The first space probe to the moon in 25 years braked safely into lunar orbit a few hours ago, successfully completing the first leg of a 12-month mission which could open up a new era in space travel.
Launched by the US National Aeronautics and Space Administration and dubbed the Lunar Prospector, its purpose is to search the moon's surface for a staple of human life - water. Find water and you have found refreshment, but also oxygen to breathe and energy for heat, light and fuel for further space travel.
The moon provides an ideal way-station for those seeking a jumping-off point for other planets. Despite its harsh environment, the moon's gravitational pull is much less than on Earth so less fuel is needed to blast off on visits to other planets.
Discussions of such efforts smack more of science fiction than reality, however, and clearly won't come to pass for decades.
A key part of the probe's mission is to look for water at the lunar poles using a neutron spectrometer. The instrument will be able to measure amounts of water available at up to a metre under the surface.
It also carries a gamma ray spectrometer that will scan not for water but for metals and other elements. It could identify deposit of iron, aluminium or uranium if they are there and close enough to the surface.
The neutron spectrometer detects neutrons - particles from the nucleus of an atom - which escape into space when hit by energetic particles known as cosmic rays. The device, built by the Los Alamos National Laboratory in New Mexico, can measure the neutrons' energy levels.
The neutrons pick up energy from the cosmic rays but smack into other surface materials before escaping into space. If they hit heavy elements they will bounce away, retaining their energy, but if they hit lighter elements, such as hydrogen, the neutrons will lose some of their energy before reaching the detector. If these lower-energy neutrons are seen then, indicate the presence of a hydrogen compound such as water would be indicated.
A similar process is involved with the gamma ray spectrometer, which watches for the spectrum of gamma rays and neutrons emitted by the contents of the moon's crust. Each element has a tell-tale spectrum to give researchers a very clear picture of the crust's make-up.
There is also an alpha particle spectrometer on board the Lunar Prospector which should give information about seismic activity on the moon. The hardening lunar crust would have contained uranium-238 which, in turn, produces radon gas, just as it does on Earth.
If "moonquakes" vent radon to the surface, the spectrometer will capture the evidence by looking for the alpha particle signatures of radon's radioactive decay.
The Lunar Prospector will carry out a series of high-altitude orbital manoeuvres this week as it brings itself into its mapping orbit 63 miles above the lunar surface, although it will dip as low as six miles for some measurements. Its spectrometers are already taking data and will do so for at least 12 months, all going well.
The probe will also provide the most accurate map yet of the lunar surface. It will provide an atlas detailing every named feature of the moon.
Despite the high return expected from Lunar Prospector, NASA includes it among its Discovery programme of low-cost, highly-focused space exploration missions.
Discovery includes a series of launches to Mars over the next few years, continuing the work begun with the Pathfinder/Sojourner mission last summer which saw a shoebox-sized space buggy touring the Martian surface and the Global Surveyor mission which will begin detailed mapping of the "Red Planet" in March using a probe already in orbit around Mars.
