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Lunar GRAIL

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May 22, 2008: Meet MIT professor of physics Maria Zuber. She's dynamic, intelligent, intense, and she's on a quest for the Grail.

No, not that Grail.

Zuber is the principal investigator of the Gravity Recovery and Interior Laboratory — "GRAIL" for short. It's a new NASA mission slated for launch in 2011 that will probe the moon's quirky gravity field. Data from GRAIL will help scientists understand forces at play beneath the lunar surface and learn how the moon, Earth and other terrestrial planets evolved.

"We're going to study the moon's interior from crust to core," says Zuber. "It's very exciting."


Above: An artist's concept of GRAIL in action.

Here's how it works: GRAIL will fly twin spacecraft, one behind the other, around the moon for several months. All the while, a microwave ranging system will precisely measure the distance between the two satellites. By watching that distance expand and contract as the two satellites fly over the lunar surface, researchers can map the moon's underlying gravity field1.

Scientists have long known that the moon's gravity field is strangely uneven and tugs on satellites in complex ways. Without course corrections, orbiters end their missions nose down in the moondust! In fact, all five of NASA's Lunar Orbiters (1966-1972), four Soviet Luna probes (1959-1965), two Apollo sub-satellites (1970-1971) and Japan's Hiten spacecraft (1993) suffered this fate.

The source of the gravitational quirkiness is a number of huge mascons (short for "mass concentrations") buried under the surfaces of lunar maria or "seas." Formed by colossal asteroid impacts billions of years ago, mascons make the moon the most gravitationally lumpy major body in the solar system. The anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. A plumb bob held at the edge of a mascon would hang about a third of a degree off vertical, pointing toward the central mass. Moreover, an astronaut in full spacesuit and life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's center.

To minimize the effects of mascons, satellite orbits have to be carefully chosen. GRAIL's gravity maps will help mission planners make those critical decisions. Moreover, the maps GRAIL scientists will construct are essential to NASA's intended human landing on the moon in the next decade. The gravity of the moon's far side and polar regions, where future landings are targeted, is least understood.

The GRAIL team aims to map the moon's gravity field so completely that "after GRAIL, we'll be able to navigate anything you want anywhere on the moon you want," says Zuber. "This mission will give us the most accurate global gravity field to date for any planet, including Earth."

Above: A gravity map of the moon made by the Lunar Prospector spacecraft in 1998-99. Mascons are shown in orange-red. The five largest all correspond to the largest lava-filled craters or lunar "seas" visible in binoculars on the near side of the Moon: Mare Imbrium, Mare Serenitatus, Mare Crisium, Mare Humorum and Mare Nectaris. Image reference: Alex S. Konopliv et al, Icarus 150, 1–18 (2001). [more]

GRAIL will also help students learn about gravity, the moon, and space. Each satellite will carry up to five cameras dedicated to public outreach and education. Undergraduate students supervised by trained adults will remotely operate the cameras from a facility at the University of California, San Diego, that currently operates similar cameras on the International Space Station.

Middle school students from all over the country will also get to join in the excitement of lunar exploration. "We'll have an interactive website where the middle school students can make recommendations for targets to photograph and then view the pictures of their suggested targets," she says. "This just has incredible potential to engage students."

Clearly, this is no ordinary Grail quest. Stay tuned to Science@NASA for updates as the adventure unfolds.

Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA

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1How does the distance between satellites reveal the underlying gravitational field of the moon? Zuber explains:

"The change of distance with time is velocity, the change of velocity with time is acceleration, and the unit of gravity is acceleration," explains Zuber. "If you very very precisely measure the tiny gravitational perturbations of the two satellites at various locations, and then put all those measurements together for the whole moon, you get a gravity map."

It's not as simple as it sounds. Like any good quest, this one has its challenges. For starters, in making all their calculations, Zuber and company will have to correct for pesky factors such as atmospheric drag, gravitational pull from other planets and general relativity, just to name a few.

But according to Zuber, the biggest challenge to GRAIL is operating two spacecraft at the same time. "They are launched together, but they have separate trajectories. At some point they have to hook up and range to each other. This requires great precision. We'll do a lot of testing and simulations to make sure everything is ready."

Zuber proposed the concept for the GRAIL mission to meet an objective NASA identified in a 2006 Announcement of Opportunity for NASA's Discovery Program. Her proposal was selected from among 24 submissions. She hand picked a science team to carry out the investigation, and she chose the Jet Propulsion Laboratory to support the mission and JPL's Dave Lehman to serve as program manager. NASA's Marshall Space Flight Center will oversee the mission, with Bryan Barley as mission manager there.

EXTRA! TEACHER WORKSHOP: Teachers in the Southeast U.S. are invited to apply to attend a workshop entitled "Paving the Way to the Moon and Beyond," held June 12 – 14, 2008 in Huntsville, Alabama. The two day workshop, sponsored by the Lunar Precursor Robotic Program (LPRP), will focus on content that will explain the who, the what, and the why of lunar exploration. Beginning with the Lunar Reconnaissance Orbiter (LRO) and the Lunar Crater Observation and Sensing Satellite (LCROSS), teachers will research mission design and scientific goals and experiments through hands on activities. Included in the workshop will be a tour of NASA science labs and an opportunity to talk with scientists involved in exploration related activities. All materials will be appropriate for elementary and middle school preservice and inservice teachers and will be aligned with national standards. Example activities: Earth-Moon comparisons and motions, craters and lunar soils, solar influences on the Moon. Most activities will take place at the Educator Resource Center, located at the Space and Rocket Center. Housing costs for three nights will be provided (at the University of Alabama in Huntsville). Stipends also will be provided. To attend, contact Mitzi Adams 256 961 7626 or mitzi dot adams @ nasa dot gov.

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