth Asteroid 2012 TC4
Abstract
Near-Earth asteroid 2012 TC4 approached within 0.00035 au (6.9 Earth radii of Earth's surface) on 2017 October 12. This was one of the closest near-Earth asteroid encounters known in advance and it provided an outstanding opportunity to investigate this object's physical properties. Here we report radar observations obtained between Oct. 9-14 with the 70 m DSS-14 and 34 m DSS-13 antennas the Goldstone Deep Space Communications Complex and with the 100 m Green Bank Telescope. 2012 TC4 has an absolute magnitude of 27, suggesting a diameter within a factor of two of 15 meters. Despite its diminutive size, the flyby was so close that we expected to observe extremely strong radar signal-to-noise ratios (SNRs) that would permit imaging at the highest range resolutions currently attainable at Goldstone. Monostatic detection of the asteroid occurred at DSS-14 (8560 MHz, 3.5 cm) on October 9 but at weaker SNRs than expected, possibly indicating that the diameter was significantly smaller than 15 meters. Monostatic DSS-14 delay-Doppler imaging on Oct. 11 at 3.75 m resolution placed only two range pixels on the object. The highest resolution images were obtained on Oct. 12 when transmissions were at DSS-13 (7190 MHz, 4.2 cm) and reception was at Green Bank. The images achieved a range resolution of 1.875 m/pixel that is the finest ever obtained at Goldsone for any near-Earth asteroid. The images reveal an elongated, angular, asymmetric, and rapidly rotating object and place lower bounds on its long and intermediate axes of about 6 x 12 meters. The rotation evident in the images is consistent with the 12 minute period that is prominent in the lightcurves obtained by W. B. Ryan (pers. comm.). The ratio of same-sense circular (SC) to opposite-sense circular (OC) polarization, SC/OC, is about 0.6, which probably indicates that this object has an unusually rough surface at decimeter spatial scales. Circular polarization ratios have been obtained for hundreds of near Earth asteroids, and the ratio for 2012 TC4 overlaps the upper end of the values reported for C- and SQ-class NEAs and the lower end for E- and V-class objects. Combined with an effective diameter in the realm of 7-10 meters, the implication is that 2012 TC4 is an optically-bright object. The radar images are suitable for estimation of the 3D shape and spin state, which may have changed due to tides during the extremely close approach. When combined with the area/mass ratio estimated independently from orbit fitting (D. Farnocchia, pers. comm.), the physical model will be used to obtain the mass and the bulk density, information that is generally not available for solar system objects in this size regime.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E.278B