Even with enough fuel, the second concern comes in—the lunar surface is covered in a material called regolith, a mixture of dust, rock, and even glass fragments. During the crewed Apollo missions to the Moon, there was even some worry that a large spacecraft might sink down into the surface. But the real problem facing astronauts is that the dust tends to get everywhere with much gravity helping hold it down. That applies to landings, too: When a spacecraft is landing autonomously, its rocket thrusters could throw up dust that interferes with its sensors and leads to the wrong maneuver, or turns a flat landing area into a crater.