Steins is an E-type asteroid that was visited by ESA's Rosetta spacecraft on September 5, 2008, with the closest approach taking place at 18:38:20 UTC at a distance of 803 kilometers. Rosetta's OSIRIS high-resolution camera system imaged about 60 percent of Steins' surface during the flyby. Unfortunately, due to stringent limits set on onboard fault protection software, the narrow-angle camera component of OSIRIS (the highest-resolution camera instrument on Rosetta) went into "safe mode" before closest approach, but the wide-angle camera returned photos throughout the encounter. This description of the asteroid is based upon an article published in the January 8, 2010 issue of Science by H. Uwe Keller and numerous coauthors and on a related press release (in German).
Related stories on planetary.org.
Rosetta's images showed Steins to have the shape of a brilliant-cut diamond, with overall dimensions of 6.67 by 5.81 by 4.47 kilometers, equivalent in volume to a sphere with a radius of 2.65 kilometers.
Although the wide-angle camera system on Rosetta is capable of color imaging and took color data on Steins, the resulting images still look gray; Steins shows no significant color variations across its surface. Like other E-type asteroids, it has a high geometric albedo of about 0.4 at visible wavelengths, meaning that it reflects about 40 percent of the light that strikes it. (For comparison, this is roughly four times more reflective than the Moon.)
The south pole of Steins (which is at the top in the Rosetta images) is dominated by a large impact crater 2.1 kilometers in diameter. The impact crater is big enough that if Steins were a solid body, the impact should have shattered it; therefore, Steins was likely already fractured when the crater formed, and is probably even more rubbly in the crater's aftermath. Trailing from the crater to the north is a chain or "catena" of seven circular indentations. Although they look like impact craters, their similar size and geometric alignment suggest that they are not. Rather they are more likely to be collapse pits, where dusty surface material has drained into a subsurface fracture. There is a north-south groove located on the opposite side of Steins as well.
Steins' impact craters can be used to explore its history. Like Ida and Mathilde (visited by Galileo and NEAR, respectively), Steins is not "saturated" with craters, meaning that there are uncratered surfaces in between impact craters. That implies that Steins' surface has been modified since it formed as part of the main belt of asteroids. It is quite likely that the large south polar impact crater -- whenever the impact occurred -- "reset" the surface. Different models for impact cratering processes in the asteroid belt produce widely varying estimates for when this might have happened, ranging from 150 million to 1.5 billion years.
Notably, there are few small impact craters on Steins' surface. Small craters on Steins may be erased due to landslides. Being a small body, Steins is affected by YORP, an acronym for the Yarkovsky-O'Keefe-Radzievskii-Paddack effect, a variety of different mechanisms by which solar radiation exerts torques on small celestial bodies. YORP causes the rotation rate (and even the pole position) of small asteroids to change with time, often increasing the rotation it to very high speeds. The high-speed rotation can alter the shape of "rubble pile" asteroids. In fact, the conical shape of Steins' northern hemisphere matches the shapes acquired by other small asteroids after they have been spun up by YORP; presumably the southern hemisphere also once had this shape before the large impact occurred. Curiously, though, Steins' present rotation rate of once per 6.05 hours is too slow to cause such shape changes.
Rosetta saw Steins at a wide range of phase angles -- from zero ("full") phase to a crescent-like 132 degrees. Studying how the brightness of Steins varies with phase provides clues to the nature of Steins' surface. Steins' surface roughness was found to be similar to Gaspra, and higher than for typical C- and S-type asteroids.
Spectral data from the flyby match well with observations made from Earth and confirm that Steins is an E-type asteroid. E-type asteroids have low iron content and surfaces made of iron-poor minerals such as enstatite (magnesium-rich pyroxene), forsterite (magnesium-rich olivine) and feldspar. Further, the OSIRIS data confirm the presence of an unusual and poorly understood absorption feature in Steins' spectrum at a wavelength of 490 nanometers, which some workers attribute to the presence of sulfides. E-type asteroids are linked with a type of meteorite called an aubrite that is theorized to have formed deep within the mantle of a parent body as crystals that settled out of a solidifying body of rock melt at a temperature above 1000 degrees Celsius. Therefore, Steins is likely a chunk from deep within some primordial planetesimal that was blasted apart in a destructive impact.