NASA's Deep Impact
Films Earth As An Alien World
(17 July 2008) NASA's Deep Impact
spacecraft has created a video of the moon transiting (passing in front of)
Earth as seen from the spacecraft's point of view 50 million kilometres (31
million miles) away.
Scientists are using the video to develop
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," said
University of Maryland astronomer Michael A'Hearn, principal investigator for
the Deep Impact extended mission, called Epoxi.
Deep Impact made history
when the mission team directed an impactor from the spacecraft into comet
Tempel 1 on July 4, 2005. NASA recently extended the mission, redirecting the
spacecraft for a flyby of comet Hartley 2 on Nov. 4, 2010.
Epoxi is a
combination of the names for the two extended mission components: a search for
alien (extrasolar) planets during the cruise to Hartley 2, called Extrasolar
Planet Observations and Characterization (EPOCh), and the flyby of comet
Hartley 2, called the Deep Impact eXtended Investigation (DIXI).
During
a full Earth rotation, images obtained by Deep Impact at a 15-minute cadence
have been combined to make a colour video. During the video, the moon enters
the frame (because of its orbital motion) and transits Earth, then leaves the
frame. Other spacecraft have imaged Earth and the moon from space, but 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.
"To image Earth
in a similar fashion, an alien civilisation would need technology far beyond
what Earthlings can even dream of building," said Sara Seager, a planetary
theorist at the Massachusetts Institute of Technology, Cambridge, Mass., and 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. 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." said Seager.
"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. Deming is deputy principal investigator for Epoxi, and leads the EPOCh
observations. "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." That happens because plants reflect
more strongly in the near-infrared, Deming explained. Hence the video
illustrates the potential for detecting vegetated land masses on extrasolar
planets by looking for variations in the intensity of their near-infrared light
as the planet rotates.
The University of Maryland is the Principal
Investigator institution, leading the overall Epoxi mission, including the
flyby of comet Hartley 2. NASA Goddard leads the extrasolar planet
observations. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages Epoxi
for NASA's Science Mission Directorate, Washington. The spacecraft was built
for NASA by Ball Aerospace & Technologies Corp., Boulder,
Colo.
(source: NASA Jet Propulsion Laboratory)
