The moment mission controllers at JPL realised they had a problem

The Mars Climate Orbiter was due to go into orbit around the Red Planet at about 0950 BST (0850 BST). It completed a successful burn of its main engine and then went behind Mars, as planned.

Mission controllers at the Jet Propulsion Laboratory in Pasadena then waited for the orbiter to come out the other side and contact Earth.

Mars Climate Orbiter was designed to be the first interplanetary weather satellite

This should have happened at about 1027 BST (0927 GMT) but the signal was never picked up.

"We are still attempting to establish contact with the spacecraft," JPL spokesman Frank O'Donnell said. "We have not heard from it since the burn."

It is believed the spacecraft is in orbit around Mars, and attempts are now underway to re-establish communications.

The Mars Climate Orbiter reached the Red Planet after travelling some 670 million kilometers (416 million miles), following its launch into space on a Delta II rocket last December.

The orbiter was designed to be the first interplanetary weather satellite, studying the ebb and flow of carbon dioxide frosts and giant dust storms over a whole Martian year.

Internal computer

Nasa says the lack of contact does not necessarily mean the craft is lost forever. It could be that the craft's antennae have not locked-out properly, preventing any communication with Earth.

Nasa's Richard Cook: We hope to hear from the satellite within a few hours

Engineers hope that the spacecraft has entered a "safe mode" with its internal computer executing commands designed to put it back into normal operation.

They speculate that it may have lost its 'inertial reference' which means that it no longer knows in which direction in space it should be pointing.

The recovery plan for this is that the craft will automatically begin a process called "sun-coning" during which it will try to find the Sun and then track around it in order to pick up a signal from Earth.

Controllers hope the craft is in safe mode

Signals will also be sent from NASA's Deep Space Tracking antenna at Canberra, Australia. They will be sent over a broad range of frequencies in the hope that one of them will be detected by the spacecraft.

With the 22 minute delay on the round trip to Mars due to the speed of radio signals, scientists are facing a frustrating and worrying few hours as they wait for Mars Climate Orbiter to get back in touch.

If it remains lost, it will be a disaster. A few years ago another major spacecraft, the Mars Observer, was lost as it attempted to enter Martian orbit.