Article Posted on : - Dec 28, 2011

LONDON (PTI): Nearly three months after they were launched, the twin solar-powered NASA spacecraft are set to enter Moon's orbit on the New Year's Eve in a mission designed to map the interior of the Earth's satellite.

The washing-machine-sized Grail spacecraft, launched from the Florida coast in September, will map the Moon's interior 100 to 1000 times more accurately than any past lunar mission.

Maria Zuber, the Grail mission head and a scientist at the Massachusetts Institute of Technology, said the precise lunar gravity measurements will help scientists understand better how the Moon evolved over the past 4 billion years.

The findings also should help identify the composition of the Moon's core: whether it's made of solid iron or possibly titanium oxide, Zuber said.

Another puzzle that Grail may help solve, she said, is whether Earth once had a smaller second moon. Astronomers have speculated that some of the "landmarks" on the Moon's surface are a result of a collision with a second, smaller moon.

"Both spacecraft have performed essentially flawlessly since launch, but one can never take anything for granted in this business," Zuber said.

On New Year's Eve, one of the Grail probes -- short for Gravity Recovery and Interior Laboratory -- will fire its engine to slow down so that it can be captured into orbit and this move will be repeated by the other the following day, the scientists said.

The chances of the probes -- officially known as Grail-A and Grail-B -- overshooting each other are slim since their trajectories have been precise. But things could still go wrong, the scientists said.

Cosmic rays might interfere with the probes' engine burn and they won't get boosted into the right orbit, they feared.

"I know I'm going to be nervous," said project manager David Lehman of NASA?s Jet Propulsion Laboratory which manages the USD 496-million, three-month mission.

Once in orbit, the spacecraft will spend the next two months flying in formation and chasing one another around the Moon until they are about 35 miles above the surface with an average separation of 124 miles.

Data collection will begin in March.

Past missions have tried to map lunar gravity with mixed success. Grail is the first mission dedicated to this goal.

As the probes circle the moon, regional changes in the lunar gravity field will cause them to speed up or slow down.

This in turn will change the distance between them. Radio signals transmitted by the spacecraft will measure the slight distance gaps, allowing researchers to map the underlying gravity field, the researchers said.

Using the gravity information, scientists can deduce what's below or at the lunar surface such as mountains and craters and may help explain why the far side of the Moon is more rugged than the side that faces Earth.