Space twins near Moon after three-month trip

“We’re on our way there,” said project manager David Lehman of the Nasa Jet Propulsion Laboratory, which manages the $496 million (£323m) mission.

The Grail – Gravity Recovery And Interior Laboratory – probes will not land on the Moon’s surface. Instead, they were poised to slip into orbit to study the uneven lunar gravity field.

Grail-A was scheduled to arrive tomorrow, followed by Grail-B on New Year’s Day. Mr Lehman said team members would not celebrate until both probes were safely in orbit.

It has been a long voyage for the near-identical spacecraft, which travelled more than two million miles since launching in September. Though the Moon is relatively close at about 250,000 miles away, Grail took a roundabout way to save on costs by launching on a small rocket.

The probes will spend the next two months tweaking their positions before collecting data in March. The pair will fly in formation 34 miles above the surface and 124 miles apart.

The mission’s chief scientist, Maria Zuber, of the Massachusetts Institute of Technology, said many aspects of the Moon remained a mystery despite being well studied.

“We actually know more about Mars … than we do about our own Moon,” Ms Zuber said.

One puzzle scientists hope to solve is why the Moon’s far side is more hilly than the side that always faces Earth. Research published this year suggested that Earth once had dual moons that collided and formed the Moon that people gaze at today.

Despite the wealth of knowledge expected from the mission, Nasa has no plans to send astronauts back to the Moon.

The Obama administration last year rejected the idea in favour of landing astronauts on an asteroid and eventually Mars.