Nasa bid to unlock Moon mystery

NASA is counting down the seconds until its twin spacecraft bound for the Moon make back-to-back arrivals this weekend.

The washing machine-sized probes have been cruising independently toward their destination since launching in September aboard the same rocket on a mission to measure lunar gravity.

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. The Grail-A was poised to fire its engine for more than a half hour to slow itself and get captured into orbit yesterday. Grail-B will follow suit today.

Deep space antennas in the California desert and Madrid will track the tricky manoeuvres and feed real-time updates to ground controllers. “The anxiety level is heightened,” project manager David Lehman of the Nasa Jet Propulsion Laboratory said last week.

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

For example, it remains a mystery why the Moon is slightly lopsided with the far side more mountainous than the side that always faces Earth. A theory put forth last year suggested the Earth once had two Moons that collided early in the solar system’s history, producing the bumpy region.

Grail is expected to help researchers better understand why the Moon is asymmetrical and how it formed by mapping the uneven lunar gravity field that will indicate what’s below the surface. “We think that the answer is locked in the interior,” said chief scientist Maria Zuber of the Massachusetts Institute of Technology.

Previous lunar missions have attempted to study the Moon’s gravity – which is about one-sixth of Earth’s – with mixed results. Grail is the first mission devoted to this goal. Once in orbit, the near-identical spacecraft will spend two months refining their positions until they are just 34 miles above the surface and flying in formation. Data collection will begin in March.

The $496 million (£320m) mission will be closely watched by schoolchildren. An effort by Sally Ride, the first American woman in space, will allow pupils to use cameras aboard the probes to zoom in and pick out their favourite lunar spots to photograph.

Despite the latest focus on the Moon, Nasa won’t be sending astronauts back any time soon. The Obama administration has ruled out a lunar return in favour of landing humans on an asteroid and eventually Mars.