The Lunar Atmosphere and Dust Environment Explorer (LADEE) is due for launch in September 2013 from the Mid-Atlantic Regional Spaceport. LADEE will study the composition and structure of the tenuous lunar atmosphere, including dust that may be lofted up from the surface. It will also undertake a demonstration of a laser-mediated communications system.
In a thick atmosphere like Earth’s, particles are constantly colliding with each other and hence moving in random and frequently-changing directions. The Moon is thought to have a surface boundary exosphere, which is a thin, collision-free atmosphere in which particles follow largely uninterrupted paths.
A key goal of the LADEE mission is to understand the dynamics of a surface boundary exosphere and how it changes over time, as external conditions vary. A surface boundary exosphere is probably the most common atmosphere found around celestial bodies in the Solar System, including Mercury, the majority of the large asteroids, the moons around our major planets, and the majority of large Kuiper Belt Objects. So, getting a better understanding of the Moon’s surface boundary exosphere is a good start to understanding those environments as well.
Indeed, there is a certain imperative to undertake this research now. The increasing interest in the Moon by a number of nations will lead to a steadily increasing frequency of lunar exploration missions that could change the natural composition of the lunar atmosphere, both through the stirring up of surface dust and by the addition of rocket exhaust components.
The Moon’s tenuous atmosphere was sampled by Apollo 17’s Lunar Atmospheric Composition Experiment which found helium and argon, as well as traces of neon, ammonia, methane, and carbon dioxide. Subsequent spectroscopic analyses from Earth telescopes have since found sodium and potassium atoms as well. These components of the lunar atmosphere may have been released by solar wind particles knocking atoms from the lunar surface, perhaps changing their chemistry in the process. Comet and meteoroid impacts, as well as a degree of out-gassing from the Moon’s interior, also contribute to the lunar atmosphere. LADEE’s objectives include determining the relative importance of these different contributions.
LADEE will also investigate the curious atmospheric rays reported by Apollo astronauts, which preceded a sunrise and followed a sunset as they orbited the Moon. The same phenomena were photographed by Surveyor probes from the lunar surface. These rays might result from light diffraction by dust particles or from the glow of energised sodium atoms.
As well as pursuing these science objectives, LADEE will undertake the Lunar Laser Communication Demonstration (LLCD) to test the feasibility of communicating data using laser light instead of radio. The case for using optical or near-optical laser light is strong, since its shorter wavelength means it can carry more information at the speed of light than longer wavelength radio, and this can be achieved without increasing the mass, volume, and power usage of the transmission equipment. The infrared laser module on LADEE will be able to aim at one of three ground terminals in New Mexico, California, and Spain.
The potential for laser communication between the Earth and the Moon was demonstrated in January 2013 when the data needed to build a high-resolution image of da Vinci’s Mona Lisa was received from Earth by the Lunar Reconnaissance Orbiter. High data rate transmission is becoming increasingly important in space exploration, as our spacecraft become increasingly able to undertake higher-resolution observations. Decreasing the time needed to transmit data back to Earth will increase the amount of time that can be dedicated to new investigations and discovery.
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