NASA headed quietly Friday into the final hours of Columbia's fifth countdown for an astronomy mission that has been on hold for half a year because of a leaky shuttle.

Columbia is scheduled to blast off at 11:28 MST tonight with seven astronauts and an astronomical observatory. The shuttle will orbit 218 miles above Earth for 10 days."We're going to go do it this time," astronaut Sam Durrance said after arriving from Johnson Space Center in Houston late Thursday.

Meteorologists were predicting a 65-percent chance of good weather at launch time. The main concerns are low clouds and high winds.

Coincidentally, the Soviets plan to send two cosmonauts and a Japanese television journalist into space on Sunday. Their destination is space station Mir.

Columbia's seven-member crew has been trying to reach space since late May, when the shuttle was grounded by hydrogen leaks. The mission originally was scheduled for March 1986, but was delayed until this year because of the Challenger accident.

Three of Columbia's four previous launch attempts were called off because of leaking hydrogen, including the latest one in mid-September. Another was halted because of problems with one of the four telescopes that constitute the $150 million Astro observatory.

Each time, a comet has been among Astro's planned targets. Comet Levy will be viewed once during the mission; it is moving rapidly out of the solar system and not nearly as visible as it would have been in September.

"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," astronaut Jeffrey Hoffman said, smiling.

So far, only "nickel and dime problems" have popped up during the countdown that began Thursday, said NASA test director Al Sofge. "They're the type of thing you would expect during a launch countdown," he said.