Lance A. M. Benner, Michael C. Nolan, Steven J. Ostro, Jon D. Giorgini, Donald P. Pray, Alan W. Harris, Christopher Magri, and Jean-Luc Margot 2006. Icarus 182, 474-481. [pdf]
Arecibo (2380 MHz, 13 cm) radar observations of 2005 CR37 provide detailed images of a candidate contact binary: A 1.8-km-long, extremely bifurcated object. Although the asteroid's two lobes are round, there are regions of modest topographic relief, such as an elevated, 200-m-wide facet, that suggest that the lobes are geologically more complex than either coherent fragments or homogeneous rubble piles. Since January, 1999, about 9% of NEAs larger than ~200 m imaged by radar can be described as candidate contact binaries.
This collage shows a sequence of 38 runs spanning an interval of 1.6 hours. Images are arranged chronologically, with time increasing from left to right and from top to bottom. In each image, range increases from top to bottom and Doppler frequency increases from left to right, so rotation is counterclockwise. The resolution is 0.1 usec (15 m) x 0.07 Hz (~14 m for an equatorial view).
Close-up showing features on the surface.
The complex surface morphology on 2005 CR37 is somewhat more rugged than the generally smooth shapes produced by simulations of collisions or tidal distortion of agglomerates of gravitationally-bound spheres. The raised facet suggests to us that 2005 CR37 might be at least partially fractured and hence not monolithic. Perhaps its internal structure consists of an assemblage of irregularly-shaped blocks that span a distribution of sizees.
How abundant are contact binaries in the NEA population? Since January, 1999, about 9% of radar-imaged NEAs larger than ~200 m have bifurcated shapes and can be considered candidate contact binaries. The abundance of true binaries above the same size threshold imaged by radar during the same interval is about 17%. If contact binaries and true binaries form by the same mechanisms, as has been suggested theoretically, then both types of objects should exist. Is there really a two-fold difference in the abundance of binaries and contact binaries in the NEA population?
Which way is the evolution of 2005 CR37 going? Is it a failed binary that collapsed back into a single object? Or is it a rubble pile being spun up by YORP or planetary tides that is on the verge of fission into a true binary?