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Mars Rovers Watching Solar Eclipses By Twin Martian Moons

(L) Deimos Mar 4 - Phobos Mar 7
Pasadena - Mar 09, 2004
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.

Phobos and the smaller, more distant satellite, Deimos, were discovered in 1877 by Asaph Hall, an astronomer at the United States Naval Observatory in Washington, D.C. Hall had been hunting for martian satellites for some time, and was about to abandon the search when he was encouraged by his wife to continue.

In honor of her role in the discovery, the largest crater on Phobos was named Stickney, her maiden name. In a strange case of prescience, author Jonathan Swift wrote a century and a half earlier in his Gulliver's Travels (1726) that " [The astronomers]..have likewise discovered two lesser stars, or satellites, which revolve about Mars."

Related Links
Mars Rovers at JPL
Mars Rovers at Cornell
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Aloha Mars - The Advanced Course
Honolulu - Feb 13, 2004
My friend Dr. X is really getting interested in Mars. Yesterday he challenged me about those mysterious spherical beads at the Opportunity landing site, writes Jeffrey Bell.



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