NASA Rovers Watching Solar Eclipses by Mars Moons
March 8, 2004
NASA's Mars Exploration Rovers have become eclipse watchers.
Though the Viking landers in the 1970s observed the shadow of
one of 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,
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."
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 travels 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, D.C.
Images of the March 4 and March 7 eclipses are available
online at
http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20040308a.html. Other images from the rovers and additional
information about the project are available from JPL at
http://marsrovers.jpl.nasa.gov and from Cornell University at
http://athena.cornell.edu.
