Mars Express Phobos Fly-by (Wednesday)

The European Space Agency's Mars Express probe will be doing the closest ever approach to the Martian moon Phobos. On Wednesday night the spacecraft will come within 67 km of Phobos. They will use a very cool radio technique to gather data about the internal structure of the object (see below).
Mars Express carried the Beagle II lander to the red planet. I was in the UK when they made that brave attempt to put the probe on the surface, and shared in the disappointment when no signals came back. Later, I had the privilege of meeting Beagle II's creator, Colin Pillinger. Colin gave me signed copies of his wonderful books on Mars.

Here's some more info on Wednesday's fly-by (from ESA):

1 March 2010
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.

	Preparing for closest approach to Phobos

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.

After the closest flyby, the work is not over. Mars Express will sweep past Phobos a further seven times before the campaign is complete. 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.

The MARSIS radar is already taking data

“All the experiments on Mars Express have something to say about Phobos,” says Olivier Witasse, Mars Express Project Scientist, ESA. This is a bonus for science, considering that none of them were originally designed to study Phobos the moon, only Mars the planet. The science results from these flybys are expected in subsequent weeks or months, when the various teams have had time to analyse the data.



All photos from ESA.