The flyby will be so close and fast that Mars Express will not be able to take any images, but instead it will yield the most accurate details yet of the moon's gravitational field and, in turn, provide new details of its internal structure.
As the spacecraft passes close to Phobos, it will be pulled slightly off course by the moon's gravity, changing the spacecraft's velocity by no more than a few centimetres per second.
These small deviations will be reflected in the spacecraft's radio signals as they are beamed back to Earth, and scientists can then translate them into measurements of the mass and density structure inside the moon.
Knowing the structure of the roughly 27 x 22 x 18 km Phobos will help to solve a big mystery concerning its origin and that of its more distant sibling, Deimos, which orbits Mars at approximately three times greater distance.
The leading theories propose that the duo are either asteroids captured by Mars, or that they were born from debris thrown up from giant impacts on Mars.
In addition to probing the gravitational field of Phobos during its close approach, Mars Express will be making measurements of how the solar wind influences the moon's surface.
Furthermore, ground stations around the world will track the spacecraft for a total of 35 hours in the lead up to, during, and after the flyby to ensure that the position of Mars Express is precisely known.
--ANI (Posted on 24-12-2013)