How Do You Land on a Moon With Almost No Gravity?

Last August, looking up from the edge of an ancient river delta on Mars, the Perseverance rover’s electronic eye spied an oblong shadow crossing the distant, glowing Sun. Phobos, the Martian moon named for the Greek god of fear and panic, was transiting the solar disk. Too small to darken the pale, dust-tinged sky, but orbiting close enough to eclipse a significant fraction of the Sun, Phobos passed in less than a minute. NASA’s Perseverance rover captured Phobos passing in front of the Sun from the surface of Mars.ASU/MSSS/SSI/JPL-Caltech/NASAThe tiny moon, with its murky origins, is an enigma, and one whose makeup and history could tell us much about Mars and the wider solar system. And now the international Martian Moons eXploration (MMX) mission, which successfully launched on 20 October, seeks to explore that mystery with an unprecedented series of landings on the unknown surface of Phobos.This mission brings unusual challenges—for example, designing an orbit to study a mi