"Europe has arrived at the Moon, we're in lunar orbit," said Professor David Southwood, director of science for the European Space Agency (Esa).

On Monday, Smart 1 began firing its ion engine to bring it into lunar orbit.

Continuous engine firing until Thursday will push the European probe over a "hump" in order to complete its capture by the Moon's gravity.

The probe will spiral ever closer to the surface until reaching its final orbit on 1 February 2005.

When Smart 1 reaches a stable elliptical orbit it will range between 3,000km and 300km from the Moon's surface.

Mission scientists made the announcement at a conference on Tuesday.

Dr Bernard Foing, Esa chief scientist, said the data gathered by Smart 1 would be used by the next generation of space scientists.

Smart 1 is testing a highly efficient solar-electric propulsion system as one of its key mission objectives. The engine works by expelling a beam of charged xenon atoms - ions - from the back of the probe.This produces thrust in the opposite direction, pushing the spacecraft forward. The energy to feed the system comes from the solar panels, hence the term "solar-electric".

Fuel savings due to better than projected power and engine performance mean that Smart 1 has arrived much earlier than expected.

"We didn't expect to arrive this early," said mission control manager Octavio Camino. "This means we can start the science tasks sooner than expected."

When it begins its scientific investigations in January 2005, Smart will deploy an X-ray spectrometer called D-Cixs which will comprehensively map chemical elements on the Moon's surface. This will help scientists test theories of its birth and evolution.

"We believe that the Moon is the daughter of the Earth and it was created [4.5 billion years ago] when a planetary embryo [half] the size of Mars impacted the Earth," Dr Foing explained.

"This sent some mantle of the Earth into orbit and the debris re-condensed to form the Moon."

Perpetual light

Conversely, studying the Moon's origins and evolution could also shed light on the composition of the early Earth.

One target for D-Cixs is the biggest impact crater in the Solar System - a massive hole on the far side of the Moon that is 2,500km (1,550 miles) across.

By looking down the hole, it should be possible to analyse the composition of rocks deep within the Moon's interior.

Smart 1 will also survey a flat region known as the Peak of Eternal Light near the south pole, which is thought to be bathed in perpetual sunlight and also appears to be flat.

This could render it a particularly suitable spot for future manned lunar bases.

"We will look at types of future landing activities for robotic and human bases," said Dr Foing.

"Smart-1 can be used as a precursor for long-term exploration of the planets."

The temperature in this region is stable at -20C; much more acceptable than the equator where the temperature varies from 120C to -170C.

Smart 1 will also look for water-ice in very deep craters nearby which some scientists think could be used both to drink and to create oxygen for moonbase dwellers.

It will also search for possible building materials for a future moonbase.

"Another thing you need is building blocks. You need to find bits of the Moon that are good for turning into concrete for building your lunar base," said Professor Manuel Grande, lead scientist for D-Cixs, from the Rutherford-Appleton Laboratory in Didcot.