NASA is hiring out space station supply runs to American companies now that the shuttles are museum relics. The shuttle fleet was retired in 2011 after 30 years so the space agency could focus on human trips beyond low-Earth orbit; the destinations include asteroids and Mars.
Space station partners Russia, Japan and Europe also launch cargo ships, but those vessels are filled with trash and destroyed during descent. NASA scientists eagerly are awaiting nearly 500 samples of astronauts' blood and urine that have been stockpiled aboard the complex since Atlantis visited for the last time more than a year ago.
NASA's human exploration and operations chief, Bill Gerstenmaier, called the returning specimens "unbelievably unique and precious" and a major reason for going commercial in the post-shuttle era.
"There were a lot of skeptics at the beginning, but as evidenced today, I think you're starting to see that this can work and can move forward," Gerstenmaier said.
SpaceX is working to make its Dragon capsule safe enough to carry astronauts, possibly in three years. For now, NASA is paying the Russian Space Agency tens of millions of dollars to launch astronauts to the space station. Other U.S. companies also are vying for crew-carrying rights.
The space station currently houses three astronauts from America, Russia and Japan. Another American and two more Russians will arrive in two weeks.