NASA?s Deep Impact spacecraft has made a movie of the moon passing in front of the Earth from the probe's vantage point millions of miles away.
Astronomers plan to use the video to develop techniques to look for Earth-like worlds in other solar systems.
"Making a video of Earth from so far away helps the search for other life-bearing planets in the universe by giving insights into how a distant, Earth-like alien world would appear to us," said Michael A'Hearn of the University of Maryland and the principal investigator for the Deep Impact extended mission.
Deep Impact, which sent an impactor into comet Tempel 1 on July 4, 2005, is currently 31 million miles away from Earth on its way to a flyby of comet Hartley 2 on Nov. 4, 2010.
During its cruise to Hartley 2, Deep Impact will be searching for extrasolar planets.
Deep Impact took several images of the Earth during a full planetary rotation; these images have been combined into a color video. During the video, the moon enters the frame as it orbits the Earth and then is shown transiting, or passing in front of, the Earth.
While other spacecraft, including Voyager 1 and Galileo, have imaged Earth and the moon from space, Deep Impact is the first to show a transit of Earth with enough detail to see large craters on the moon and oceans and continents on Earth.
"Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars," said Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Md., and the deputy principal investigator for the extended mission. "A 'sun glint' can be seen in the movie, caused by light reflected from Earth's oceans, and similar glints to be observed from extrasolar planets could indicate alien oceans."
The team used infrared light to look at the Earth because plants reflect more strongly in the near-infrared, so the video will help scientists evaluate the potential for detecting vegetated land masses on alien planets.
Most of the nearly 300 extrasolar planets that have been found to date are Jupiter-sized behemoths, though a few "super-Earths," around four to nine times the mass of our planet, were recently detected.
NASA is currently studying planet-characterizing telescopes that would observe extrasolar planets as a single point of light and would distinguish land masses and oceans by changes in the total brightness, said planetary theorist Sara Seager of MIT and a co-investigator on the Deep impact extended mission.