UPDATED DECEMBER 17, 2012 5:45 P.M. CST (2345 UTC). NASA has named the crash site of twin GRAIL moon probes in honor of the late astronaut, Sally K. Ride, who was America’s first woman in space and a member of the probes’ mission team. The twin probes – named Ebb and Flow – were sent crashing into the side of a lunar mountain earlier today (Monday, December 17, 2012).

On December 14, Ebb and Flow were given a command to descend into a lower orbit around the moon. That orbit would result in the December 17 impact on a mountain near the moon’s north pole. The two probes – which moved one behind the other in orbit – hit the lunar surface as planned at 4:28 p.m. CST (22:28 UTC) and 4:29 p.m. CST at a speed of 3,760 mph (1.7 kilometers per second).

The location of the Sally K. Ride Impact Site is on the southern face of an approximately 1.5-mile (2.5-kilometer) tall mountain near a crater named Goldschmidt.

The impact marked a successful end to the GRAIL mission, which was NASA’s first planetary mission to carry cameras fully dedicated to education and public outreach. Ride, who died in July after a 17-month battle with pancreatic cancer, led GRAIL’s MoonKAM (Moon Knowledge Acquired by Middle School Students) Program through her company, Sally Ride Science, in San Diego. Sen. Barbara Mikulski of Maryland said:

Sally Ride worked tirelessly throughout her life to remind all of us, especially girls, to keep questioning and learning. Today her passion for making students part of NASA’s science is honored by naming the impact site for her.

Fifty minutes prior to impact, the spacecraft fired their engines until the propellant was depleted. The maneuver was designed to determine precisely the amount of fuel remaining in the tanks. Ebb fired its engines for 4 minutes, 3 seconds and Flow fired its for 5 minutes, 7 seconds. Data from these final firings of the twin probes’ engines will help NASA engineers validate computer models to improve predictions of fuel needs for future missions.

The mission team deduced that much of the material aboard each spacecraft was broken up in the energy released during the impacts. Most of what remained probably is buried in shallow craters. The craters’ size may be determined when NASA’s Lunar Reconnaissance Orbiter returns images of the area in several weeks. GRAIL project manager David Lehman of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena said:

We will miss our lunar twins, but the scientists tell me it will take years to analyze all the great data they got, and that is why we came to the moon in the first place So long, Ebb and Flow, and we thank you.

ORIFINAL POST – DECEMBER 17, 2012

NASA’s twin GRAIL moon gravity probes – named Ebb and Flow – have nearly run out of fuel. NASA intends to crash the probes into the side of a mountain near the moon’s north pole today (Monday, December 17, 2012) and you can watch it happen live on NASA Television or via livestream from NASA’s website. The show starts with live commentary from NASA scientists at around 4 p.m. CST (2200 UTC). The crash landing will take place around 4:28 CST (2228 UTC) on December 17.

NASA TV streaming video, schedule and downlink information:

https://www.nasa.gov/ntv

The coverage will also be streamed live on Ustream:

https://www.ustream.tv/nasajpl2

Join the conversation on Twitter by following the hashtag #GRAIL.

GRAIL stands for Gravity Recovery and Interior Laboratory. The two craft launched in September 2011 and arrived in lunar orbit about three months later. The two probes have been orbiting the moon ever since, one following the other, mapping the moon’s gravity in unprecedented detail. They completed a new map showing the moon’s gravity, shown below.

The map shows areas of concentrated mass including basin rings and volcanic structures. Because of the GRAIL spacecraft, we now know that the bulk density of the moon’s highland crust is much less than previously believed.

The image below is GRAIL’s moon map. Remember, it’s a map of the moon’s gravity. Looking at it is like seeing tiny variations in how strongly the moon can pull across its surface.

Here’s how GRAIL’s gravity-mapping ability works. As any spacecraft orbits a larger body, changes in the larger body’s topography – its hills and valleys, for example – minutely affect the orbital path of the craft by slightly increasing or decreasing the amount of gravity exerted on it. As they orbited the moon, the GRAIL B craft followed GRAIL A. After the proper orbit was established, an instrument on board each craft measured relative changes in velocity, which could then be translated to map lunar gravity. Scientists said GRAIL’s instruments worked so precisely they could detect a change in the distance between the two GRAIL orbiters the diameter of a red blood cell.

The mission was expected to increase knowledge of far-side gravity a thousandfold, and near-side a hundredfold, and I suspect it did that. The new knowledge is essential for planning future moon landings. It will also contribute to our understanding of the moon’s history of heating and cooling, which will provide insights into how the moon, and therefore other bodies in our solar system, formed.

And now the mission is ending spectacularly, with a crash landing of each craft into a mountain at the moon’s north pole. As Geekosystem quipped:

NASA scientists will be livestreaming and carefully monitoring the event, presumably so they can make sure the impact near the lunar north pole doesn’t hurt Moon Santa.

And we all know that’s important.

Bottom line: NASA will crash its twin GRAIL spacecraft into the side of a mountain on the moon on Monday, December 17, 2012 at 2228 UTc (4 p.m. CST). There are links in this post to livestream of the event.