In March 2010 that superb but underappreciated probe Mars Express made a series of very close passes by the Martian moon Phobos. On one occasion it came as close as 107 km to the tiny world’s surface. The purpose of these encounters was to determine information on Phobos’ interior in a clever way. Basically the probe would fly past the tiny moon broadcasting to Earth with its radio transmitter all the time, as it flew by the moon its gravity would subtly alter the spacecraft’s trajectory. Exactly how it was altered can be measured by observing changes in the frequency of the radio signal received on Earth (the Doppler effect). Analysing the results will provide data on how the mass of Phobos is distributed under its surface.
This set of experiments are so precise that nothing can allowed to disturb them so even the probe’s cameras were turned off (some of the passes were made while the face of Phobos approached was in darkness so imaging would have been impossible any way). However some images were returned on 7 March and have now been released by ESA. The smallest features visible in the image above (which you can enlarge by clicking on it) are about as big as a house and the probe was moving at about 10 000km/h relative to the surface.
Discovered in 1877, Phobos is a dark, lumpy pile of rubble about 27km long. It is unusually light for its size indicating that it is not made of solid rock but may be partially made of ice or have a porous structure. Its origin is mysterious too; it could be a captured asteroid, ejecta from an asteroid impact on the planet’s surface or debris left over from the planet’s formation. The visits by Mars Express may help resolve these puzzles. It is also due to be visited by Russian probe called Phobos Grunt in the next few years. This robot explorer will hopefully land on the moon’s surface, scoop up a sample and return it to Earth.