ESA Mars Express: Phobos Fly-by

Credits: MaRS team/Observatoire Royal de Belgique

This animation shows how the orbit of Mars Express has been influenced by the gravitational influence of Phobos during the spacecraft’s 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 (1.072 1016 kg, or about one billionth the mass of the Earth).

ESA’s Mars Express will skim the surface of Mars’ largest moon Phobos on Wednesday evening. Passing by at an altitude of 67 km, precise radio tracking will allow researchers to peer inside the mysterious moon.

Mars Express is currently engaged in a series of 12 flybys of Phobos. At each close pass, different instruments are trained towards the mysterious space rock, gaining new information. The closest flyby will take place on 3 March at 21:55 CET (20:55 GMT).

From close range, Mars Express will be pulled ‘off-course’ by the gravitational field of Phobos. This will amount to no more than a few millimetres every second and will not affect the mission in any way. However, to the tracking teams on Earth, it will allow a unique look inside the moon to see how its mass is distributed throughout.

How will the ground teams make these tremendously sensitive measurements? Ironically, they will turn off all data signals from the spacecraft. The only thing that the ground stations will listen out for is the ‘carrier signal’ – the pure radio signal that is normally modulated to carry data.

With no data on the carrier signal, the only thing that can modulate the signal is any change in its frequency caused by Phobos tugging the spacecraft. The changes will amount to variations of just one part in a trillion, and are a manifestation of the Doppler effect – the same effect that causes an ambulance siren to change pitch as it zooms past.

To read the full article on the ESA Mars Express Blog site, Click Here....