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Earth filmed as “alien” world

July 25, 2008
Courtesy NASA
and World Science staff

The first space­craft from Earth to have stud­ied a com­et up-close has tak­en on a new proj­ect: film­ing our own plan­et from a vast dis­tance. The aim is to partly ap­prox­i­mate what we might some­day see if we found an­oth­er plan­et with life.

NASA’s Deep Im­pact space­craft has cre­at­ed a vi­deo of the moon pass­ing be­fore Earth as seen from the space­craft’s point of view 31 mil­lion miles (50 mil­lion km) away.

A still from a NA­SA vi­deo show­ing Earth from 31 mil­lion miles away. The di­a­gram at low­er left clar­i­fies the po­si­tion of Earth in this view.  The down­load­able vi­deo (Quick­time for­mat) exists in two ver­sions cor­res­pond­ing to dif­fer­ent wave­lengths of light; the films can be seen here and here. (Cred­it: Don­ald J. Lindler, Sig­ma Space Cor­po­ra­tion/GSF; EPOCh/DIXI Sci­ence Teams)


“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 Uni­ver­s­ity of Mar­y­land as­tron­o­mer Mi­chael A’Hearn, prin­ci­pal in­ves­ti­ga­tor for the Deep Im­pact ex­tend­ed mis­sion.

Deep Im­pact made his­to­ry when sci­en­tists on its mis­sion team di­rect­ed an im­pactor from the un­manned craft in­to com­et Tem­pel 1 on July 4, 2005. The agen­cy lat­er ex­tend­ed the mis­sion, redi­rect­ing the craft for a fly­by of com­et Hart­ley 2 on Nov. 4, 2010. 

The ex­tend­ed mis­sion, called EPOXI, is a com­bina­t­ion of the names for the two ex­tend­ed mis­sion com­po­nents: a search for plan­ets be­yond our so­lar sys­tem dur­ing the cruise to Hart­ley 2, and the fly­by of that comet. 

In the film­ing of Earth, im­ages ob­tained by Deep Im­pact at 15-minute in­ter­vals were com­bined to make a col­or vi­deo in which the moon en­ters the frame, passes by Earth, then leaves the frame. Oth­er space­craft have im­aged Earth and moon from space, but Deep Im­pact is the first to show a moon trans­it with enough de­tail to see large craters on the moon and oceans and con­ti­nents on Earth.

“To im­age Earth in a si­m­i­lar fash­ion, an al­ien civ­il­iz­a­tion would need tech­nol­o­gy far be­yond what Earth­lings can even dream of,” said Sara Sea­ger, a plan­etary the­o­rist at the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy and co-in­ves­ti­ga­tor on the ex­tend­ed mis­sion. While seem­ingly huge, the dis­tance to the craft dur­ing its film­ing was ti­ny in cos­mic terms: less than one-hundredth the dis­tance to the fur­thest plan­ets in our own so­lar sys­tem, let alone oth­ers.

Yet “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,” Sea­ger said. “The vi­deo will help us con­nect a var­y­ing point of plan­etary light with un­der­ly­ing oceans, con­ti­nents, and cloud­s—and find­ing oceans on ex­tra­so­lar plan­ets means iden­ti­fy­ing po­ten­tially habita­ble worlds.”

“Our vi­deo shows some spe­cif­ic fea­tures that are im­por­tant for ob­serva­t­ions of Earth-like plan­ets or­bit­ing oth­er stars,” said Drake Dem­ing of the agen­cy’s God­dard Space Flight Cen­ter in Green­belt, Md., dep­u­ty prin­ci­pal in­ves­ti­ga­tor for EPOXI. 

“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 col­or com­pos­ite im­ages, and that makes the land mass­es much more vis­i­ble.” That hap­pens be­cause plants re­flect more strongly in near-in­fra­red light, a type of light with slightly less en­er­gy than the type vis­i­ble to the na­ked eye, Dem­ing ex­plained. 

The Uni­ver­s­ity of Mar­y­land leads the over­all EPOXI mis­sion, while NASA God­dard leads the extra­so­lar plan­et ob­serva­t­ions.


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The first spacecraft from Earth to have studied a comet up-close has taken on a new project: filming our own planet from a vast distance. The aim is to partly approximate what we might someday see if we found another planet with life. NASA’s Deep Impact spacecraft has created a video of the moon passing before Earth as seen from the spacecraft’s point of view 31 million miles (50 million km) away. “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 University of Maryland astronomer Michael A’Hearn, principal investigator for the Deep Impact extended mission. Deep Impact made history when scientists on its mission team directed an impactor from the unmanned craft into comet Tempel 1 on July 4, 2005. The agency later extended the mission, redirecting the craft for a flyby of comet Hartley 2 on Nov. 4, 2010. The extended mission, called EPOXI, is a combination of the names for the two extended mission components: a search for planets beyond our solar system during the cruise to Hartley 2, and the flyby of that object. In the filming of Earth, images obtained by Deep Impact at 15-minute intervals were combined to make a color video in which the moon enters the frame, passes by Earth, then leaves the frame. Other spacecraft have imaged Earth and moon from space, but Deep Impact is the first to show a moon transit with enough detail to see large craters on the moon and oceans and continents on Earth. “To image Earth in a similar fashion, an alien civilization would need technology far beyond what Earthlings can even dream of,” said Sara Seager, a planetary theorist at the Massachusetts Institute of Technology and co-investigator on the extended mission. While seemingly huge, the distance to the craft during its filming was tiny in cosmic terms: less than one-hundredth the distance to the furthest planets in our own solar system, let alone others. Yet “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,” Seager said. “The video will help us connect a varying point of planetary light with underlying oceans, continents, and clouds—and finding oceans on extrasolar planets means identifying potentially habitable worlds.” “Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars,” said Drake Deming of the agency’s Goddard Space Flight Center in Greenbelt, Md., deputy principal investigator for EPOXI. “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 color composite images, and that makes the land masses much more visible.” That happens because plants reflect more strongly in near-infrared light, a type of light with slightly less energy than the type visible to the naked eye, Deming explained. The University of Maryland leads the overall EPOXI mission, while NASA Goddard leads the extrasolar planet observations.