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One twin enters lunar orbit, the other inbound Sunday BY WILLIAM HARWOOD STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION Posted: December 31, 2011 
After a round-about 2.6-million-mile voyage, the first of two small NASA spacecraft successfully braked into orbit around the moon Saturday, a welcome New Year's Eve milestone for an ambitious $496 million mission to map out the moon's hidden interior.
If all goes well, a second 677-pound Gravity Recovery and Interior Laboratory -- GRAIL -- satellite will follow suit New Year's Day . Over the next two months, flight controllers will carefully maneuver them into the same orbit at an altitude of just 34 miles above the moon's poles where they will fly in close formation over the course of an 82-day mission. By precisely measuring the distance between the two spacecraft as they fly along -- and thus the subtle effects of the moon's gravity as they sail over visible and sub-surface geologic structures -- scientists will be able to determine the nature of the moon's enigmatic core and perhaps confirm or refute theories about how the moon formed at the dawn of the solar system. "GRAIL's a journey to the center of the moon," Maria Zuber, the principle investigator, told reporters earlier this week. "It'll use exceedingly precise measurements of gravity to reveal what the inside of the moon is like. And that information will be combined with the plethora of other remarkable observations of the moon that have been taken by satellites before and that are being taken right now (to) enable us to reconstruct the moon's early evolution." Despite more than 100 missions to the moon, including six manned landings, Zuber said more is known about Mars than Earth's nearest neighbor. Major questions, she said before launch, include "is there a core? How did the core form? How did the interior convect? What are the impact basins on the near-side flooded with magma and give us this man-in-the-moon shape whereas the back side of the moon doesn't have any of this?" "These are all mysteries that despite the fact we've studied the moon before, we don't understand how that has happened. GRAIL is a mission that is going to tell us that." Along the way, it also may help inspire school kids to take more of an interest in science. Each satellite is equipped with four cameras sponsored by former shuttle astronaut Sally Ride's science education company that can be used by students around the world to photograph the lunar surface. Zuber said more than 2,100 schools had signed up to participate in the "MoonKAM" program. "Students, in collaboration with their teachers, will be able to look at where the trajectory of the two spacecraft are and then when the school's turn comes up they will be able to target images and then take their own images of the moon," Zuber said. "We've had a great response to the MoonKAM project, we're still accepting applications. We can accommodate another several hundred schools into the program if there's interest in doing that."
The so-called low-energy trajectory was set up to allow about a day between the arrival of GRAIL-A and GRAIL-B. Following programmed instructions, GRAIL-A's main engine fired at 4:21 p.m. Saturday, burning for nearly 40 minutes to slow the craft by some 427 mph, just enough to let the moon's gravity to capture it in an 11.5-hour elliptical orbit. GRAIL-B is programmed to fire its braking rocket at 5:05 p.m. Sunday. (LIVE Mission Status Center coverage of the burns) "Just because we're in lunar orbit doesn't mean we're good to go for the science phase," said David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "There's still work to be done. Following lunar orbit insertion, the spacecraft will perform a series of intricate burns that take about two months. These are required before we can get both spacecraft down to a 34-mile altitude. Once that's done, that's when the science for GRAIL can begin." If all goes well, science operations will begin around March 8. Flying in formation at distances ranging from about 46 to 140 miles, the two spacecraft will send radio pulses and timing signals back and forth to precisely measure the distance between them. Sailing over buried mass concentrations, craters, mountain ranges, basins and other geologic features, the satellites will ever so slightly speed up and slow down, one after the other. The ranging system is accurate enough to detect differences of as little as one micron, or the width of a red blood cell. By carefully analyzing those changes, scientists can determine the distribution of mass within the moon to gain insights into its hidden interior structure and the nature of its deep interior. The primary mission must be completed by early June when the solar-powered satellites will experience an extended eclipse. The spacecraft were not designed to survive that long without power, but Zuber said the systems are performing better than expected and it now appears an additional, extended mapping mission may be possible.
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VIDEO:
THE FULL LAUNCH EXPERIENCE 