Radar observations and the shape of near-Earth asteroid 2008 EV5

Abstract

We observed the 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 equator that is broken by a concavity 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.

An investigation of the low-ΔV near-Earth asteroids (341843) 2008 EV5 and (52381) 1993 HA

“…Indeed, the only available spectrum of 2008 EV5 is typical of primitive Ctype asteroids and shows a weak (∼3%) spectral absorption at 0.48 µm, suggesting the presence of alteration minerals with similarities to the CI primitive meteorite Orgueil (Reddy et al 2012). The albedo of 2008 EV5 (0.12±0.04, Busch et al 2011) is also consistent with a C-type taxonomic classification.…”

Shape model and surface properties of the OSIRIS-REx target Asteroid (101955) Bennu from radar and lightcurve observations

“…The overall shape of Bennu is similar to that of 1999 KW 4 and other binary near-Earth asteroids despite the slower rotation rate and lack of a satellite. This result is not unprecedented: 2008 EV 5 has a similar shape and also does not have a detected satellite (Busch et al, 2011). The smooth equatorial region and sloped regions towards the poles may be affected by migration of loose material in a similar way, only without the additional tidal effects of a satellite.…”

“…This extreme difference in pole solution between the ellipsoid solution and the more complex one was surprising, and we believe it is due to the ridged shape of Bennu. The equatorial diameter is substantially (> 10 %) larger than the "average" diameter, and as a result, the limb-to-limb distance that determines the bandwidth is not in general the same as the range depth, which is not well-represented by an ellipsoid shape (Busch et al, 2011). To address this difficulty, we chose a nominal rotation rate within the uncertainty, and searched a grid of possible poles allowing the period to float over a narrow range so that it would "lock in" an acceptable (though not necessarily correct) number of complete rotations with the viewing aspect the same in the two observing windows.…”

“…However, the orientation of the axis in the plane of the sky is always undetermined in a single radar image, and must be determined from the whole set of images and the object's motion over the entire period of observation. More irregular shapes complicate this procedure, and an iterative determination of shape and pole is generally necessary unless other data are available to further constrain the pole (Busch et al 2011). Two separate apparitions usually help enormously to determine the pole orientation (e.g., Ostro et al 2006), unless they happen to have nearly the same viewing geometry.…”

Looking into the evolution of granular asteroids in the Solar System

“…It has been previously explained that this feature could have been formed by an impact [26]. However, in the best case scenario, the probability of having an impact at that precise location and in the required time frame, taking into account the YORP effect, is extremely low (<0.01%).…”