NASA's Lunar Atmosphere and Dust Environment Explorer spacecraft slammed into the moon early Friday, as planned, but a few days earlier than NASA officials expected.
The crash, which occurred between 12:30 a.m. and 1:22 a.m. Eastern time, brought to a successful end a six-month, $280 million mission to study the tenuous envelope of gases and dust surrounding the moon.
The vending-machine-size spacecraft -- called LADEE for short -- crashed at 3,600 mph, breaking up into pieces that heated up to hundreds of degrees and partly vaporized.
"There's nothing gentle about impact at these speeds," Richard C. Elphic, the project scientist, said in a NASA news release. "It's just a question of whether LADEE made a localized craterlet on a hillside or scattered debris across a flat area. It will be interesting to see what kind of feature LADEE has created."
By design, the elliptical orbit was oriented to ensure that LADEE would crash on the far side of the moon, away from the historic Apollo landing sites. But that meant the spacecraft was out of sight and out of communication when it hit. If mission controllers can narrow down the time and place of LADEE's demise, another NASA spacecraft, the Lunar Reconnaissance Orbiter, may try to photograph the crash site.
Mission managers had expected that LADEE -- pronounced "laddie" -- would stay in orbit until Monday before meeting its destruction. On April 11, the spacecraft fired its engine one last time to swoop in closer to sample the gas and dust at the very lowest parts of the lunar atmosphere.
On Tuesday, the spacecraft passed through a four-hour eclipse, robbing its solar panels of the electricity-generating ability and draining the batteries longer than had been designed. "It was a nail-biting event," said Butler Hine, the project manager. "I've never seen that many yellow and red alarms going off simultaneously."
When LADEE emerged back into sunlight, the batteries recharged, the alarms cleared and the spacecraft continued collecting measurements.
Unevenness in the strength of the moon's gravity jostled the elliptical orbit, bringing LADEE to within a kilometer of the surface at the lowest point.
Calculations indicated that LADEE would remain just above the surface Friday and then rise again before another gravitational fluctuation sends it on a collision course early Monday. But with uncertainties about LADEE's exact position and the height of the lunar terrain, mission managers also said the spacecraft could easily crash sooner, as it did.
It is not the only piece of spacecraft litter to arrive at the moon recently. In December 2012, two small NASA spacecraft measuring the moon's gravity similarly crashed when their maneuvering fuel ran out. Last December, the Chinese landed a small rover on the moon, but it malfunctioned a couple of months later.
LADEE launched in September and started orbiting the moon Oct. 6.
Mr. Elphic said the mission had achieved all of its objectives. One of the interesting observations was of water in the atmosphere, very small amounts of about 100 molecules per cubic centimeter. "Sometimes more, sometimes less, sometimes none at all," Mr. Elphic said. "We see it fairly sporadically."
That could help explain how water traveled to the moon's poles and accumulated, over billions of years, as ice at the bottom of deep, perpetually dark craters.
What LADEE did not solve is a four-decade mystery. Some Apollo astronauts had reported seeing glowing light at the lunar horizon just before sunrise. The lunar atmosphere, just 1/100,000th the density of Earth's, is too thin to scatter the light, so scientists thought perhaps enough dust had been lofted by electrostatic forces to do so.
But LADEE observed only minuscule amounts of dust, not enough to account for the glow. That might mean the dust conditions during Apollo were somehow different, or that something else created the glow. "Maybe that signature comes and goes, and we just didn't happen to be lucky about it," Mr. Elphic said.