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GRAIL Spacecraft New Year's Moon Arrival
NASA's twin GRAIL spacecraft are due to enter orbit around the moon just in time for 2012. GRAIL will study the moon's gravity, enabling scientists to deduce the moon's structure and composition, and helping them piece together the history
of how the moon, Earth and terrestrial planets formed.

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Narrator: The moon is expecting twins—for the New Year.

I’m Jane Platt with NASA’s Jet Propulsion Laboratory in Pasadena, California.
The New Year will bring the arrival of twins—twin spacecraft, that is—at the moon.
The two GRAIL spacecraft, launched September 5th from Cape Canaveral, Florida, are scheduled to begin orbiting the moon—one on New Year’s Eve, the other New Year’s Day.  David Lehman of JPL is the GRAIL Project Manager.

Lehman: OK, On New Years’ Eve, the GRAIL-A spacecraft is going to be sent into orbit around the moon.  Alright, we’re going to go through the south pole of the moon and do about a 38-minute burn, a 38-minute maneuver to slow the spacecraft down enough so it’s going to go into orbit around the moon.   And the orbit’s about a period of 11-1/2 hours.  It takes 11-1/2 hours to go around the moon.

Narrator: It’s a tricky task, but Lehman says the team is up for the challenge….thanks to some very careful preparations.

Lehman: To capture the spacecraft into orbit takes a lot of planning and a lot of practice  and a lot of maneuvers before that happens. Like just recently, the GRAIL-A spacecraft, we did a maneuver to change the speed by .05 miles per hour.  That's how small we had to make this change in order to affect going into orbit precisely around the moon.  So it does take a lot of planning, a lot of testing and then a lot of small maneuvers in order to get ready to set up to get into this big maneuver when we go into orbit around the moon.

Narrator: So GRAIL-A enters moon orbit on New Years’ Eve, and then the next day, the team does it again—with GRAIL-B.

Lehman: The purpose of the GRAIL mission is to obtain gravity data on the moon. And with that data, the scientists are able to determine the structure of the lunar interior, from crust to core.

Narrator: This info will help scientists figure out how the planets in our solar system were formed.

Lehman: Five billion years ago, the Earth was formed, and all the other terrestrial planets were formed, and the moon was formed.  But since that time, the Earth evolved, there’s earthquakes, there’s erosion, there’s lots of rain. And so when we study the earth now, we really don’t have good feeling for what happened because it changed so much.

Narrator: So they hope to fill in pieces of the puzzle by studying how the moon was formed, using not one, but two spacecraft.

Lehman: The reason we need two spacecraft is we use the spacecraft to measure the distance between each other.  And with that information, we’re able to determine the gravity.  Because we’re gathering this information on the distance between the two and how they change over time very precisely. And with that information we can understand the gravity. For example, as the spacecraft flies over a big moon mountain, GRAIL-A will accelerate towards that mountain. And then a few minutes later, GRAIL-A passes that mountain, GRAIL-B will get close to that mountain, and it will accelerate towards that mountain. So that's how the distance between the two is changing, and with that information, you can infer the gravity. 

Narrator: And then in February, the team will start putting the two spacecraft into formation, which is a technique currently being used by the Grace mission to measure Earth’s gravity. With GRAIL—it’s the moon.

Lehman: So we have to start formation flying. And we have to align the spacecraft very precisely together. That to me is the challenging part is setting up the maneuvers so that the spacecrafts are very precisely aligned from the orbit’s standpoint.

Narrator: More information on the GRAIL mission is online at . Thanks for joining us for this podcast from NASA’s Jet Propulsion Laboratory.
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