The first images and other data from Rosetta's first asteroid flyby - the first nominal science phase of the long mission - have just been presented at a news conference at the control center: A sequence of images from the wide angle channel of the OSIRIS camera unveils a diamond-shaped body of 5.9 x 4 km with 23 craters on the side seen that are larger than 200 meters. The two biggest craters are 2 km each: The impacts that caused them must have fractured the whole body. The shape of which had been predicted very well from previous ground- and space-based photometry, but the geometrical albedo of 35±5 % is lower than expected, the body thus a bit larger.
A major mystery has already emerged: There is a crater chain (catena) of 7 craters, something seen only on our Moon and icy outer planet satellites so far. The mechanism that caused it (secondary cratering or an impact of a swarm of bodies) is pretty unclear. There is some regolith on Steins as indicated by shallow, degraded craters that seem to be filled up by some degree (so the cratering history is complex), and the overall color of Steins is grey. A lot more data from several instruments will be downlinked in the coming days, so a fuller understanding of yet another small solar system body will emerge soon from Europe's first visit to an asteroid. Already a few VIRTIS spectra are in - and data from the GIADA dust detector: no impacts at all, as expected for a flyby in 800 km distance. Which missed the target by less than 2 km.