Earth filmed as alien world
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: July 18, 2008

From a distance of 31 million miles, NASA's Deep Impact spacecraft, now operating under the name of EPOXI, has captured the transit of the Moon as it passes in front of the Earth, an observation that will help scientists develop new techniques to study alien worlds.

"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," says Michael A'Hearn, principal investigator for EPOXI.

The Moon passing in front of the Earth as seen by the EPOXI spacecraft from a distance of 31 million miles. In this video, the Earth has been observed at infrared-green-blue wavlengths. Video: Donald J. Lindler, Sigma Space Corporation/GSFC; EPOCh/DIXI Science Teams.

The spacecraft first made history as ‘Deep Impact’, when it launched a coffee-table sized penetrator into comet Tempel-1 on July 4, 2005, to learn about the composition of the comet’s surface and interior. After this highly successful mission phase was complete, Deep Impact was reborn as the EPOXI mission, which will culminate in a rendezvous with comet Hartley 2 in 2010.

EPOXI is a fusion of the names for the two extended mission components: a search for extrasolar planets during the cruise to Hartley 2, called Extrasolar Planet Observations and Characterisation (EPOCh), and the flyby of comet Hartley 2, called the Deep Impact eXtended Investigation (DIXI).

As part of the EPOCh mission phase, the spacecraft has already sought out red dwarf star GJ436, which has a Neptune-sized planet, or ‘super-Earth’ orbiting it (read story), and has now looked back towards its home planet, watching the Moon as it journeys around the Earth.

In this video, the Earth has been observed through a red-green-blue filter, to pick out differences in detail compared with the video above. Video: Donald J. Lindler, Sigma Space Corporation/GSFC; EPOCh/DIXI Science Teams.

Images were obtained at 15 minute intervals for a full Earth rotation, and were combined to make colour videos. Although other spacecraft have imaged the Earth and Moon from space, EPOXI is the first to show a transit of Earth with enough detail to distinguish large craters on the Moon and oceans and land masses on Earth. Mission scientists are using this video to learn more about locating alien worlds.

"Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars," says Drake Deming of NASA's Goddard Space Flight Centre in Greenbelt. "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. Also, we used infrared light instead of the normal red light to make the colour composite images, and that makes the land masses much more visible."

This is because plants reflect more strongly in the infrared, and so looking for variations in the intensity of near-infrared light as an extra-solar planet rotates around its parent star could reveal vegetated land masses. Combined with the ‘sun glint’ trick to betray the presence of an ocean, scientists have made an important step on the way to identifying potentially habitable worlds elsewhere in the Galaxy.

The mission team also speculate whether a hypothetical alien civilisation could use a similar technique to be able to detect the presence of life on Earth from their distant viewpoint.

"To image Earth in a similar fashion, an alien civilisation would need technology far beyond what Earthlings can even dream of building," says Sara Seager, a co-investigator on EPOXI. "Nevertheless, planet-characterising space telescopes under study by NASA would be able to observe an Earth twin as a single point of light, a point whose total brightness changes with time as different land masses and oceans rotate in and out of view.”