That's not as scary as it sounds. Deep Impact is a spacecraft that was so successful in its original mission to observe the impact of a 370 kilogram copper block into the comet Tempel 1. That was back in July of 2005. But the spacecraft itself was still operating, so NASA decided to extend its mission, now called EPOXI. It will fly by comet Hartley 2 in 2010, and to get there it needs a little boost. It'll pass by the Earth tomorrow, on December 31, to steal give us a bit of its orbital energy as we circle the Sun. This willadd take away energy from the spacecraft, changing its orbit it a bit (it'll need two more flybys to put it on the right trajectory).
This is very cool, and lots of people are looking for the tiny spacecraft as it approaches us. It's faint; at magnitude 20 it's about 0.000001 as bright as the faintest star you can see with your eyes, so you'll need a big 'scope! But it helps to have observatories look, because they can confirm the trajectory of the spacecraft and make sure everything is on track.
Tomorrow I expect some images of the little probe will be online. Stay Tuned.
Tip o' the Whipple Shield to Emily!
Here are some other Bad Astronomy posts about comet Hartley 2. Use the thumbnails and arrows to browse, and click on the images to go through to the posts with more details and descriptions.