From Earth all natural celestial objects rise in the east and set in the west.  The same is not true at Mars.  Phobos goes in reverse.  

If you were to stand on the surface of the Red Planet, Phobos would rise in the west.  It would appear about one-third the apparent size of our Moon as seen from Earth’s surface, and it would cross the Martian sky against the flow of the other celestial objects before setting in the east.  What makes Phobos so different?

Simple: it orbits unusually close to Mars, at an average altitude of around 9400 km (compared with our Moon’s distance from Earth of around 385 000 km).  To maintain such a low orbit, Phobos has to move so quickly that it out-paces the planet’s rotation.  Mars rotates once ever 24 hours 37 minutes, whereas Phobos completes an orbit in just 7 hours 39 minutes.

Hence, Phobos is constantly overtaking the surface of the planet.  All other natural celestial objects, including Mars’s second moon Deimos, are moving more slowly in relation to the surface of the planet and so are brought into view as the surface turns to face them. This causes them to rise in the east and set in the west.

From Earth, only artificial space-borne objects, such as the International Space Station, are in such low orbits and travelling so fast that they appear to rise in the west.

Back on Mars, the Phobos weirdness does not stop with its backward journey through the sky.  Because the moon is in such a low orbit above the equator, it can never be seen from inside the Martian polar circles. -- Stuart

Although emphasis is being placed on 3 March, when the closest Phobos flyby ever performed will take place, it is not the only time Mars Express will be drawing near to the mysterious moon of Mars.  The 3 March flyby is simply the high point (or should that be low point?) of a six-week campaign to study Phobos in closer detail than ever before.  It all begins on 16 February, next week, when Mars Express flies past Phobos at an altitude of 991 km.

There will be three more flybys during the fortnight after that, each drawing closer than the last, until the unprecedented 50 km flyby on 3 March.  Afterwards, a further seven flybys will be performed, each at slightly higher altitudes as the spacecraft’s orbit carries it further away from Phobos. Two of them will be used to image the proposed Phobos-Grunt landing site.  The final flyby of this campaign takes place on March 26 at an altitude of 1304 km.

During each flyby, a variety of science instruments will be used to study Phobos from different scientific viewpoints.  One of the highlights will be to measure the gravity field of Phobos, which will allow scientists to understand more about the structure of the moon.

The origin of Phobos is a mystery, in fact three scenarios are considered possible.  The first is that the moon is a captured asteroid; the second is that it formed in-situ as Mars formed below it, and the third is that Phobos formed later than Mars, from debris flung into martian orbit when a large meteorite struck the Red Planet. Among other objectives, the Phobos flybys are designed to provide clues towards answering this question. -- Stuart

This is how Phobos looked on 28 July 2008 to the HRSC camera on Mars Express.  Then, the spacecraft was 351 km from Phobos.  HRSC will take new images during this new sequence of flybys. Credits: ESA/ DLR/ FU Berlin (G. Neukum)