On June 1, 2001, Mars Express watched as Phobos (the inner and larger of Mars' two moons) slipped past distant Jupiter. Mars Express was studying Phobos to help the ill-fated Russian Phobos-Grunt mission prepare to land on the moon and grab a sample for return to Earth. Phobos is only 23 kilometers in diameter, while Jupiter is 142,000 kilometers across! The inset image shows that the camera was sensitive enough to spot all four of Jupiter's moons as well, once the contrast was greatly enhanced.

ESA / DLR / FU Berlin (G. Neukum)