Odyssey spacecraft nears orbit around Red Planet

LOS ANGELES (AP) -- The 2001 Mars Odyssey spacecraft closed in on Mars on Tuesday on a mission that NASA hoped would mark a comeback after a pair of failures.

Odyssey was designed to map minerals and search for frozen reservoirs of water on the planet's dusty surface.

The probe was programmed to fire its main engine late Tuesday, following a six-month crossing from Earth, in order to ease it into orbit around the red planet.

Entering orbit would be the probe's riskiest move during the $297 million mission. Two of the last three vehicles the National Aeronautics and Space Administration sent to Mars, Climate Orbiter and Polar Lander, failed just before or upon arrival in 1999.

Odyssey's engine burn was programmed to last just under 20 minutes, enough to consume 579 pounds of propellant or more than one-third of the spacecraft's weight.

A successful firing would place the vehicle in an elliptical, 20-hour orbit around Mars. It would then take Odyssey until late January to settle into a circular, mapping orbit where it would join the Global Surveyor craft, which has been mapping the planet since 1997.

Odyssey's instruments include one that uses infrared imagery to chart the distribution of minerals on the planet's surface. Another will measure gamma radiation coming from the surface to pinpoint specific elements, including hydrogen, most likely in the form of buried deposits of water ice.

Odyssey was designed to use those instruments to build up the first inventory of the planet's global makeup.

It was upon entering Mars' orbit that Climate Orbiter was lost in September 1999. A mix-up of English and metric units used in calculating trajectory sent the spacecraft too close to Mars, causing it to burn up in the atmosphere. Just three months later, Polar Lander plummeted to the surface, probably because a software error cut off its engines prematurely.

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On the Net:

Odyssey: http://mars.jpl.nasa.gov/odyssey/