PASADENA, Calif. (AP) — A pair of NASA spacecraft tumbled out of orbit around the moon and crashed back-to-back into the surface on Monday, ending a mission that peered into the lunar interior.

Engineers commanded the twin spacecraft, Ebb and Flow, to fire their engines and burn their remaining fuel. Ebb plunged first, slamming into a mountain near the moon’s north pole. Its twin, Flow, followed about a half minute later and aimed for the same target.