Update for 6 p.m ET: One year after their arrival at the moon, NASA's twin Grail spacecraft got a grand sendoff into oblivion, climaxing with a well-orchestrated crash onto a crater's rim. The place where they crashed will be named after Sally Ride, America's first woman in space, who passed away this summer.
Ride was in charge of the Grail mission's MoonKam project, which let students from around the world select targets for the probes' cameras. MIT's Maria Zuber, the mission's principal investigator, announced just after today's double whammy that her team received clearance from NASA to name the crash site after Ride.
"Sally was all about getting the job done, whether it be in exploring space, inspiring the next generation, or helping make the Grail mission the resounding success it is today," Zuber said in a NASA news release. "As we complete our lunar mission, we are proud we can honor Sally Ride's contributions by naming this corner of the moon after her."
The late astronaut's sister, Bear Ride, said the name is a fitting tribute to the space pioneer and her students. "It's really cool to know that when you look up now at the moon, there's this little corner of the moon that's named after Sally, and we hope that kids will really be inspired by that as well," she said during NASA's webcast of the Grail endgame.
The webcast showed mission team members applauding, shaking each other's hands and trading congratulations to mark the end of spacecraft operations at NASA's Jet Propulsion Laboratory. But the main event — the actual crash into the rim of a crater in the moon's north polar region — went unseen. Each of the probes was the size of a washing machine, which means that even though they were going 3,760 mph (1.7 kilometers per second), they didn't make a fiery impact. What's more, at the time of the crash, 5:28 p.m. ET, the impact site was in shadow.
Eventually, NASA's Lunar Reconnaissance Orbiter will take stock of the crash site, NASA spokesman D.C. Agle said. Mission managers don't expect to see any wreckage — just a couple of fresh holes in the ground. However, there's a chance that the blast may have kicked up some water ice or something similarly interesting that the orbiter could detect.
The Grail mission's spacecraft entered lunar orbit on Jan. 1 and successfully completed their $496 million mission to map the moon's gravitational field in unprecedented detail. ("Grail" is an acronym standing for Gravity Recovery and Interior Laboratory.)
Earlier this month, the scientists behind the mission reported evidence that the moon was significantly more battered than they thought, early in its history. The fracturing goes deep into the crust and perhaps down further, into the mantle. Researchers also determined that the moon's crust goes 25 miles (40 kilometers) deep, which is not nearly as deep as previously thought.
Such findings are expected to shed light on the process that influenced the formation of the moon as well as Earth, billions of years ago. Before the mission, some scientists suspected that two moons smashed together to form the modern-day moon, in an ancient event nicknamed the "Big Splat." Grail's findings provided no evidence to back up that hypothesis, however.
The Grail mission also made a huge contribution to education and public outreach: The two probes were named Ebb and Flow by elementary-school students from Bozeman, Mont., who participated in a nationwide contest. Both Ebb and Flow were equipped with cameras that could be pointed at targets selected by students from around the world as part of Ride's project.
Although the spacecraft are gone, the job of analyzing the images and data produced by the mission is far from complete.
"Ebb and Flow have removed a veil from the moon, and removing this veil will enable discoveries about the way the moon formed and evolved for many years to come," Zuber said.
Avoiding Apollo sites
NASA opted for a controlled crash primarily to make sure that the Grail didn't end up hitting a historic site on the moon, such as the landing zones for the Apollo lunar modules or unmanned U.S. or Soviet probes.
The rocket burns that set the stage for the crash were successfully executed on Friday. Grail project manager David Lehman said that before the burns, mission navigators calculated a seven-out-of-a-million chance that one of the probes would hit a historic site. "Now, after these two successful rocket firings, there is zero chance," he said in a status report at the time.
Ebb was the first of the two spacecraft to go down. "Impact in 3, 2, 1, zero," a member of the systems team declared. Flow's crash followed about 25 seconds later. Mission navigators estimated that the two crash sites were separated by 2 miles (3 kilometers).
During their final hours of existence, the two probes were used for one last experiment. Mission engineers had the spacecraft fire their engines until all their remaining fuel was gone, to compare the computer models for fuel consumption against the actual figures.
"Fuel gauges in space are rather challenging, because fuel doesn't sit on the bottom like it does in car tanks on Earth," Agle explained. Grail's last experiment should help engineers get a better handle on the fuel requirements for future missions to the moon, Mars and other cosmic destinations.
Grail project manager Dave Lehman told NBC News that "we have all the data in" from the fuel-tank experiment, and that his team would analyze the data over the weeks ahead. Like many others on the Grail team, Lehman voiced mixed emotions about the end of a mission that went so well from start to finish.
"It's sad," he said, "but things turned out much better than expected."
More about Grail and the moon:
- Kids get their very own 'Earthrise'
- Watch the moon evolve in 3 minutes
- How Ebb and Flow got their names
- Grail snags first video of moon's far side
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.