Bad weather was the only major concern as NASA made final preparations on its second attempt to launch the Atlantis and its astronauts Thursday and start a robot on its 806-million-mile journey to Venus.

The space shuttle was to lift off at 11:48 a.m. MDT with a five-member crew. Six hours later, they were to deploy the $550 million Magellan - America's first deep space probe in 11 years - and then spend four days orbiting 184 miles above Earth.Forecasters Thursday said there was a 40 percent chance that showers or crosswinds at a Kennedy Space Center emergency landing site could force a postponement.

Before dawn, technicians started pumping more than a half-million gallons of liquid hydrogen and oxygen into the spacecraft. The crew was to board four hours later.

The final preparations followed a frenzied effort by engineers to correct problems that on Friday stopped the countdown just 31 seconds from liftoff. A flawed pump and a suspicious hydrogen line were replaced.

"A real problem stopped us," shuttle chief Richard Truly said. "We've fixed that. We are ready to fly again."

Today's launch opportunity, determined by the path Magellan must follow to reach Earth's mysterious neighbor, was to end at 12:52 p.m. MDT. If the launch is postponed, there are daily launch opportunities until May 28. At that point, Earth and Venus will have moved out of alignment and the Magellan mission will not be possible for two years.

Magellan then will rocket itself out of Earth orbit and start a 15-month voyage that carries it 1 1/2 times around the sun before reaching Venus.

Magellan marks the beginning of a new era in a U.S. planetary exploration program that has not had a launch since 1978.

Venus, Earth's nearest planetary neighbor, has fascinated humans since they first began scanning the sky. It is the brightest object in the nighttime sky, except for the moon. It appears in season as either the morning or the evening "star," but Venus is actually a planet that orbits 67 million miles from the sun. The Earth orbits 93 million miles from the sun.