But weather could still intercede.

Latest forecasts give a 60 percent chance of favorable weather conditions for the launch aboard a Delta II rocket from the military base at Cape Canaveral, Florida, abutting the Kennedy Space Center.

Liftoff of the rocket carrying the Mars Expedition Rover (MER-B), named "Opportunity," is tentatively set for 23:56 and 16 seconds local timeplus 16 GMT Sunday), with a second window, if needed, at 00:37 and 59 seconds (0437 plus 16 GMT) Sunday.

"The team has done everything possible to make this mission successful," NASA associate administrator Ed Weiler told a press conference.

But he was reserved.

"Mars is a graveyard of spacecraft," he said. "The job is just beginning. It's not time for champagne quite yet. That time will come about 3 months after landing," programmed for next January.

The first of the twin probes, MER-A, or "Spirit," was launched on June 10, beginning a three-month, 500 million km (310 million mile) voyage, to end on January 3 2004 in the Gusev Crater, 15 degrees south of the equator of Mars.

"Spirit is behaving in a excellent fashion, it is in perfect condition," said Peter Theisinger, Mars Exploration Rover project manager of NASA's Jet Propulsion Laboratory in Pasadena, California.

The second probe is heading for the Meridiani Planum, a zone containing an concentration of ferrous oxide situated two degrees south of Mars' equator, where it is scheduled to set down on January 25 2004 after a 491 million km (305 million mile) journey.

Although both probes will set down relatively close the the martian equator, they will be on virtually opposite sides of the planet, some 6,000 miles (9600 km) apart.

NASA is investing a total of 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," said Weiler. "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."

Weiler had enthusiastic words for the European Space Agency, which also has a probe -- "Mars Express" -- bound for Mars and scheduled to arrive next January.

"We look at this as an opportunity to join forces as two major space agencies and share data and the public excitement," he said.

"It would be great to have three landers on Mars at once. So far, I haven't detected any sense of competition. Ultimately, if we send humans to Mars, I have the feeling it's going to be an international effort so we've got to learn how to work together and I think we are doing that here."

SPACE.WIRE