Yesterday, Deep Impact performed a trajectory correction maneuver, firing its thrusters to line up for a flyby seven years from now. Deep Impact, you might recall, launched January 12, 2005 and flew past comet Tempel 1 on July 4 of the same year. It went on to a bonus encounter with Hartley 2 on November 25, 2010. At the time of the Hartley 2 flyby, we were told that Deep Impact was too low on fuel to be able to set up a third encounter. Since then, though, navigators have identified a target close to Deep Impact's trajectory: asteroid 163249 (2002 GT). The October 4, 2012 rocket burn steered Deep Impact toward a January 4, 2020 flyby of the asteroid.
Asteroid 163249 is pretty small, probably around 800 (plus or minus 400) meters across. That would make it the second-smallest minor planet to be visited by a spacecraft, between Itokawa and Hartley 2 in size -- but by the time Deep Impact gets to it, OSIRIS-REx will have visited a similar-sized one, 101955 (1999 RQ36), which is, by the way, the subject of the Name That Asteroid contest, still ongoing.
Asteroid 163429's trajectory classifies it as a "potentially hazardous asteroid." Not a lot is known about its physical properties, but information should be better by this time next year. That's because it will have a reasonably close approach to Earth next year, passing within 18 million kilometers on June 26. As a possible target of future exploration, there will undoubtedly be lots of telescopes pointed in its direction.
I asked Jessica Sunshine for some details on this proposed encounter, and here's the basic outline (although details could and probably would change). Deep Impact's possible January 4, 2020 flyby would be a nice one, at a close-approach distance of around 200 kilometers and a relative velocity of 7 kilometers per second. That's both closer and slower than the Hartley 2 flyby (which was at 700 kilometers and 12 kilometers per second). Deep Impact would approach the asteroid from its night side, seeing it at very high phase, which means the best science would happen on the way out, with the asteroid at lower phase of around 40 degrees. The encounter would happen while OSIRIS-REx is operating at asteroid 101955, a fun juxtaposition much like the dual comet encounters that happened with Hartley 2 and Tempel 1 two years ago.
But do take notice of the conditional tense I'm using here. A mission extension that would allow Deep Impact to be kept in operation through January 2020 has been proposed, but not yet approved. And we all know the state of NASA's planetary exploration budget. I know that, historically, Deep Impact has done a stellar job of sticking within budget, devising operational tools that keep ongoing costs relatively low. And it would be really cool to be able to compare and contrast asteroids 163429 and 101955. The Hartley 2 encounter was extremely productive, scientifically; there's every reason to expect asteroid 163249 to be full of surprises. But while yesterday's rocket firing keeps Deep Impact's options open, it does not guarantee that NASA will exercise that option.
Deep Impact is now on course to fly close by asteroid 163429 no matter what. I hope that we can watch the encounter through its camera when it does!