Launch is now scheduled for 23:46 Sunday (0356 GMT Monday) with a second launch window, if needed, set for 00:28 (0428 GMT) Monday at the Air Force base adjacent to the Kennedy Space Center here.

The weather is expected to improve overnight Sunday to Monday, and forecasters see a 70 percent chance of favorable conditions, he said.

Opportunity is waiting to follow its twin, the Mars Expedition Rover "Spirit," which was launched June 10 and is scheduled to land in early January 2004 in the Gusev Crater, 15 degrees south of the equator of Mars.

If it is launched successfully on Sunday night, Opportunity will be off on a seven month, journey 491-million kilometer (305-million mile) to the Meridiani Planum, a zone containing an concentration of ferrous oxide situated two degrees south of Mars' equator.

Although both probes will set down relatively close to the Martian equator, they will be on virtually opposite sides of the planet, some 6,000 miles (9,600 kilometers) apart.

NASA is investing some 800 million dollars on the two six-wheeled vehicles, which for three months are to probe sites thought to be geologically important, roaming in search of clues to whether Mars has or could support life.

"These missions are not designed to find life on Mars," NASA associate administrator Ed Weiler told a press conference Friday. "They are not designed to find water on Mars. They are designed to answer a critical question in the search for life.

"We know Mars has water, we know it had it in the past and may have it in the present," he said. "What we don't know is how long the water persisted in any given place.

"If it stayed there for tens of millions of years, then there is a good chance that life might have evolved. Because on Earth, wherever we find water and energy and organic compounds, we find life, no matter what the conditions are."

SPACE.WIRE