Spacecraft Makes Closest Ever Pass by Mars' Moon Phobos

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

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

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

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

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

"This is a unique experiment and requires the spacecraft to be entirely passive, so that the only deviations to its motion are produced by the gravitational field of Phobos (which for Mars Express is just one-billionth the strength of Earth?s gravity at the surface of our planet)," 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 expected after 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