It seems like every spacecraft headed away from Earth to some distant solar-system target takes a moment to look back and record its home planet for posterity. Most often there's a bit of science involved — the imaging instruments use Earth or the Moon as a calibration target. But what gets released to the news media is pure PR.What's different about the image at right is that the NASA spacecraft responsible for it, Deep Impact, is nowhere near us. In fact, this photo shoot took place in late May when the spacecraft was 31 million miles (50 million km) away. That's only a skosh closer than the minimum separation of Mars and Earth during a really good year (as 2003 was).
You might recall that Deep Impact had its 15 minutes of fame when it slammed an artillery-size copper bullet into Comet Tempel 1. That was three years ago. So why is the spacecraft still taking snapshots of the inner solar system?
Deep Impact has been given a second life as a combination comet chaser (next up is Hartley 2 in 2010) and extrasolar-planet sleuth. This new mission has been dubbed EPOXI, for convoluted reasons. The spacecraft recently took a time-lapse video of the Moon transiting in front of a "first-quarter" Earth. Mission scientists think that such long-range photography may give them an edge when it comes to identifying Earthlike worlds around other suns.
Regardless, the result is very cool. Deep Impact took images every 15 minutes throughout a full Earth rotation, and the Moon steals the show during a 4½-hour dash across center stage.