In a few days, the ESA spacecraft dubbed Mars Express will streak past the largest moon of Mars, Phobos. The spacecraft will make the closest approach to Phobos ever made at only 28 miles above the surface of the moon. The catch is that Mars Express will be traveling so fast it won't be able to take photos.
The flyby will happen on December 29 and while the spacecraft won't be able to shoot photos of its close encounter with Phobos, it will be able to take some scientific measurements. The measurements that the spacecraft will take will offer the most accurate details on the moon's gravitational field ever taken.
Those details will help scientists gather new information on the internal structure of Phobos. Mars Express will pass close enough to the moon that it will be pulled slightly off course by its gravity. The spacecraft will only loose velocity to the tune of a few centimeters per second.
That slight loss of velocity will mean some minor deviations in the radio signals the spacecraft sends back to Earth. The scientists controlling the mission will be able to translate those slight changes in velocity and signal into measurements that shed light on the mass and density of Phobos. Photos captured by the spacecraft for the last several weeks as it approaches Phobos and photos taken after the spacecraft passes the moon will be used to determine the precise location of Phobos needed to make accurate calculations.