Radar Observations and the Shape Near-Earth Asteroid 2008 EV5

Michael W. Busch, Steven J. Ostro, Lance A. M. Benner, Marina Brozovic, Jon D. Giorgini, Joseph S. Jao, Daniel J. Scheeres, Christopher Magri, Michael C. Nolan, Ellen S. Howell, Patrick A. Taylor, Jean-Luc Margot, Walter Brisken

Icarus 212, 649-660 (2011)


We observed near-Earth asteroid 2008 EV5 with the Arecibo and Goldstone planetary radars and the Very Long Baseline Array during December 2008. EV5 rotates retrograde and its overall shape is a 400 ± 50 m oblate spheroid. The most prominent surface feature is a ridge parallel to the asteroid's equa- tor that is broken by a concavity about 150 m in diameter. Otherwise the asteroid's surface is notably smooth on decameter scales. EV5's radar and optical albedos are consistent with either rocky or stony- iron composition. The equatorial ridge is similar to structure seen on the rubble-pile near-Earth asteroid (66391) 1999 KW4 and is consistent with YORP spin-up reconfiguring the asteroid in the past. We interpret the concavity as an impact crater. Shaking during the impact and later regolith redistribution may have erased smaller features, explaining the general lack of decameter-scale surface structure.

A companion website describing radar speckle tracking is available here.

Fig. 1. Arecibo delay-Doppler images of 2008 EV5 from 2008 December 23-27. Resolution is 0.05 us x 0.0625 Hz (7.5 m x 7.9 mm/s). Within each image, Doppler frequency increases from left to right and distance from Earth from top to bottom. These images are sums of six Arecibo runs each and each covers 7° of rotation phase. The asteroid appears to rotate counter-clockwise. In the collage, time increases from top to bottom and left to right.

Fig. 2. Enlargements of several images from Fig. 1, selected to show the concavity (marked with arrows) and its position on the object. The labels give the observation time and rotation phase (Table 1).

Fig. 3. Delay-Doppler images of 2008 EV5 from Goldstone and Arecibo used in our shape modeling, corresponding synthetic images generated from the shape model, and plane-of-sky projections of the model. Time increases from top to bottom and left to right. Arecibo images cover 7° of r otation; Goldstone images cover 5° of rotation and are spaced tens of degrees apart . Delay-Doppler images are oriented as in Fig. 1. Plane-of-sky images are oriented with north up and east to the left.

Fig. 4. Principal axis views of our 2008 EV5 shape model. The model is viewed from six orthogonal directions, along its principal axes. Rotation is around the z-axis, with +z in the direction of the angular momentum vector. Yellow-shaded regions were seen only at incidence angles >45° or not seen at all.

Fig. 5. Top: Geopotential mapped as equivalent velocity over the surface of the EV5 shape model, assuming a bulk density of 3 g/cm^3 (equivalent velocity = (2 x geopotential)^-1/2). Viewing directions as in Fig. 4. The equatorial ridge is at higher potential than the mid-latitudes. Note the gravitational minimum centered on the concavity (outlined by the black dashed line). The lowest potential point in the model is slightly offset to the north of the equator, but the true low point may be elsewhere in the concavity (the position of the minimum is sensitive to sub-resolution changes in the shape). Bottom: Facet-scale gravitational slope (angle between the local acceleration and inward normal vectors), mapped across the shape model's surface and viewed from the +x axis. The highest slope point occurs along the eastern edge of the concavity, but the only systematic slope feature is the generally lower slopes in the mid- latitudes relative to the ridgeline and to the poles.

Last update: 2011 October 06