Science Correspondent Christine McGourty: "It could help to identify landing sites for future manned missions to Mars"

After a nine-month journey, Nasa's Mars Climate Orbiter (MCO) began manoeuvring into orbit around the Red Planet at about 0950 BST (0850 GMT).

It will be the first weather satellite to orbit a planet other than the Earth.

The scientists behind the MCO want to observe the ebb and flow of carbon dioxide frosts and giant dust storms over a whole Martian year.

The orbiter has been fitted with a Pressure Modulator Infrared Radiometer, constructed at Oxford University in England by a team headed by Professor Fred Taylor.

Where did all the water go?

It will probe the temperatures, dust, water vapour and clouds in the thin Martian atmosphere.

They are trying to work out what happened to reserves of water, which Mars is widely believed to have had in the past.

Scientists believe they may be contained in polar ice caps or held in a huge subterranean reservoir.

Professor of Atmospheric Physics, Oxford, Frederick Taylor: "It's time to look at another planet"

Prof Taylor, who spent 10 years working on the design of the sensor, said: "This has implications for future landings.

"Humans would need to land where the atmosphere is relatively moist and it is possible to drill for water."

The probe's flight operations manager at Nasa, Sam Thurman, says: "After traveling 670 million kilometres (416 million miles) during the last nine months, MCO is now ready for its most dramatic moment, the orbit insertion burn."

Dr Thurman, speaking from Nasa's Jet Propulsion Laboratory in Pasadena, California, said: "The spacecraft is on a course that will pass just 193 kilometres (120 miles) over the north pole of Mars, at which point it will fire its main engine to break into orbit."

Eye in the sky

The unmanned probe is to observe the seasons from March 2000, to January 2002 - 687 Earth days or one Martian year.

The MCO's solar panel is 5.5 metres long

Dr Richard Zurek, the project scientist at Pasadena, said: "MCO will do what weather satellites do.

"It will take pictures of clouds, look for storms, and try to understand the atmospheric winds by measuring temperature and pressure, and by watching how the atmospheric distributions of dust and water vapour change with time."

MCO also will study how water is distributed across the Martian landscape, where temperatures rarely rise above freezing and often sink as low as -88C (-126F).

But MCO has another crucial role to play.

It will serve as a communications relay station for the Mars Polar Lander (MPL) which is due to touch down on 3 December 1999. MPL will search for sub-surface water and then send data to MCO, which will relay it back to Earth.

Calling home

The MPL mission will end in February 2000 but when the MCO finishes its observation mission two years later, it will still be used for communications between Earth and any future spacecraft that land on Mars.

MCO was launched last December and is travelling at 11,900 kilometres (7,380 miles) per hour.

As well as the radiometer, it is also carrying a colour imager, which will gather horizon-to-horizon images at up to kilometre-scale (half-mile) resolutions, which will then be combined to produce daily global weather images.

The camera will also image surface features and produce a map with 40-metre (130-foot) resolution in several colours.