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Lunar dust mission still chasing mystery of ‘horizon glow’

LADEE at moon credit NASA


Posted on behalf of Alexandra Witze.

NASA is preparing one last blast for its expired Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft — a controlled crash into the Moon’s surface, probably on 21 April. But before it goes, LADEE will take a final shot at unravelling one of the main mysteries it went to the Moon to uncover.

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.

LADEE will try one more time to unravel the horizon-glow mystery. As it gets closer and closer to the lunar surface, it will point its star tracker towards the Moon’s horizon to try to replicate the angle and conditions under which the astronauts saw the glow. The star tracker is not designed for high-resolution imaging, but Elphic says that it’s worth looking.

This weekend, mission managers will guide LADEE on a trajectory just 3 kilometres above the Apennine mountains on the Moon’s near side. The goal is to see what sort of dust LADEE can spot so close to the surface. Then it will move slightly higher for its remaining few weeks before plunging to its doom. It is destined to follow the natural decay of its orbit and vaporize itself on the lunar far side.

LADEE scientists have plenty of science to distract them from mourning. The spacecraft made the best measurements ever of the Moon’s dusty envelope, generated as tiny meteorites bombard its surface. The mission also discovered exotic atoms such as neon, magnesium and aluminium in the Moon’s outer atmosphere.


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