Unlike the gas giants Saturn and Jupiter, both of which have dozens of moons, and Earth, which boasts a massive moon relative to its size that helps stabilize the planet, Mars doesn't have much in the way of lunar companions. Its two moons, Phobos and Deimos, are small, odd-shaped lumps that may be asteroids captured by Mars's gravity. Phobos, the larger of the two, is just 27 kilometers across in its broadest dimension, whereas Deimos is only about 15 kilometers across.

On November 5, the European Space Agency's Mars Express orbiter, circling the Red Planet in a polar orbit since 2003, was perfectly aligned to photograph both moons simultaneously. As Mars Express cycled through its orbit, Phobos [right] passed through its line of sight roughly 11,800 kilometers away, partially obscuring Deimos as the smaller moon cruised more slowly through its orbit some 26,200 kilometers from the spacecraft. The ESA has a stunning video of the unusual partial "eclipse," cobbled together from 130 images taken over the roughly 90-second-long event.