The European Rosetta mission is on its way to rendezvous with a comet (and deploy a lander!), but on its way there it's making a flyby of an asteroid. Called 2867 Steins (wondering about the title of this post?), the rock is about 5 kilometers across (roughly as big as the mountains I see out my window), and Rosetta will pass just 800 kilometers from it. The encounter will happen on Friday September 5 at 20:58:16 CEST (18:58 UT or 14:58 Eastern US time).
Emily, as always, has the info. That link goes to a specific blog post, but keep her blog handy on Friday and this weekend. She'll be posting more, as will I, including pictures from the flyby. Rosetta also has a blog of its own, which will no doubt have even more.