If the twin spacecraft known as Ebb and Flow had any glimmer of awareness, they would have realized on Friday that something ominous was going on. For nearly a year, the two probes have been orbiting the moon in tandem, one behind the other, working tirelessly to map the gravity field of that cratered orb. It was always something of a thrill ride: the pair, known collectively as the GRAIL (Gravity Recovery and Interior Laboratory) mission has been screaming along at thousands of kilometers per hour, dipping to as low as 10 km above the surface.
But on Friday, NASA controllers ordered Ebb’s rockets to life, and then, 16 seconds later, sent Flow an identical command, and both spacecraft began to descend. The glide path was far gentler than that of a passenger jet, but the end would be a lot more abrupt: between 5:28 p.m. and 5:29 p.m. Eastern time on Monday, an unnamed mountain will suddenly loom dead ahead, and first Ebb, then Flow, will smash headlong into its flank at a blistering 6,051 km/h, almost literally vaporizing themselves. Last week, the GRAIL probes made history. This week, they are history.
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The reason for the controlled crashes near the moon’s north pole is that the probes, having completed their mission, are nearly out of fuel. They’d be going down anyway, but NASA wanted to avoid even the small chance that they might damage the sites of the Apollo landings, where footprints, flags and other artifacts remain, perfectly preserved in the lunar vacuum.
It isn’t the first time interplanetary spacecraft have been crashed deliberately either. Back in the early 1960s, a series of Ranger probes took nosedives into the moon, furiously snapping photos as they went and giving scientists the very first closeups of the lunar surface. During the Apollo program, discarded rocket boosters were crashed so that seismometers on the surface could record the impacts. In 2009, the LCROSS probe smashed into a permanently shadowed crater floor to see if traces of water vapor would emerge from the fireball, proving there was ice on the moon’s surface (there was). And further from home, the Galileo probe was sent plunging into Jupiter in 2009 rather than take the chance that it might crash into Jupiter’s satellite Europa instead, contaminating a place where life could plausibly exist with Earthly bacteria.
Fortunately for scientists, Ebb and Flow have completed their mission with impressive success, mapping the subtle differences in gravity from one part of the lunar surface to another with extraordinary precision. When the pair approached a place with extra material — say, a mountain range — the extra gravity would tug on Ebb, lengthening the distance between the two craft. When they approached a place with less material than average, such as a crater or a crevice, Ebb would slow down, letting Flow catch up just a bit.
Those gravity differences let the GRAIL mission create the most detailed topographic map ever of the moon’s surface. But Ebb and Flow also responded to areas of high or low density below the surface — and those revealed something nobody had imagined: the moon is crisscrossed with underground cracks, up to 483 km long and up to 40 km wide, filled with solidified magma. The cracks were created billions of years ago, as the moon’s hot interior expanded against its cool, solid surface; the fact that they’re now buried shows that the surface was later pulverized by a bombardment of asteroids.
There’s plenty more as well: the enormous volume of data sent back by Ebb and Flow (they were named by elementary-school kids in Bozeman, Mo., who won a contest) led to three separate papers published on Dec. 5 in Science — but they’re just the beginning of what could be a revolution in our understanding of the moon.
Once they’re gone, space lovers will remember that Ebb and Flow lived fast, died young — and while they didn’t leave a pair of good-looking corpses, they certainly left a gorgeous scientific legacy.