Adrastea is the second-closest moon of Jupiter, moving just one Jovian radius above Jupiter's cloud tops. It is extremely small and irregular in shape. Like its inner neighbor, Metis, it orbits faster than Jupiter spins on its axis, a situation that results in orbital instability and will eventually cause Adrastea to spiral into Jupiter's atmosphere. Adrastea is a shepherd moon of Jupiter's main ring and, together with Metis, appears to be the source of the material making up this ring.