Spacecraft takes express tour of Martian moon

New images and gravity data reveal details of Phobos

The sharpest images yet taken by the Mars Express spacecraft of Mars’ tiny moon Phobos reveal features as small as 4.4 meters across, the European Space Agency announced March 15.

PEERING AT PHOBOS This portrait of Mars’ moon Phobos shows features as small as 4.4 meters in diameter and was taken by the Mars Express spacecraft on March 7. G. Neukum/FU Berlin, ESA, DLR
IN FOR A LANDING Inset of this new, high-resolution portrait of the Mars moon Phobos shows the proposed 2011 landing sites for a Russian craft called Phobos-Grunt. G. Neukum/FU Berlin, ESA, DLR

Some of the new images taken March 7 during one of several recent close flybys of the moon home in on the proposed landing site for a Russian mission, Phobos-Grunt (meaning Phobos soil), that is expected to launch next year. 

During the three flybys on March 7, 10 and 13, researchers measured the moon’s tug on Mars Express by examining changes in the frequency of radio signals beamed by the spacecraft to Earth. The frequency shifts indicate that the craft has sped up or slowed down by a few millimeters per second due to the moon’s gravity.

When combined with images, the gravity data may provide new clues about Phobos’ composition and origin. According to one theory, Mars captured the moon from the nearby asteroid belt. Alternatively, Phobos may have formed where it now resides and could be a direct leftover from the planet-making era. With dimensions of 27 by 22 by 19 kilometers, the moon is the larger of Mars’ two moons.

The Mars Express flybys, which happen every five months, may also determine if Phobos is a fragile pile of rocky fragments stuck together — what planetary scientists refer to as a rubble pile — or  solid through and through, says Mars Express scientist Gerhard Neukum of the Free University of Berlin. He notes that due to orbital maneuvers that had to be performed on relatively short notice, scientists missed the opportunity to take even higher-resolution images with another camera on Mars Express, which would have revealed features on Phobos as small as a meter across.

The craft will make two more passes by Phobos before the end of March, but they will not come as close as the March 7 flyby.

This animation simulates the Mars Express’s March 7 flyby of the Martian moon Phobos. Credit: G. Neukum/FU Berlin, ESA, DLR

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