The Large Crater on Asteroid Steins: Is It Abnormally Large?

Introduction: The Rosetta mission to comet 67/P Churyumov-Gerasimenko is also scheduled to do two asteroid fly bys. The first of these was on Sept. 5 th , 2008, when the spacecraft flew within 800 km of asteroid 2867 Steins. The timing of the encounter meant that the surface of the asteroid (as seen from the spacecraft) was well illuminated, providing good imaging conditions. As well as providing an opportunity for checking out the instruments on the Rosetta spacecraft, the encounter also provided high quality data concerning this E type asteroid. Full publication of this data is eagerly awaited, but early reports noted a major crater like feature on the asteroid. Asteroid Steins is a small, irregularly shaped object. Before the encounter, its size had been estimated as typically 4.6 km based on its light curve and albedo [1-3]. Estimates of the albedo vary somewhat, an initial 0.45±0.10 was determined by polarimetry [1] but was later lowered to 0.34±0.06 based on modeling of the spectrum over the range 5 – 38 μm [3]. Features in its spectra suggest an enstatite like composition and an E[II] type asteroid classification [4-5]. The asteroid is located at 2.36 AU, i.e., well within the main belt, and rotates with a period initially estimated as 6.052±0.007 hrs [6], and later refined as 6.04681±0.00002 hrs [3]. Based on 26 visible light curves, obtained from ground telescopes and the Rosetta spacecraft itself, a shape model of the asteroid was made in [3] which gave the 3 axis dimensions as 5.73±0.52, 4.95±0.45, and 4.58±0.41 km, where the ratios of the main axes were given as a/b = 1.17 and a/c = 1.25 and total volume as 64.3 km 3 . One aspect taken from the shape model in [3] is shown in Fig. 1. Observations: Shortly after the encounter the Rosetta team publicly showed images taken of the asteroid during the flyby. A typical montage (source the ESA web site accessed Dec 2008: http://www.esa.int/esammg/mmg.pl?mission=Rosetta&type=I) is shown in Fig. 2. The agreement with the shape model developed preencounter is testament to the precision of that work. However, a major type of surface feature can now be observed which could not be previously modeled based on light curves, namely craters Craters: Three types of craters were noted on the surface of asteroid Steins by the Rosetta team in their initial press announcements; small craters randomly distributed across the surface, a chain of (small) craters Fig. 1 One view of the pre-encounter 3 dimensional shape model of asteroid Steins. Taken from Fig. 12 in [3].