Spacecraft Makes Closest Ever Pass by Mars' Moon Phobos

A European probe made its closest-ever swing by Mars' moonPhobos Wednesday on a quest to learn more about the inner structure of the mysteriousMartian satellite.

The European Space Agency (ESA)'s Mars Express spacecraftflew in about 42 miles (67 km) above the rocky moon's surface. The pass was oneof 12 scheduled flybys of Phobos,each with its own scientific objective.

"I'm very happy that it's working so well," GerhardSchwehm, head of ESA's Solar System Science Operations Division, said after themaneuver. "This close encounter by MarsExpress has been a great chance to learn more about the inner structure ofPhobos."

Phobos is the larger of Mars' two diminutive moons (theother is Deimos). With a diameter only about 14 miles (22 km) wide, Phobosis not massive enough to have rounded into a sphere when it formed. Rather, itis shaped irregularly, more like a lumpy asteroid than a moon.

The goal of this pass was to study the mass distribution andgravitational field of this body. So any kind of movement by the spacecraft,even the miniscule changes caused by using its cameras to snap pictures, couldthrow the measurements off.

"This is a unique experiment and requires thespacecraft to be entirely passive, so that the only deviations to its motionare produced by the gravitational field of Phobos (which for Mars Express isjust one-billionth the strength of Earth?s gravity at the surface of ourplanet)," wrote ESA spokesperson Stuart Clark on the Mars Express Blog.

Sadly, that means no pretty pictures from this flyby.

Luckily, new high resolution pictures of Phobos are expectedafter subsequent approaches starting March 7, ESA officials said.

  • Image Gallery: Mars Express - A Year of Discoveries
  • Video - Two Moons of Mars Seen Together
  • Mars in 3D: Images from Mars Express

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Clara Moskowitz
Assistant Managing Editor

Clara Moskowitz is a science and space writer who joined the team in 2008 and served as Assistant Managing Editor from 2011 to 2013. Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She covers everything from astronomy to human spaceflight and once aced a NASTAR suborbital spaceflight training program for space missions. Clara is currently Associate Editor of Scientific American. To see her latest project is, follow Clara on Twitter.