March 8, 2004

Donald Savage 
Headquarters, Washington 
(Phone: 202/358-1547)

Guy Webster 
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-5011) 

RELEASE: 04-085

NASA ROVERS WATCHING SOLAR ECLIPSES BY MARS MOONS

NASA's Mars Exploration Rovers have become eclipse watchers. 

Though the Viking Landers in the 1970s observed the shadow of one 
Mars' two moons, Phobos, moving across the landscape, and Mars 
Pathfinder in 1997 observed Phobos emerge at night from the shadow of 
Mars, no previous mission has ever directly observed a moon pass in 
front of the sun from the surface of another world.

The current rovers began their eclipse-watching campaign this month. 
Opportunity's panoramic camera caught Mars' smaller moon, Deimos, as 
a speck crossing the disc of the sun on March 4. The same camera then 
captured an image of the larger moon, Phobos, grazing the edge of the 
sun's disc on March 7. 

Rover controllers at NASA's Jet Propulsion Laboratory (JPL), Pasadena, 
Calif., are planning to use the panoramic cameras on both Opportunity 
and Spirit for several similar events in the next six weeks. Dr. Jim 
Bell of Cornell University, Ithaca, N.Y., lead scientist for those 
cameras, expects the most dramatic images may be the one of Phobos 
planned for March 10. 

"Scientifically, we're interested in timing these events to possibly 
allow refinement of the orbits and orbital evolution of these natural 
satellites," Bell said. "It's also exciting, historic and just plain 
cool to be able to observe eclipses on another planet at all," he 
said.

Depending on the orientation of Phobos as it passes between the sun 
and the rovers, the images might also add new information about the 
elongated shape of that moon. 

Phobos is about 27 kilometers long by about 18 kilometers across its 
smallest dimension (17 miles by 11 miles). Deimos' dimensions are 
about half as much, but the pair's difference in size as they appear 
from Mars' surface is even greater, because Phobos flies in a much 
lower orbit. 

The rovers' panoramic cameras observe the sun nearly every martian day 
as a way to gain information about how Mars' atmosphere affects the 
sunlight. The challenge for the eclipse observations is in the 
timing. Deimos crosses the sun's disc in only about 50 to 60 seconds. 
Phobos moves even more quickly, crossing the sun in only 20 to 30 
seconds. 

Scientists use the term "transit" for an eclipse in which the 
intervening body covers only a fraction of the more-distant body. For 
example, from Earth, the planet Venus will be seen to transit the sun 
on June 8, for the first time since 1882. Transits of the sun by 
Mercury and transits of Jupiter by Jupiter's moons are more common 
observations from Earth.

From Earth, our moon and the sun have the appearance of almost 
identically sized discs in the sky, so the moon almost exactly covers 
the sun during a total solar eclipse. Because Mars is farther from 
the sun than Earth is, the sun looks only about two-thirds as wide 
from Mars as it does from Earth. However, Mars' moons are so small 
that even Phobos covers only about half of the sun's disc during an 
eclipse seen from Mars. 

JPL, a division of the California Institute of Technology in Pasadena, 
manages the Mars Exploration Rover project for NASA's Office of Space 
Science, Washington. Images and additional information about the 
project are available from JPL at:

Images of those two events are available online at: 

http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040308a.html 

http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040308a.html 

http://marsrovers.jpl.nasa.gov 

and from Cornell University at: 

http://athena.cornell.edu 

	
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