PASADENA, Calif. -- For the first time in history, humans saw a solar eclipse that was out of this world.
Opportunity's panoramic camera, or Pancam, developed and built at Cornell University, captured Deimos (Mars' small moon) crossing the lower part of the sun on March 4, and then saw Phobos (Mars' large moon) grazing the sun's edge on March 7.
Scientists on the Mars rover missions use the Deimos and Phobos eclipses -- or transits, where smaller bodies cross in front of a much larger one -- to refine orbital timing and determine the shapes of the Martian moons. "It's also exciting, historic and just plain cool to be able to observe eclipses on another planet at all," said Jim Bell, Cornell associate professor of astronomy and the lead scientist on the Pancams.
Opportunity's first Deimos image was taken at 3:04 Universal Time on March 4, or 10:04 p.m. Eastern Standard Time on March 3. The first Phobos image was taken at 2:46 Universal Time on March 7, or 9:46 p.m. EST.
Deimos appeared as a tiny speck against the solar disc, much the way Venus and Mercury look from Earth when those planets transit the sun. At its longest dimension, Deimos is only 9 miles in diameter. Phobos, on the other hand, bites a large of chunk of sun when it crosses the solar disc. Although Phobos is only 17 miles in diameter, it is closer to the red planet and so appears larger against the sun. And because Mars is farther from the sun than Earth, the sun appears only about two-thirds as large in the Martian sky.
Rover controllers at NASA's Jet Propulsion Laboratory are planning to use the panoramic cameras on Opportunity and Spirit for similar events in the next six weeks. Bell believes the most dramatic solar eclipse images await as Phobos crosses the sun again on March 10.
Phobos and Deimos enjoy a history as colorful as the red planet. Astronomer Asaph Hall at the U.S. Naval Observatory in Washington, D.C discovered them separately in August 1877. They were the first major cosmic bodies discovered with an American-built telescope. Hall used the observatory's 26-inch refracting telescope -- the largest telescope in the world for a decade -- to make the find. Alvan Clark & Sons, Cambridgeport, Mass. made the telescope, at a cost of $50,000.
The telescope is still used today to measure the parameters of double stars, which is information vital for sending missions to distant worlds.
Each of the Martian moons is in the shape of a triaxial ellipsoid, or roughly the shape of a football. On both moons the long axis, or the football tip, points toward Mars. Deimos and Phobos lie in the equatorial plane of Mars. The rovers Opportunity and Spirit are roughly at the Martian equator, making the solar eclipses easy to spot. Deimos and Phobos are named for the sons of the Greek god Ares (the Greek name for Mars) and the goddess Aphrodite (whom the Romans called Venus.)
Space historians will note that the Viking landers in the mid 1970s observed the shadow of Phobos moving across the Marscape. The Mars Pathfinder mission in 1997 observed Phobos emerging at night from the shadow of Mars. Thanks to Opportunity's Pancam, this is the first time anyone has directly observed a moon passing in front of the sun from the surface of another world.
The Pancams aboard both Spirit and Opportunity observe the sun every sol, or Martian day, to determine how the planet's atmosphere affects the sunlight. To capture these rare events, the scientists and engineers must be quick shutterbugs, since Deimos crosses the sun in about 50 to 60 seconds, while Phobos crosses the sun in only 20 to 30 seconds.
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