NASA counted down Friday to the launch of an unmanned spacecraft that aims to study the Moon's atmosphere, the US space agency's third lunar probe in five years.
The Lunar Atmosphere and Dust Environment Explorer (LADEE) was to lift off at 11:27 pm (Saturday 0327 GMT) aboard a Minotaur V rocket -- a converted Air Force missile -- from NASA's Wallops Flight Facility in Virginia.
The main goal of the mission is to learn more about the atmosphere and dust on the Moon by circling it over the course of several months.
When US astronauts last walked on the moon four decades ago, they learned that Moon dust could be a huge problem for sensitive spacecraft and equipment, said space expert John Logsdon.
"If we were ever to go there with people for long duration, the dust gets in everything. It's not smooth dust like a piece of sand on the beach. It's made of very, very small fragments," said Logsdon, a NASA advisor and former director of the Space Policy Institute at George Washington University.
"All the Apollo crews complained about the lunar dust getting everywhere."
US astronauts first walked on the Moon in 1969, and the last explorers of the Apollo era visited in 1972.
The Moon's atmosphere is so thin that its molecules do not collide, in what is known as an exosphere.
Exploring that exosphere will be a $280 million solar and lithium battery-powered spacecraft about the size of a small car -- nearly eight feet (2.4 meters) tall and five feet (1.85 meters) wide.
After launch, LADEE aims to hurtle itself beyond Earth's orbit so it can circle the Moon.
The journey there will take about a month.
When the spacecraft first enters the Moon's orbit, it will cruise at a height of about 250 kilometers (156 miles) for 40 days, and then moving lower to 20 to 60 kilometers (12.4 - 37.3 miles) from the surface for the science portion of its mission.
It is carrying an Earth-to-Moon laser beam technology demonstration and three main tools, including a neutral mass spectrometer to measure chemical variations in the lunar atmosphere and other tools to analyze exosphere gasses and lunar dust grains, NASA said.
"These measurements will help scientists address longstanding mysteries, including: was lunar dust, electrically charged by solar ultraviolet light, responsible for the pre-sunrise horizon glow that the Apollo astronauts saw?" NASA said.
Other instruments will seek out water molecules in the lunar atmosphere.
About 100 days into the science portion of the mission, the LADEE spacecraft will do a death plunge into the Moon's surface.
The spacecraft was made in a modular design that aims to "ease the manufacturing and assembly process" and "drastically reduce the cost of spacecraft development," NASA said.
Potential future uses of this module could include unmanned probes to an asteroid or to Mars, as well as future Moon probes, though none are planned for now.
LADEE was conceived when NASA was planning to return humans to the Moon as part of the Constellation program, which President Barack Obama cancelled in 2010 for being over budget and redundant in its goals.
NASA's next big human exploration project aims to send humans to Mars by the 2030s.
Recent NASA robotic missions such as the Lunar Reconnaissance Orbiter have returned troves of images detailing the Moon's cratered surface, while NASA's Gravity Recovery and Interior Laboratory (GRAIL) revealed how being pummelled by asteroids resulted in the Moon's uneven patches of gravity.
A previous NASA satellite, the Lunar Crater Observation and Sensing Satellite(LCROSS) discovered water ice when it impacted in 2009, the space agency said.