Columbia's seven astronauts expressed optimism the shuttle's fifth countdown would lead to liftoff as the clock ticked toward this weekend's launch.

"This time, why, I think we have a very good feeling. We're ready to go," Columbia's commander Vance Brand said after arriving with his crew late Thursday from Johnson Space Center in Houston.Columbia is scheduled to blast off at 11:28 p.m. MST Saturday. The countdown proceeded uninterrupted Thursday, and meteorologists forecast a 70 percent chance of good weather at launch time.

NASA has been trying since May to send up Columbia with the $150 million Astro observatory, which will be operated by the crew.

Because of hydrogen leaks, three previous countdowns came to an abrupt halt hours before liftoff. Numerous repairs were needed before the shuttle was pronounced safe. Problems with the observatory also forced a postponement.

Comet Levy, one of the objects to be observed, is moving rapidly out of the solar system and is not nearly as visible as it would have been in September, during the previous launch attempt.

"You've all heard comets are harbingers of bad news and bad luck. Well, this time we have no comet so we're going to go," said astronaut Jeff Hoffman, one of the crew's four astronomers.

Among those afraid to get their hopes up are scientists who have been working on the Astro mission for years.

The goal of the flight is to study X-rays and ultraviolet light from stars and galaxies that cannot penetrate Earth's atmosphere, high-energy radiation that provides priceless clues about the chemical and physical processes that govern the structure and evolution of the universe.