European spacecraft Smart 1 made its closest initial approach to the surface of the Moon at 1748 GMT on Monday - its first so-called perilune.
Smart 1 began firing its ion engine at 0524 GMT on 15 November to help bring it into orbit around the Moon.
Continuous engine firing over the next four days should push the probe over a "hump", beyond which it will be captured by the satellite's gravity.
After this, it spirals closer to the surface until reaching its final orbit.
Eventually, Smart 1 reaches a stable elliptical orbit where it will range between 3,000km and 300km from the Moon's surface.
"Up until now, Smart 1 has been in an orbit around Earth which has been getting bigger and bigger. Then after Thursday it will be in an orbit around the Moon which will be getting smaller and smaller," Professor Manuel Grande, lead scientist on the D-Cixs X-ray spectrometer aboard Smart 1, told the BBC News website.
"You just need the ion drive to tickle you over that hump."
Professor Grande added scientists would be firing the ion engine "pretty much constantly" until the probe reached its final orbit on 1 February.
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".
When it begins its scientific investigations in early 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 the size of Mars impacted the Earth," Dr Foing explained.
GIANT IMPACT THEORY
Suggests Mars-sized object crashed into early Earth
Debris thrown into space aggregated into the Moon
Supported by similar composition of Earth and Moon rocks
"This sent some mantle of the Earth into orbit and the debris re-condensed to form the Moon."
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 in the Moon more than 2,000 miles across on its far side.
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 Grande from the Rutherford-Appleton Laboratory in Didcot.
HOW SMART 1'S ION ENGINE WORKS
Xenon gas atoms are pumped into a cylindrical chamber, where they collide with electrons from the cathode. The electrons - which are negatively charged - knock electrons off the xenon atoms, creating xenon ions - which are positively charged.
Coils outside the chamber create a magnetic field, which causes electrons from the cathode to spiral and become trapped at the mouth of the chamber.
The build-up of negatively-charged electrons at the mouth of the chamber attracts the positively charged ions, accelerating them out of the chamber.
The stream of accelerated ions leaving the chamber thrusts the spacecraft forward. Although the force is small, over time it creates great speed in the frictionless environment of space.