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For the teams working on the Mars Express mission, one of the most important activities during the current fly-by phase is generating and refining estimates of the spacecraft's orbit, which must be as accurate as possible.
This is mandated by the fact that Phobos is really not that big - its mean radius is just 11 km - and the fly-by at closest approach happens rather quickly. It will be over in just a few moments as Mars Express zips by at around 3 km/second.
As a result, the camera and other instruments on board the spacecraft - and indeed the spacecraft itself - must be pointed in precisely the correct direction at precisely the correct time in order to get any useful results at all.
These pointings and timings, in turn, depend on knowing the spacecraft's trajectory very accurately - and estimating this is at least as much an art as it is a science.
The trajectory estimates make use of astrophysical data including the gravity of Mars and Phobos as well as data from the star trackers on board Mars Express (which provide a picture, literally, of the background star field) and radiometric ...