The Moon, long described as airless, carries a measurable exosphere of helium, neon, and argon so sparse that individual atoms travel vast distances without ever meeting another. Scientists studying this vanishingly thin envelope have found it to be a living record — of solar wind, of radioactive decay deep within lunar rock, of meteoroid strikes — constantly assembled and disassembled at the boundary between world and void. Now, as human activity returns to the lunar surface, the very instruments meant to read that record face a new challenge: the presence of people may overwhelm the signal t