NASA's Twin Grail Spacecraft Reunite in Lunar Orbit

PASADENA, Calif. The second of NASA's two Gravity Recovery And Interior Laboratory (GRAIL) spacecraft has successfully completed its planned main engine burn and is now in lunar orbit. Working together, GRAIL-A and GRAIL-B will study the moon as never before.

"NASA greets the new year with a new mission of exploration," said NASA Administrator Charles Bolden. "The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet. We begin this year reminding people around the world that NASA does big, bold things in order to reach for new heights and reveal the unknown."

GRAIL-B achieved lunar orbit at 2:43 p.m. PST (5:43 p.m. EST) today. GRAIL-A successfully completed its burn yesterday at 2 p.m. PST (5 p.m. EST). The insertion maneuvers placed the spacecraft into a near-polar, elliptical orbit with an orbital period of approximately 11.5 hours. Over the coming weeks, the GRAIL team will execute a series of burns with each spacecraft to reduce their orbital period to just under two hours. At the start of the science phase in March 2012, the two GRAILs will be in a near-polar, near-circular orbit with an altitude of about 34 miles (55 kilometers).

During GRAIL's science mission, the two spacecraft will transmit radio signals precisely defining the distance between them. As they fly over areas of greater and lesser gravity caused by visible features such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly.

Scientists will translate this information into a high-resolution map of the moon's gravitational field. The data will allow scientists to understand what goes on below the lunar surface. This information will increase knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today.

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Twin GRAIL Spacecraft to Map the Moon's Gravity

Circling the Earth at a distance of only 240,000 miles, the moon has captivated humanity's collective imagination since ancient times. Humans have studied the moon for hundreds of years first with telescopes, then with robotic probes, even sending twelve American astronauts to the lunar surface. But many mysteries remain.

The Gravity Recovery and Interior Laboratory mission, or GRAIL, features twin spacecraft embarking on a challenging mission to map the moon's gravity.

"Trying to understand how the moon formed, and how it evolved over its history, is one of the things we're trying to address with the GRAIL mission," says Maria Zuber, principal investigator for GRAIL from the Massachusetts Institute of Technology. "But also, (we're) trying to understand how the moon is an example of how terrestrial planets in general have formed."

"GRAIL is a mission that will study the inside of the moon from crust to core," Zuber says.

The mission's two spacecraft will fly in formation above the lunar surface to measure the variations in gravity. The mission seeks to reveal clues about our moon's thermal history, and how the inner solar system's rocky planets developed.

GRAIL is set to depart from Space Launch Complex 17B at Cape Canaveral Air Force Station in Florida on Sept. 8 at 8:37 a.m. Prelaunch processing and the final countdown are managed by NASA's Launch Services Program (LSP) at nearby Kennedy Space Center.

"Our team, especially, gets excited whenever we leave Earth orbit, and going to the moon excites us and excites the public," says LSP's Tim Dunn, the NASA Launch Manager for GRAIL.

The two spacecraft called GRAIL-A and GRAIL-B are riding into space side-by-side aboard a powerful Delta II Heavy rocket built by United Launch Alliance. It's a rocket with an impressive reliability record.

"NASA has a terrific history with the Delta family of rockets," Dunn says. "If we just look at the Delta II rocket, which is the version of the vehicle that we fly today, NASA has a perfect launch record, 48 for 48."

The payload for NASA's most recent lunar mission, called LRO-LCROSS, weighed in at 6,600 pounds and was the size of a minivan. It launched in 2009 aboard a massive Atlas V rocket. Compare that to the GRAIL spacecraft, which together weigh only about 1,600 pounds. Each unit is about the size of a washing machine, designed to be compact and rugged.