04-26-2014, 06:39 PM
NASA’s Lunar Atmosphere and Dust Environment Explorer spacecraft slammed into the moon early Friday morning, as planned, but a few days earlier than NASA officials expected.
At about 12:30 a.m. Eastern time, radio signals from the spacecraft abruptly cut off just as it passed over to the far side of the moon. Mission managers believe that is the moment when it ran into a crater rim — they suspect they even know which crater rim — but that has not been confirmed yet.
The crash brought to a successful end a six-month, $280 million study of the tenuous envelope of gases and dust surrounding the moon. With impact at 3,600 miles per hour, the vending-machine-size spacecraft, called Ladee (pronounced LAD-ee), broke up into pieces that heated up to hundreds of degrees and partly vaporized. “It’s just a question of whether Ladee made a localized craterlet on a hillside or scattered debris across a flat area,” Richard C. Elphic, the project scientist, said in a NASA news release.
Mission managers had expected that Ladee would stay in orbit until Monday.
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. 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.
Unevenness in the strength of the moon’s gravity jostled the elliptical orbit. Calculations indicated that Ladee would remain just above the surface on Friday and then rise again before another gravitational fluctuation would send it on a collision course early Monday morning. But with uncertainties about Ladee’s exact position and the height of the lunar terrain, mission managers also said that the spacecraft could easily crash sooner.
On Thursday night, Ladee dipped lower and lower on each successive orbit, and mission controllers scrambled to send remaining data to Earth. “What you don’t want to do is leave valuable science data on the spacecraft when you impact,” Butler Hine, the project manager, said in an interview.
The last data file was sent about a minute before the cutoff of the radio signal. Ladee, which launched in September and started orbiting the moon on Oct. 6, is not the only piece of spacecraft debris to arrive at the moon recently. In December 2012, two small NASA spacecraft measuring the moon’s gravity similarly crashed after their maneuvering fuel ran out. Last December, the Chinese landed a small rover on the moon, but it malfunctioned a couple of months later.
One of Ladee’s 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,” Dr. Elphic said in an interview. “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 was a four-decade-old mystery: The glowing light at the lunar horizon just before sunrise that some Apollo astronauts had reported seeing. 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. However, 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,” Dr. Elphic said.
Source: NY Times