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A Dramatic End for the Twin Moon Spacecraft

This artist’s concept shows one of the GRAIL satellites inserting itself into orbit. Image by NASA/JPL-Caltech.

The twin GRAIL robots, the washing-machine sized satellites that have been zipping around the moon for nearly a year, will end their lives magnificently on Monday in a kamikaze crash into the moon’s North pole. Since Friday, they’ve been burning off fuel and descending toward the moon.

By noon ET, they were already waving distance from the lunar surface — about a mile away, said mission project scientist Sami Asmar. And just after 5 p.m. ET, their fuel finally depleted, they’re expected to collide at speeds of nearly 4,000 mph into a lunar mountain near the Goldschmidt crater.

You can see NASA’s livestream of mission control at the Jet Propulsion Laboratory during the event here:

Video streaming by Ustream

It was on New Years Eve a year ago when the first satellite, named Ebb, fired its engine and maneuvered into orbit around the moon after its three-month, 2.5 million-mile journey. A day later, Flow, the second craft, joined its twin, trailing 10 miles behind.

Since that time, they’ve been making scientific observations and measuring changes in the moon’s gravitational field. Together, they generated the highest-resolution gravity field map of any celestial body, according to NASA.

They did this by sending constant radio signals back and forth, providing scientists with precise measurements of the separation distance between the two spacecraft. The mass of geological formations like mountains, valleys or deep basins, creates a slight gravitational pull or push on the orbiting objects, changing the distance of the orbiters slightly. Materials and forces beneath the lunar surface that can’t be seen on a topographical map do the same thing. Thus by measuring distance, scientists were able to also measure these gravitational changes. The moon’s far side is rough and mountainous, while the near side is smoother, with more dips and valleys.

Here’s how Asmar explained it earlier this year:

“If you’re orbiting the moon and you fly over a mountain, the spacecraft will literally sense additional pull,” Asmar said. “A mountain has more mass than its surroundings. If you fly over a valley, you literally sense less gravity, you sense a gravity low. We try to correlate that with the topography.”

This post by Universe Today lays out the results of the mission, which has provided details “that are up to five orders of magnitude better than previous studies of the Moon.” Features revealed by the gravity map include tectonic structures, volcanic landforms, basin rings, an unusual gravity field and an inner crust “almost completely pulverized.”

Results also showed that the moon went through a brief expansion period and that its crust is thinner and significantly more porous than thought, Asmar said.

Now, low in fuel and too close in orbit to the moon’s surface, the only real option for the spacecrafts is to dispose of them.

“The responsible thing is not to let them crash randomly in the moon and hit ‘holy’ sites like the Apollo landing or the flag,” Asmar said. “It’s better to do a controlled crash.”

But they’ll do it with mixed feelings, he added. “Especially for me, who’s been on it from Day 0, and seen it grow like raising a child.” The team will continue to work on data processing for another year.

You can watch their projected flightpath here:

Ebb, the first to orbit, will also be the first to go.


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The science series Vsauce set out to study the five-second rule for dropping food. After one minute on the floor, your food has 10 times the bacteria it had after five seconds, he reports. More on this in the video below:

Jeremy Blackman contributed to this report.

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