As the NASA lunar probe LADEE nears its planned end — where it will crash onto the Moon — the scientists running it admit that they have as yet been unable to solve its primary scientific question about levitating lunar dust.

A major goal of the mission was to understand a bizarre glow on the Moon’s horizon, spotted by Apollo astronauts just before sunrise. “So far we haven’t come up with an explanation for that,” project scientist Rick Elphic, of NASA’s Ames Research Center in Moffett Field, California, said at a media briefing on 3 April. One leading idea is that the Sun’s ultraviolet rays cause lunar dust particles to become electrically charged. That dust then lofts upwards, forming a cloud that caught the light and the astronauts’ eyes.

LADEE carries an instrument that measures the impact of individual dust particles, as well as the collective signal from smaller particles. Lunar scientists had expected a certain amount of tiny dust to explain what the Apollo astronauts saw. But LADEE didn’t find it. “We did measure a signal that indicates that the amount of lofted dust has to be at least two orders of magnitude below the expectations that were based on the Apollo reports,” says Mihály Horányi, the instrument’s principal investigator, who is at the University of Colorado. Perhaps the dust lofting happens only occasionally, he suggests, and the astronauts were in just the right place at the right time to see it.

This remains an important question. Knowing what caused that horizon glow and knowing how often it occurs is essential knowledge for any future lunar base or research station.