European Probe Swings Close by Martian Moon
An image of Phobos by the High-Resolution Stereo Camera on board Mars Express in Jan. 2008. The larger and inner of the two martian moons is seen here floating just above the Martian limb. The image has been enhanced slightly to bring out the detail on the moon.
Credit: ESA/DLR/FU Berlin (G. Neukum)

A European spacecraft is making its closest ever pass by the Martian moon Phobos today to scan never-before-seen regions of the small, rocky satellite.

The European Space Agency?s (ESA) Mars Express spacecraft will skim just 60 miles (97 km) above the surface of Phobos, one of Mars? two diminutive moons, during today?s pass. The flyby is the third — and closest — in a series of five swings past Phobos this summer to study the moon in unprecedented detail.

With a diameter of just 13.5 miles (22 km), Phobos is larger than Mars? second moon Deimos (7.4 miles, or 12 km, along its long axis). Both satellites were first spotted by astronomer Asaph Hall in 1877, but researchers are still unsure whether the moons are actually captured asteroids, ancient planetismals from the birth of the solar system, or the remains of a massive Martian impact.

Mars Express made its first summer swing by Phobos on July 12 at a distance of about 350 miles (563 km), with subsequent passes bringing the spacecraft within 60 miles (97 km) of the moon at the closest and out as far as 412 miles (664 km) on the final flyby set for Aug. 3. The probe has flown by Phobos before, such as its August 2004 pass at a range of about 124 miles (200 km), but never as close as today?s visit, ESA officials said.

Mars Express has turned its full complement of science instruments on Phobos for its flyby series, including a high-resolution camera for mapping and three-dimensional images, a subsurface-probing radar to study the moon?s interior and other tools to take detailed measurements of Phobos? mass, composition and other features.

Mission managers said Mars Express may also catch a glimpse of the planned destination for Russia?s Phobos-Grunt lander, a spacecraft slated to launch in 2009 to collect samples of the small Martian moon.

Mars Express is not the only spacecraft to take a look at Phobos this year.

In April, NASA?s Mars Reconnaissance Orbiter turned its camera eyes on the pitted moon from a distance of about 3,600 miles (5,800 miles) and returned some of the most detailed views of Phobos to date with its High-Resolution Imaging Science Experiment (HiRISE) camera.