Mars Express will skim over the surface of Mars’ largest moon on Wednesday, making the closest flyby of Phobos by any spacecraft. Passing at just 67 km above the surface, precise radio tracking will allow researchers to virtually peer inside the mysterious moon. You can follow the flyby in “real time,” — allowing for the current 6 minute and 30 second light time delay from Mars (13 minutes round trip) – on the Mars Express blog. The flyby will take place on March 3, at 20:55 GMT.
The straight-line distance between Mars Express and Earth is now about 116 million km.
Flying by at such close range, Mars Express will be pulled ‘off-course’ by the gravitational field of Phobos. This will amount to no more than a few millimeters 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. Phobos’ shape is 27 km × 22 km × 19 km, and has a mass of 1.072 x 1016 kg, or about one-billionth the mass of Earth.
To make the very sensitive measurements of Phobos’ interior, all the data signals from the spacecraft will be turned off. 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.
Two dress rehearsals for this exacting operation have already taken place, allowing ground station personnel and spacecraft controllers to practice.
Originally, the closest flyby was going to only 50 km above the surface, but a slight ‘over performance’ during a maneuver last week had put the spacecraft on a trajectory that included an occultation by Phobos. This meant that Mars Express would pass behind Phobos as seen from Earth. As this would jeopardize the tracking measurements, it was decided to perform another maneuver to position the flyby at a slightly higher altitude than originally planned.
In addition to the tracking experiment, known as MaRS for Mars Radio Science, the MARSIS radar has already been probing the subsurface of Phobos with radar beams. “We have performed a preliminary processing of the data and the Phobos signature is evident in almost all the data set,” says Andrea Cicchetti, Italian Institute of Physics of Interplanetary Space, Rome, and one of the MARSIS team.