The crew of space shuttle Discovery, from left, mission specialist Clayton Anderson, Japan Aerospace Exploration Agency astronaut Naoko Yamazaki, mission specialists Stephanie Wilson, Dorothy Metcalf-Lindenburger, Rick Mastracchio, pilot James Dutton and commander Alan Poindexter arrive at the Kennedy Space Center in Cape Canaveral, Fla., Thursday, April 1, 2010. Discovery is scheduled to launch April 5 on a mission to the International Space Station.(AP Photo/John Raoux)

(AP) -- NASA's countdown clocks began ticking Friday for next week's launch of shuttle Discovery on a space station supply run, one of only four remaining.

Discovery is scheduled to blast off just before dawn Monday with seven astronauts and a load of science experiments and spare parts for the International Space Station. Forecasters put the odds of good weather at 80 percent.

This is scheduled to be the last space shuttle launch in darkness. Only three shuttle missions remain after this one.

NASA already is relying on the Russians for rides to the space station. In fact, U.S. astronaut Tracy Caldwell Dyson hitched a ride aboard a Soyuz rocket early Friday. The space agency hopes U.S. commercial rockets will take over this taxi job, but that's still several years away. As for cargo, Russian, Japanese and European spacecraft will fill the void, though none is as big as the shuttle.

"Every one of the remaining missions is an important mission for us. Because we only have a few left, we have to get everything that we're going to get uphill on these next few missions," said NASA test director Steve Payne.

"The team out here really loves launching space shuttles. So we're enjoying everything we can, while we've got it," he added.

Launch time will be 6:21 a.m., nearly an hour before sunrise.

If the visibility is good, sunlight should reflect off the shuttle as it rises and, a little later, create a rainbow of colors in the exhaust plume.

This wasn't supposed to be a night flight. But the mission was delayed two weeks after this winter's unusually cold weather stalled preparations.

Three spacewalks are planned during Discovery's 13-day flight.

Explore further Countdown begins for Tuesday space shuttle launch

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