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.

Moon transits Earth

A single frame from a 24-hour time-lapse video shows the Moon approaching Earth's night side. The full Quicktime video is here (1 MB), and a version using a near-infrared filter is here. Notice how much darker the Moon's surface is than Earth's — something we rarely see compared so well.

D. Lindler (Sigma Space / NASA-GSFC) / EPOXI science teams

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.

Learn the how and why of it from NASA's press release, or just view/download the QuickTime video here. (A second version, utilizing an infrared channel that makes landmasses more obvious, is here.)


Image of Lee Tune

Lee Tune

July 23, 2008 at 5:05 pm

This is a terrific article about a great mission. Its readers should also know that Deep Impact and its follow-on mission EPOXI are both University of Maryland-led NASA missions with UM astronomer Michael A'Hearn the principal investigator for both missions. Numerous other University of Maryland scientists also are, or have been, deeply involved in these missions. NASA deserves great kudos for these missions. But so do the University of Maryland scientists whose knowledge and passion are most responsible for Deep Impact and its new mission, and who have overseen almost every step along the way and devoted years of work to them. It’s true that I’m completely biased on this subject as I’ve been covering/promoting Deep Impact for the university for ten years. However, to me, talking about Deep Impact without mentioning the University of Maryland is rather like failing to mention Christopher Columbus and attributing the discovery of America solely to Queen Isabella because she supplied the ships and money for the trip.

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