Goldstone radar evidence for short-axis mode non-principal axis rotation of near-Earth asteroid (214869) 2007 PA8

Wednesday, 17 December 2014
Marina Brozovic1, Lance Benner1, Christopher Magri2, Michael Busch3, Daniel Jay Scheeres4, Jon D Giorgini1, Vishnu Reddy5, Michael D Hicks1, Joseph S Jao1, Clement G Lee1, Lawrence G Snedeker6, Marc A Silva6, Martin A Slade1 and Kenneth J Lawrence1, (1)NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States, (2)University of Maine at Farmington, Farmington, ME, United States, (3)SETI Institute Mountain View, Mountain View, CA, United States, (4)University of Colorado Boulder, Boulder, CO, United States, (5)Planetary Science Institute Tucson, Tucson, AZ, United States, (6)SAITECH, Goldstone Deep Space Communication Complex, CA, United States
We report Goldstone radar (8560 MHz, 3.5 cm) observations and shape and spin state modeling of near-Earth asteroid (214869) 2007 PA8. We observed 2007 PA8 on 16 days between Oct 16-Nov 13, 2012 when the asteroid was within 0.14 AU of Earth. Closest approach was on Nov 5 at a distance of 0.043 AU (17 lunar distances). Images obtained with Goldstone's new chirp system achieved range resolution as fine as 3.75 m, placing thousands of pixels on the asteroid’s surface and revealing that 2007 PA8 is an elongated, asymmetric object. Surface features include angularities, multiple facets, and a concavity approximately several hundred meters in diameter. We used the Shape software [1, 2] to estimate the asteroid's 3D shape and spin state. The initial shape of the asteroid was parameterized as an ovoid with dimensions of 1.85 kmx1.25 kmx1.20 km in principal axis (PA) rotation with rotational rates of 80-100 deg/day to match the progression of features visible in the images. This yielded two candidate spin states: one near the south ecliptic pole and another near ecliptic longitude and latitude of (270, +17) deg. However, PA spin state models predict that images from Oct 31 and Nov 11 should be very similar, but the images on those two days appear dramatically different. As a result, we expanded the spin state search to include non-principal axis (NPA) rotation. The best fit was obtained with NPA rotation in short-axis mode with an average period of precession by the long axis around the angular momentum vector of 4.25 days and an oscillatory period around the long axis of 20.16 days. The amplitude of rolling around the long axis is 42 deg. The angular momentum vector points within 10 deg of ecliptic longitude and latitude of (273, +16) deg. 2007 PA8 is only the second confirmed short-axis mode NPA rotator found in the near-Earth asteroid population, after (99942) Apophis [3].


[1] Hudson, S., 1993. Remote Sens. Rev. 8, 195-203.

[2] Magri, C. et al., 2007. Icarus 186, 152-177.

[3] Pravec et al., 2014. Icarus 233, 48-60.