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Moon mission: Can it solve the mystery of the glowing horizon?

LADEE (Lunar Atmosphere and Dust Environment Explorer) is set to launch Sept. 6 – NASA's third moon mission in four years. This time the focus is on the moon's atmosphere, a gap in recent exploratory efforts.

By Staff writer / September 1, 2013

Pigeons are silhouetted by a rising, nearly full moon as they perch on the roof of a barn near Taylorsville, Md. LADEE (Lunar Atmosphere and Dust Environment Explorer) is set to launch Sept. 6 – NASA's third moon mission in four years. This time the focus is on the moon's atmosphere, a gap in recent exploratory efforts.

Dylan Slagle/Carrol County Times/AP

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NASA is getting set to launch its third mission to the moon in four years – the last vestige of what was envisioned as a series of orbiters and robotic landers in advance of returning astronauts to the moon by the end of the decade.

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Dubbed LADEE, for Lunar Atmosphere and Dust Environment Explorer, the $280 million mission aims to analyze a tenuous lunar atmosphere as well as to solve the mystery of the glowing horizon.

Horizon glow was detected by NASA's Surveyor landers in the mid- to late-1960s, as well as by astronauts during Apollo missions.

Surveyor landers recorded it as a thin band of light above the lunar surface along the moon's day-night line, or terminator. When Surveyor spotted the bands, they appeared to hover one or two meters above the surface, says Paul Spudis, a senior scientist at the Lunar and Planetary Institute in Houston.

Astronauts orbiting the moon, however, reported seeing streamers of light that extended hundreds of kilometers above the surface.

Light reflecting off suspended lunar dust is the lead explanation for the phenomenon. But when the Clementine orbiter hunted for the glow during its time at the moon in 1994, it detected nothing.

"One of the things LADEE wants to try to ascertain is: Is this really happening, or is this just somebody's imagination?" Dr. Spudis says.

LADEE is scheduled for launch Sept. 6 on a US Air Force Minotaur V rocket, a converted intercontinental ballistic missile. The mission marks the maiden flight for the five-stage rocket. Moreover, it will be the first deep-space mission ever launched from the Goddard Space Flight Center's Wallops Island launch facility, on the Virgina coast.

The mission fills a gap in recent US lunar-exploration missions. Since 2007, seven spacecraft have visited the moon to measure space weather around it; map its surface and the distribution of potential mineral resources, including water, there; and measure its gravity field to reveal the moon's interior structure.

The atmosphere is the only layer of this onion left largely unexplored.

The notion that the moon has an atmosphere runs counter to what most people learned in school about Earth's companion, noted Sarah Noble, the mission's program scientist, during a recent briefing at NASA headquarters.

But it does have one, she explained. "It's just really, really thin."

(Wait for it, wait for it....) How thin is it?

"It's so thin that the individual molecules that make up the atmosphere molecules are so few and far between that they don't interact with each other; they never collide," she said.

On Earth, a cubic centimeter of atmosphere at the surface hosts about 100 billion billion molecules. On the moon, the atmosphere – or, more properly, its surface "exosphere" – has perhaps 100,000 to 10 million molecules in each cubic centimeter. [Editor's note: This paragraph originally included the wrong atmospheric density for Earth and the moon.]

Those molecules come from several sources, Spudis explains. Gas escapes from the moon's interior through faults on the surface. Tiny micrometeoroids smack into tiny grains on the moon and vaporize them via the heat from the collision. This allows some atoms and molecules to escape as gas before the material chills and returns to solid form. And charged particles in the solar wind, which constantly stream from the sun, strike the lunar surface to form other gases.

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