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GRAIL probes set to begin lunar orbit Saturday,Sunday

Los Angeles, Dec 31 (): NASA says its twin lunar GRAIL spacecraft that are on a mission to study the Moon’s gravitational field and internal structures are set to begin orbiting Earth’s satellite on Saturday and Sunday.

The two robotic spacecrafts — weighing about 303 kilograms each,will enter into the moon’s orbit over the weekend, about three months after they were launched in September.

The first spacecraft is set to fire its engines at 4:20 p.m. on Saturday, and begin its orbit of the moon soon after. The second craft will perform similar manoeuvres 25 hours later, around 5:05 p.m. on Sunday.

Approaching the moon from the South Pole, the Grail spacecraft – short for Gravity Recovery and Interior Laboratory – won’t land on the surface, but will survey from orbit.

Then over the next three months, the two crafts will do a series of small engine firings to reduce their orbit to within 55 kilometres of the moon’s surface, said Maria Zuber, principal investigator for the GRAIL mission and a professor of geophysics at Massachusetts Institute of Technology.

“Following the lunar orbit insertion, the spacecraft will perform a series on intricate burns that take about two months, and these are required to get both spacecraft down to a [55km; 34-mile] altitude; and once that’s done, that’s when the science for Grail can begin,” explained David Lehman, the mission’s project manager at the Jet Propulsion Laboratory (JPL) in California.

“Grail will improve our knowledge of the Moon’s near side gravity by more than 100 times over what was previously known, and by more than 1,000 times over what was known on the far-side,” the Massachusetts Institute of Technology (MIT) researcher said.

Grail will map the small variations in gravity across the Moon.

Previous lunar missions have attempted to study the moon’s gravity, which is about one-sixth Earth’s pull, with mixed results. Grail is the first mission devoted to this goal.

Once in orbit, the near-identical spacecraft will spend the next two months refining their positions until they are just 34 miles above the surface and flying in formation. Data collection will begin in March.

Grail is the 110th mission to target the moon since the dawn of the Space Age including the six Apollo moon landings that put 12 astronauts on the surface. Despite the attention the moon has received, scientists do not know everything about Earth’s nearest neighbour.