US space agency Nasa has launched an unmanned rocket to study the make-up of dust particles in the Moon's thin atmosphere.
The probe, known as the Lunar Atmosphere and Dust Environment Explorer, or LADEE, blasted off from the Wallops rocket facility in Virginia, on the US east coast, on schedule at 23:27 local time (03:27 GMT on Saturday).
The £180m ($280m) mission will spend six months orbiting the Moon before crashing into the lunar surface.
However, the mission ran into immediate difficulties after it was hit by equipment failure that will require repairs when the rocket reaches the Moon, Nasa reported.
Scientists hope to learn more about composition of atmospheric moon dust, how it changes over time and whether it rises from the lunar surface.
Programme scientist Sarah Noble said the mission's findings were likely to disprove earlier theories that the Moon had no atmosphere.
"It does. It's just it's really, really thin," said Noble.
"It's so thin that the individual molecules are so few and far between that they don't interact with each other; they never collide.
"It's something we call an exosphere. The Earth has an exosphere as well, but you have to get out past where the International Space Station orbits before you get to this condition... At the Moon, it happens right at the surface."
The fine particles are formed from the remnants of rock shattered over eons of meteorite impacts. Moon dust is considered a major hazard.
"It's not like terrestrial dust," observed Butler Hine, Nasa's LADEE project manager.
"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. One of the questions about dust on the Moon is an engineering question: how do you design things so that they can survive the dust environment."
The probe will take a month to arrive at the Moon. It is being carried by a US Air Force Minotaur rocket.
The launch site in Virginia marked a change for Nasa. All but one of its previous 40 moon missions - including the manned Apollo flights of the 1960s and 1970s - launched from Cape Canaveral in Florida.
The launch was expected to be visible from many parts of America's east coast.