Tomorrow, Mars Express is set to make the closest flyby ever of the innermost moon of Mars, the doomed Phobos. Tiny, resembling a potato, and possibly either a captured asteroid or shrapnel from a massive impact on the Martian surface, Phobos is sinking about a centimeter per year, and already is so low that on the Martian surface you may see it rise twice in one day. Little is known of the tiny world, but tomorrow’s flyby will provide the best measurements yet of its mass, as Mars Express passes a mere 45 km from the surface. This is much too close for Mars Express to photograph the world, as it will be traveling so rapidly relative to the spacecraft that it would be just a blur, but scientists monitoring the signal from the spacecraft will be able to measure tiny shifts as the gravity of Phobos deflects the spacecraft ever so slightly, providing the opportunity for the most accurate estimate of its mass yet and even allowing them to probe the interior mass distribution.
The European Space Agency released this video, computer generated from radar models of Phobos and overlain with actual imagery, to mark the occasion: