CAPE CANAVERAL, Fla. -- A private company is on the verge of launching another cargo ship to the International Space Station.
On Sunday night, California-based SpaceX will attempt to send a Dragon capsule to the orbiting lab.
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Liftoff is scheduled for 8:35 p.m. Forecasters put the odds of acceptable weather at 60 percent. Thick clouds and rain are the main concerns.
A Dragon cargo ship successfully docked to the space station last May, but that was considered a test flight. The coming mission is the first under a $1.6 billion contract with NASA that calls for a dozen resupply flights by SpaceX, essential in the post-shuttle era.
The Dragon will remain docked to the space station for most of October. Astronauts will fill the capsule with blood and urine samples, other experiments and old equipment, for its return to Earth.
The nearly 500 tubes of blood and syringes of urine have been stashed in space station freezers since the last space shuttle flight, by Atlantis, in July 2011. The decommissioned Atlantis, and sister ships Discovery and Endeavour, are now museum relics.
NASA nutritionist Scott Smith said these blood and urine samples -- part of medical studies -- will be the first to be returned since Atlantis' final voyage.
"This is the first real return vehicle for this type of sample," Scott told reporters Saturday.
The cargo ships periodically flown by Russia, Japan and Europe do not have the capability to return anything; they burn up upon re-entry. The SpaceX Dragons parachute down into the Pacific, reminiscent of NASA's old-time capsules.
"While it may seem very strange to some folks, my typical line is that, `It may be urine to you, but it's gold to us,' " Smith said. "There's a lot of science that comes out of this."
SpaceX -- or Space Exploration Technologies Corp. -- is run by billionaire Elon Musk, a co-founder of PayPal who also directs the electric car maker Tesla Motors.
His space company is working to turn the unmanned Dragon vessels into craft that could carry American astronauts to the space station in the coming years. Until SpaceX or another U.S. company can do that, NASA astronauts will have to keep riding on Russian rockets at a steep cost.