However, due to perturbation forces, resulting chiefly from Mars' inhomogeneous gravity field as well as gravity from other celestial bodies and also from light and solar wind pressure, the orbit rotates slightly so that periapsis (point of closest approach to the planet) drifts around Mars.
This is actually a big plus, because over the course of several months, the spacecraft gets a close-up view of all regions -- from the snow-covered poles over the windswept plains of the north to the heavily cratered highlands of the south. In other words, as Mars rotates beneath our 7-hour orbit, we get full close-up coverage of virtually the entire surface over time.
And, once in a while, our orbit intersects that of Phobos. The Flight Control and Flight Dynamics teams always take great care to ensure that whenever this happens, Mars Express and Phobos are never at that point of intersection at the same time (this would be sub-optimal -- Ed. :-)).
But, when the geometry is favourable and the two are going to pass close by, a carefully timed manoeuvre, provided by firing the thrusters, can bring our satellite close enough to Phobos to take pictures and other measurements of this extremely interesting moon.For his part, Thomas Ormston produced an animation showing the relationship among the orbits of Phobos, Deimos, and Mars Express, with calendar dates in the upper right corner, covering the period from orbit insertion in 2004 to the end of 2011. This is a fabulous resource, as you can use it to compare to Mars Express VMC images to get a sense of where the orbiter was as it took each set of pictures.
ESOC / video and caption by Thomas Ormston
Mars Express' shifting orbit
This video runs from the middle of Mars Express' first year at Mars, 2004, all the way through to 2011...sped up 10,000,000 times! You can see the red path of the Mars Express orbit as it swings around Mars throughout the Martian year (Earth dates are in the top right corner). Every few months this intersects with the orbit of Phobos (the inner green ring, the outer one is Deimos), giving us an opportunity for a Phobos flyby.
The lines don't always display in the correct order on this video, so sometimes it looks like we also cross the orbit of Deimos; this is just an optical illusion on the video (in reality, it orbits too far out from Mars for a close encounter with Mars Express).