Update 5:30 p.m.: It’s over. Both Ebb and Flow have made impact with the moon. After Ebb crashed, a short applause went up from the team assembled. Flow’s impact was confirmed moments later. After both crashes were confirmed, GRAIL principal investigator Maria Zuber announced that the impact site would be named after astronaut Sally Ride. Ride’s sister, the Rev. Bear Ride, was on hand to witness the event.
The goal of the mission is to map the moon’s gravitational field. To do this, NASA launched two washing-machine-size probes to orbit the moon in September 2011. After both craft achieved orbit on Jan. 1, they traveled around the moon in highly synchronized patterns at 3,600 miles per hour. The probes, named Ebb and Flow by elementary school students in Montana, could detect changes in the distance between one another down to a tenth of a micron — or half a strand of human hair. Their measurements have allowed scientists to create the highest quality gravitational field map of any celestial body, including Earth.
Two of the mission’s key findings have to do with the nature of the moon’s crust, specifically that it is much thinner than was previously believed and that “a couple of the large impact basins probably excavated the moon’s mantle,” Maria Zuber, the project’s principal investigator, said during a news conference last week. Another “surprising discovery,” she said, was how fractured the crust of the moon is because of impact.
“This is interesting because it tells us the role of impact bombardment on the evolution of an early planetary crust,” Zuber said.
The goal of the mission was to shed light on how other rocky planets in the solar system, including Earth, were formed. But the mission comes to an end Monday as NASA will send Ebb and Flow to crash into a mountain located near the moon’s north pole. Neither craft has enough fuel or altitude to continue scientific operations, having completed both their primary mission this spring and then an extended mission, which started Aug. 30.
The crash landing is being coordinated to eliminate any chance that Ebb and Flow could disrupt either the Apollo landing sites or where the Russian Luna probes touched down. The odds of that happening were determined to be seven in a million without the planned crash. But the maneuver eliminates any chance of a potential impact with the historic sites according to GRAIL project manager David Lehman.
Before the two craft slam into the mountain, one last test will be conducted. Both Ebb and Flow will fire their main engines to burn through their remaining fuel so scientists can determine how much is left in their tanks before the probes make impact. This will allow engineers to test the accuracy of models used to predict fuel consumption — data that could prove valuable for future missions. During the fuel burn, both craft will lose altitude gradually until the mountain face is directly in their path. Ebb, which arrived at the moon first, will be the first to slam into the surface. Flow is expected to make impact 20 seconds later.
“I’m hoping ... a gas station will pull up next to our space craft, refuel it and we can continue on for another six months, but I don’t think that’s gonna’ happen,” joked Lehman during the Dec. 13 press conference.
The agency will provide live coverage of the event at 5 p.m. ET. But video of the impact won’t be available, since the crafts’ final resting place will be in shadow when they make impact. This was done on purpose because, said Zuber, the point of the mission was data collection and the team wanted to collect data as long as possible. This made an impact in the dark necessary. The Lunar Reconnaissance Orbiter (LRO) has already taken an image of the site prior to impact and will do so afterward. Neither a large explosion nor flash are expected.
The project is managed by the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and Denver-based Lockheed Martin Space Systems built the twin crafts. Zuber and Lehman said the mission came in under budget.
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