The European Space Agency's Mars Express spacecraft made a flyby of the Martian moon Phobos yesterday, with its closest point of approach just 27 miles above Phobos's surface. What is unique about this flyby is that Mars Express did not take any photos of Phobos during this approach. Instead, scientists tracked the slight changes in the motion of Mars Express due to the gravitational tug of Phobos. The amount by which Phobos changed the orbit of Mars Express will help us to determine the mass of Phobos.
Why do we want to know how much Phobos weighs? At present, the way in which the two Martian moons (Phobos and Deimos) formed remains unknown. There are two main theories: one proposes that they are asteroids captured by Mars, the other suggests they are formed from debris thrown into orbit from objects hitting Mars. If the debris theory is correct, we'd expect the moons to have low density, since they'd essentially be piles of smaller rocks. Knowing the mass of Phobos will let us calculate its density. Phobos' formation isn't the only mystery regarding this moon; Phobos has lots of parallel grooves on its surface, and scientists are yet to figure out how those grooves formed.
Mars Express launched in 2003. With an initial mission length of two years, it has had its mission end date extended five times; it's currently slated to cease operations in December 2014. Its achievements include the detection of hydrated minerals on Mars that require liquid water to form, the detection of water ice in the the polar ice caps of Mars, and an estimate of the rate at which Mars is losing its atmosphere. The spacecraft has enough fuel to keep on trucking for over a decade, so hopefully it will continue to help us solve mysteries about Mars and its moons!