ESA’s Mars Express spacecraft, currently executing a series of 12 flybys of Mars’ largest moon Phobos, will pass the moon’s surface at an altitude of 67km on March 3 at 20:55 GMT. The close approach will enable scientists to gain valuable data an learn more about the mysterious moon.
As the spacecraft approaches Phobos, it will be pulled slightly off of its orbit (by a few millimeters per second). Scientists on Earth will turn off all data signals from the spacecraft ensuring the only thing affecting the signal is gravitational tug by Phobos. Although incredible tiny (only one part in a trillion, that is 1 followed by 12 zeros), the changes will be revealed via the Doppler effect.
This animation shows how the orbit of Mars Express has been influenced by the gravitational influence of Phobos during the spacecrafts fly-bys of the moon in Summer 2008. Since the orbital deviation strictly depends on the mass and shape of the moon, scientists could use this very deviation to determine the mass of Phobos with unprecedented accuracy (to about one billionth the mass of the Earth).