Officials from the National Aeronautic and Space Agency had planned to launch the late Saturday, but delayed liftoff by 24 hours due to strong upper altitude winds over Cape Canaveral, spokesman George Diller said Sunday.

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.

Space officials have tried several times to launch the probe, but have been thwarted repeatedly because of poor weather.

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 probe, 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.

The Opportunity rover -- also known as Mars Exploration Rover-B -- will look for evidence that water once flowed on the Red Planet's surface.

Researchers are seeking answers to one of space exploration's most persistent questions -- whether conditions present on the Red planet were favorable enough for life to evolve.

"We know Mars ... had water in the past," said Ed Weiler, NASA's associate administrator.

"It may have it in the present. What we don't know is how long this water persisted in any given place ... If it stayed there for tens of millions of years, then there is a good chance life might have evolved."

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."

Spirit is scheduled to arrive at Mars on January 3, while Opportunity will arrive three weeks later on January 24.

SPACE.WIRE