Asteroids With Tensile Strength: The Case of 2015 HM10

Near-Earth asteroid 2015 HM10 was discovered on 2015 April 19 with the 4-m Blanco Telescope at Cerro Tololo (MPEC 2015-H90). HM10 made a 0.00295 AU / 1.14 lunar distance flyby of Earth on July 7. This was the asteroid’s closest approach to Earth until at least 2419.We observed HM10 with radar between July 5 and July 8 using Arecibo, the 70 m DSS-14 and 34 m DSS-13 antennas at Goldstone, Green Bank, and elements of the Very Long Baseline Array (VLBA). Bistatic observations were crucial to obtain high-resolution images of HM10 due to the short round-trip travel time of the radar signal, which was as low as 2.95 s on July 7. Our finest image resolution was 3.75 m/pixel in range, obtained on July 7 with the new 80 kW C-band (7190 MHz, 4.2 cm) transmitter on DSS-13 and receiving at Green Bank with the new radar backend.Optical lightcurves obtained prior to closest approach indicated that HM10 has a spin period of ~22.2 minutes and an elongated shape (W. Ryan, pers. comm). The delay-Doppler radar images confirm the rotation period estimated from photometry and reveal that HM10 has a long-axis extent of 80-100 m with an equatorial aspect ratio of about 2:1. Radar speckle tracking transmitting from Arecibo and receiving with the VLBA on July 6 rule out any non-principal axis ‘wobble’ with an amplitude greater than ~10º.HM10’s rapid rotation implies significant cohesion, with a minimum tensile strength of 25-150 Pa required at its center to prevent disruption, assuming overall bulk density between 0.7 and 3.9 g cm^-3. This is comparable to strength predictions for rubble-pile aggregates (e.g. Scheeres, Britt, Carry, & Holsapple 2015, Asteroids IV, in press). HM10 is not necessarily a ‘monolith’.HM10’s shape is complex and irregular. The radar images show angular features and ‘facets’ up to ~30 m across. There is also a cluster of radar-bright pixels that tracks with HM10’s rotation, consistent with a high standing feature 15-20 m across. This feature is similar in appearance to radar images of decameter-scale boulders on other asteroids.