Deep Impact ejected formation at Tempel 1. From http://deepimpact.umd.edu/gallery/T1_Ejecta_Devel.html
NASA's Stardust spacecraft just performed a trajectory correction maneuver to fine tune the timing of its encounter with the comet Tempel 1. Stardust, you'll recall, returned dust samples from the comet Wild 2 and now is in an extended mission to encounter Tempel 1. That latter comet was impacted by a projectile from the Deep Impact spacecraft. The primary goal of the Stardust encounter will be to image the crater produced by the Deep Impact projectile. The depth and form of that crater will tell us a lot about the surface properties of Temple 1. Imaging the crater was a key goal of the Deep Impact mission, but the crater could not be seen through the ejected cloud that resulted from the impact.
Expected views of the Deep Impact Crater from the Feb. 14, 2011 Stardust encounter.
There is a nice two page fact sheet on the Stardust Tempel 1 encounter at http://stardustnext.jpl.nasa.gov/mission/pdfs/SD_NEXT_Fctsht.pdf. (Thanks to Emily Lakdawalla at the Planetary Society for the link.)
Meanwhile, the Deep Impact craft is on it way to its own second comet encounter with Hartley 2 on Nov. 4 of this year (http://epoxi.umd.edu/2science/objectives.shtml). Look about 2/3 of the way down this page for a nice explanation of the geology of Tempel 1, under the heading, Talps and Layers. Essentially, the surface of Tempel 1 is composed of layered piles (talps) that may have formed from low speed impacts. If you are interested in comets, this is an interesting read on the science that flyby missions can do.