Daniel and I have just been speaking to Thomas Ormston, Spacecraft Operations Engineer, and contributor to this blog.  We wanted the definitive answer on why the Mars Express camera (HRSC) would not be taking images at closest approach.  It turns out that there are three reasons – each of them alone would be show stoppers for the camera.

 Phobos on 30 August 2007. Credits:  ESA/ DLR/ FU Berlin (G. Neukum)

First is that Mars Express will fly between Phobos and Mars, placing it on the nightside of Phobos.  In fact, for a few seconds at closest approach Phobos will actually totally eclipse the Sun.  So the hemisphere of Phobos that HRSC would see will be in darkness, apart from the tiny amount of light reflected from the surface of Mars back onto the moon.

Second, at 67 km, the surface of Phobos will be moving through the camera’s field of view too quickly.  To track the surface, Mars Express would have to turn or slew as the engineers like to call it.  That is not a problem usually as Mars Express is designed to do this.  But at the speed at which Mars Express will pass Phobos, the spacecraft will have to slew faster than safety allows.  One factor determining the maximum speed is the fragile MARSIS experiment.  This is a 40-metre-wide antenna that could break if the spacecraft turns too quickly.  And MARSIS is still needed to probe beneath the surface of Mars and Phobos.

Thirdly, even if it were daylight and possible to turn the spacecraft fast enough to take the necessary images, this close flyby is specifically designed to probe the gravity field of Mars.  This is a unique experiment and requires the spacecraft to be entirely passive, so that the only deviations to its motion are produced by the gravitational field of Phobos (which for Mars Express is just one billionth the strength of Earth’s gravity at the surface of our planet).  Mars Express will therefore point its high gain antenna fixedly at the Earth for the duration of the flyby.

HRSC will swing into action at the next flyby on 7 March, when Mars Express will pass Phobos at around 107 km. The picture above was taken at a previous Phobos flyby, 30 August 2008, at a distance from the moon's centre of 2366 km. -- Stuart