For the fourth day, there was silence from the spacecraft as scientists tried to tell it to prepare its systems to enter Mars' orbit amid growing fears that it would overshoot the planet and head off into oblivion - taking with it a sizeable chunk of the Nasa space agency's budget.
Last night the first American emissary to Mars since 1976 was scheduled to start orbiting the red planet. Radio transmissions broke off on Saturday. Scientists were still unable to establish whether it was about to fire its thrusters and enter Martian skies.
The Mars Observer, which is about the size of a golf cart, was launched on 25 September on an 11-month, 450,000,000-mile voyage which was supposed to gather the most detailed information to date about the planet's terrain, seasons and weather changes. It was intended to spend more than two months manoeuvring into a 234- mile-high orbit, then undergo a month of testing before taking at least 687 days, or one Martian year, to study the planet.
Nasa engineers have been exploring several theories. They believe the spacecraft's clock may be broken, its radio could have overheated or its antenna could be askew. To correct this, they have been sending a variety of electronic orders telling the spacecraft to switch to different systems or adjust its equipment. All were ignored.
The loss of contact started as the fuel tanks were being pressurised, giving rise to the possibility that the tanks might have blown up. But Glenn Cunningham, heading the project at the Jet Propulsion Laboratory in Pasadena, California, called that a highly unlikely scenario.
'We go on the presumption these kinds of problems are caused by simple little things, and probably not a whole conglomeration of exotic problems,' he said. 'What we need to do is figure out what that simple little problem is.' But, Mr Cunningham added gloomily, if the spacecraft is lost it 'would be a great blow to the planetary science community'.
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