E-Type Asteroid (2867) Steins as Imaged by OSIRIS on Board Rosetta

Smooth Space Pebble In September 2008, on its way to meet comet 67P/Churyumov-Gerasimenko, the Rosetta spacecraft flew by asteroid Steins, a member of a very rare class of asteroids that had never been observed closely by spacecraft. Keller et al. (p. 190) analyzed the images to generate a reconstruction of the asteroid's shape. Steins is oblate with an effective spherical diameter of 5.3 kilometers, and it lacks small craters, which may have been erased by surface reshaping. Indeed, Steins's shape resembles that of a body that was spun-up by the YORP effect—a torque produced by incident sunlight, which can alter the rotation rate of a small body—that causes material to slide toward the equator. This effect may have produced Steins's distinctive diamond-like shape. Incident sunlight probably caused this asteroid to spin, which redistributed its mass and smoothed its surface. The European Space Agency’s Rosetta mission encountered the main-belt asteroid (2867) Steins while on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. Images taken with the OSIRIS (optical, spectroscopic, and infrared remote imaging system) cameras on board Rosetta show that Steins is an oblate body with an effective spherical diameter of 5.3 kilometers. Its surface does not show color variations. The morphology of Steins is dominated by linear faults and a large 2.1-kilometer-diameter crater near its south pole. Crater counts reveal a distinct lack of small craters. Steins is not solid rock but a rubble pile and has a conical appearance that is probably the result of reshaping due to Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) spin-up. The OSIRIS images constitute direct evidence for the YORP effect on a main-belt asteroid.

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