Moment of truth for Mars voyage

Scientists are hoping for better luck this time

By BBC News Online science editor Dr David Whitehouse

If all goes well, the 2001 Mars Odyssey spaceprobe will soon end its six-month journey spanning 460 million kilometres (286 million miles) and enter orbit around Mars to begin looking for frozen reservoirs of water on the Red Planet.

We're bouncing back. This mission is going to be a success

David Spencer, Nasa

For Nasa, the US National Aeronautics and Space Administration, the voyage marks not only a new phase in its mission to explore Mars, but also a test of its credibility.

The last two spacecraft Nasa sent to Mars burned-up or crashed, forcing Nasa to curtail what was once a far more ambitious programme of exploration.

"This mission is one of redemption,'" says Dr David Spencer, manager of the $297 million (£209 million) mission. "We had a couple of high-profile failures, but we're bouncing back. This mission is going to be a success."

'Nail-biting time'

If the 2001 Mars Odyssey mission fails, the repercussions could be far-reaching.

"They might call off the next bunch of missions until the war on terrorism is over," says Professor Howard McCurdy, author of "Space and the American Imagination".

For what it is worth statistics are not on Odyssey's side.

Fewer than one-third of the 30 missions launched towards the planet since 1960 have succeeded.

The most recent failures were 1999's back-to-back losses of the Climate Orbiter and Polar Lander spacecraft. Because of them a landing spacecraft that was to have accompanied the orbiting Odyssey was scrapped.

It's going to be either a real psychological boost for all of us or one more downer

Lou Friedman, The Planetary Society

As Odyssey nears the north polar region of Mars at 0330 GMT on Wednesday it is scheduled to begin firing its engine for 19.7 minutes to allow it to be captured into an elliptical orbit. Ten minutes later, the probe will slip behind Mars and go silent. "That will be nail-biting time," Spencer says.

It should reappear 20 minutes later and within another half-hour, mission engineers should be able to determine whether the spacecraft has begun to orbit the planet as planned.

"It's going to be either a real psychological boost for all of us or one more downer," says Lou Friedman, executive director of The Planetary Society.

'What are we missing?'

In 1999 the Mars Climate Observer burnt-up when entering Mars' orbit: a mix-up of English and metric units used in calculating trajectory sent the spacecraft too close to Mars.

If that was not bad enough barely three months later the Mars Polar Lander plummeted to the surface, probably because a software fault silenced its engines prematurely.

So, as Odyssey approaches Mars, engineers continue to worry about the spacecraft. Project members say Odyssey is among the most scrutinised missions ever launched by Nasa.

"We're looking at everything that could affect our success," said Matt Landano, the Odyssey project manager. "We're constantly asking ourselves, 'What are we missing?' "

Odyssey will carry out a surface survey

The plan calls for Odyssey to initially orbit Mars once every 20 or so hours. That period will be reduced as it brushes the atmosphere of Mars for a section of each orbit. The atmospheric drag will be used to further slow it in a fuel-saving process called aerobraking.

Limited mapping operations should begin within days of arrival. But the aerobraking will last until late January, at which point Odyssey will whip around Mars once every two hours about 250 miles above the planet's surface. It will then begin its mapping in earnest.

When Odyssey turns its three scientific instruments toward Mars, it will join another Nasa satellite, the Global Surveyor, which is already at work.

Global Surveyor has mapped Mars since 1997, taking more than 78,000 images of the planet.

Among them are high-resolution pictures that suggest water may have coursed across the surface of Mars in the recent geological past, raising the tantalising possibility that the planet harbours life.