Moondust mystery: LADEE mission into the lunar atmosphere

Lunar atmosphere

Buzz Aldrin on the moon in 1969

There used to be a pub near my house called The Other Side of the Moon but it was closed down due to a lack of popularity - people said there was no atmosphere. While that statement is entirely true, the same cannot be said of the real Moon that orbits our planet. Contrary to popular opinion, a lunar atmosphere does exist, even though it is very thin. Its density is equivalent to that at the outer fringes of Earth's atmosphere, where the International Space Station orbits.

While space exploration has been focusing recently on the Curiosity rover on Mars and the Cassini mission to Saturn’s moon Titan, NASA has been quietly preparing for a mission to the Moon to study its atmosphere. It has been a few decades since we last took a trip there but it is about to be revisited. The Lunar Atmosphere and Dust Environment Explorer (LADEE) will not actually land on the surface but will orbit the Moon while taking measurements of the atmosphere and the dust that floats near the surface.

Launched on September 6th, LADEE is currently undergoing a precise series of elliptical orbits around the Earth while its systems are checked out and it gets into the correct position to come under the influence of lunar gravity. Here it will orbit around 20-50 km above the surface taking measurements for about 100 days. The mission had been threatened by the ongoing budget crisis in the US but the spacecraft will not be shut down as it is "in a critical phase."

Analytical payload

The dual aims of the missions, analysing the atmosphere and the dust that floats around, will be carried out using three instruments aboard LADEE. A neutral mass spectrometer (NMS), a UV-visible spectrometer and a dust detector have been fitted into a modular spacecraft body, which was designed in an attempt to move away from custom-built craft for each mission and reduce mission costs.

Drawings of the dust streamers by Apollo 17 astronauts

Click on image to enlarge

The NMS design was the same as that from the ill-fated Comet Nucleus Tour (CONTOUR) mission of 2002 which lost contact with scientists at the control centre a few months after launch. Luckily, the machine fit into the new modular design body. Principal Investigator Paul Mahaffy told spectroscopyNOW.com "It was not hard to get the LADEE NMS to fit – the panel we mounted [it] to was just the right size for what was needed."

The NMS is a high-sensitivity quadrupole mass spectrometer with mass range up to 150 Da, sufficient for the species that NASA expects to find. "We believe we should for sure find helium and argon. We'll look for other species such as OH and metal compounds or atoms," declared Mahaffy. So far, only helium, argon-40, potassium, sodium and radon-222 have been found in the lunar atmosphere but the NMS will also look for hydroxyl radicals, sulphur, xenon, methane and other metals like iron, aluminium and magnesium.  

The UV-visible spectrometer will also look for atmospheric species. Measurements from this and the NMS will be taken at different times of the lunar day at various positions around the moon to deduce spatial and temporal patterns and see if there are any periodic releases from the surface. Some of these might derive from the dust present on the moon, for which the third instrument, the Lunar Dust Experiment (LDEX) is on board.

Moondust

Moondust is different to the dust on our own planet. "Terrestrial dust is like talcum powder. On the Moon, it’s very rough. It’s kinda evil. It follows electric field lines; it works its way into equipment," said Butler Hine, the project manager of LADEE. It is this same dust that forms high wispy clouds at dawn which baffled the Apollo astronauts who landed on the Moon. The dust also produced streamers of light stretching away from the surface, as the astronauts’ own sketches show (see opposite).

It is now believed that dust particles on the moon's surface become electrically charged by solar UV light and lift off, causing a glow in the distance. The LDEX is an impact detector that will measure the mass and density of the dust particles in these clouds and help to map their patterns of formation. Similar instruments have been deployed into space previously, on HEOS 2 (investigated interplanetary space), Ulysses (surveyed the areas around the poles of our sun), Galileo (to Jupiter and its moons), and Cassini missions.

In a new development on the communications front, LADEE will transmit to Earth using lasers rather than the traditional radio waves, allowing data transfers to occur at broadband speeds.

The mission will end when the fuel runs out and LADEE will make a suicidal dive into the surface of the moon.