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.
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.
Indeed, this process of the solar wind interacting with the lunar surface is thought to contribute to the moon's water cycle – a "hey, Martha!" aspect of lunar exploration over the past decade. India's Chandrayaan-1 lunar orbiter, as well as NASA's Lunar Reconnaissance Orbiter, have found water bound up in minerals all over the moon's surface, even as the LRO's LCROSS companion kicked up water ice when it crashed into the permanently shadowed side of a crater near the moon's south pole.
The original source for the water is thought to be comets, with continuing contributions from water-bearing meteoroids that strike the moon's surface. The solar wind also is thought to form hydroxyl molecules – basically water that's one oxygen atom shy of a full deck – when hydrogen in the wind combines with oxygen atoms on the lunar surface.
Whatever their sources, these molecules are thought to bounce around in the atmosphere during the long lunar day, then return to the surface during the lunar night. Over time, these molecules are thought to migrate to permanently shadowed interiors of craters near the poles, which host some of the coldest spots in the solar system.
LADEE, with its suite of four instruments, is designed to look sideways as it orbits, rather than straight down at the lunar surface. It will be hunting for this molecule migration, as well as identifying other constituents of the lunar atmosphere.
The atmosphere already is known to have sodium atoms in it. The only way to get sodium on the moon is to vaporize lunar rocks, Spudis says. But lunar scientists are interested in drawing up a more complete list of ingredients.
"What we don't know is the detailed chemical makeup, or how it varies with time," he says.
As for the mysterious horizon glow, researchers say the dust could be levitated via electrical charges that build up along the moon's terminator, as well as during the moon's travels into and out of Earth's magnetotail – a feature resembling a comet's tail but consisting of Earth's magnetic field. It stretches deep into space from the night half of the planet as the solar wind, with its magnetic fields, sweeps past.
Lunar water and dust also are of engineering interest, as space agencies and private companies consider exploring the moon and exploiting its resources.
Water contains the basic components of rocket fuel, in addition to its critical role as a thirst quencher. Suspended dust, on the other hand, represents a potential source of trouble for technologies such as solar panels or heat-shedding radiators. A buildup of dust on each can reduce their performance. So understanding the dust hazard will be important to human exploration efforts there.
Indeed, with exploration in mind, the clock is ticking on studying the lunar atmosphere and its processes in relatively pristine form – processes the moon may share with planets like Mercury or asteroids like Ceres or Vesta.
Fresh dust is ground from rock and kicked up all the time on the moon as meteoroids collide with it, notes Dr. Noble. "Something the size of LADEE hits the month once a month," she says.
But when spacecraft land instead of crash, she adds, they use a lot of fuel to control their landings.
"That actually can add a lot of material to the atmosphere," she continues, noting that the gases in rocket exhaust could become a significant component of the atmosphere.