How Mars Express' orbit shifts with time

While I was writing yesterday's blog entry on Mars Express' Phobos flybys I realized that I didn't understand Mars Express' orbit very well. So I sent an inquiry to the Mars Express blog, which they answered in a blog entry today (quick work, thanks to Daniel, Hannes, and Thomas!)

To explain, Spacecraft Operations Engineer Hannes Griebel wrote:

Mars Express is on a highly eccentric, polar orbit around Mars. This means that our spacecraft flies over the poles during each orbit, as can be seen in this full-orbit movie captured in May 2010.

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.