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Looking Up by Bob Eklund

Close Encounters of the Phobos Kind

The Mars Express spacecraft, which has been orbiting the Red Planet since 2004, recently began a series of close flybys of Phobos, the larger of Mars’ two moons. The campaign will reach its crescendo on March 3, when the spacecraft will set a new record for the closest pass to Phobos, skimming the surface at just 50 km. The data collected could help untangle the origin of this mysterious moon.

The Phobos flyby campaign began February 16, when Mars Express drew to within 991 km of Phobos’ airless surface. The flybys will continue at varying altitudes until March 26 when Phobos moves out of range. They offer prime chances for doing additional science with Mars Express, a spacecraft that was designed to study the red planet below rather than the grey moon alongside.

The names of Mars’ two moons, Phobos (fear) and Deimos (dread) are taken from Greek mythology. In Homer’s Iliad, Phobos and Deimos were two sons of Ares (Mars), the god of war. The moon Phobos is slightly irregular in shape and about 22 km in diameter.

“Because Mars Express is in an elliptical and polar orbit with a maximum distance from Mars of about 10,000 km, we regularly pass Phobos” says Olivier Witasse, Mars Express Project Scientist. “This represents an excellent opportunity to perform extra science.”

Back in 2009, the mission team decided that the orbit of Mars Express needed to be adjusted to prevent the closest approach of the spacecraft drifting onto the planet’s nightside. The flight control team at the European Space Operations Centre in Darmstadt, Germany, presented a number of possible scenarios, including one that would take the spacecraft to just 50 km above Phobos. “That was the closest they would let us fly to Phobos,” says Witasse.

Precise Gravity Measurements

Heavy emphasis is being placed upon the closest flyby because it is an unprecedented opportunity to map Phobos’ gravity field. At that range, Mars Express should feel differences in the pull from Phobos, depending on which part of the moon is closest at the time. This will allow scientists to infer Phobos’ internal structure.

Previous Mars Express flybys have already provided the most accurate mass yet for Phobos, and the craft’s High Resolution Stereo Camera has provided the volume. When calculating the density, this gives a surprising figure because it seems that parts of Phobos may be hollow. The science team aims to verify this preliminary conclusion.

In particular, Mars Express’ radar will operate in a special sequence to try to see inside Phobos, looking for structures or some clue to the internal composition. “If we know more about how Phobos is built, we might know more about how it formed,” says Witasse.

The origin of Phobos is a mystery. Three scenarios are possible. The first is that it is a captured asteroid. The second is that it formed in situ as Mars formed below it. The third is that Phobos formed later than Mars, out of debris flung into Martian orbit when a
large meteorite struck the red planet (which is the way Earth’s moon is believed to have been formed).

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