Scott Smith, a nutritionist at NASA's Johnson Space Center in Houston, knows exactly what he'd like to bring back from space -- astronauts' urine.
Unraveling the mysterious impacts of microgravity on the human body takes careful and repeated analysis of urine, blood and other scientific samples, most of which have been stranded aboard the International Space (ISS) Station for more than a year.
"We have not brought any samples back since the last shuttle flight," Smith told reporters at a press conference before Space Exploration Technologies (SpaceX) launched its Dragon cargo capsule earlier this month.
Unlike other freighters that service the station, Dragon is designed for round-trip flights, enabling scientists and engineers to bring back experiments and equipment no longer needed aboard the outpost.
It's a service that hasn't been available since the space shuttles stopped flying more than a year ago.
"When NASA knew the shuttle was going to retire, we actually flew extra freezers to the space station so the crews could continue to collect samples on orbit knowing we would bring them back when we had the chance," Smith said.
The wait is nearly over. Astronauts aboard the station are preparing to release the Dragon capsule that has been parked at the outpost for more than two weeks, the first of at least 12 supply runs the company known as SpaceX plans to conduct for NASA under a $1.6 billion contract.
Dragon, which arrived Oct. 10, already has been packed with about one ton of cargo for the return trip home.
"(Dragon) essentially replaces that capacity that we lost when the shuttle retired," said lead space station scientist Julie Robinson.
In addition to returning biological and physical sciences samples, Dragon will be bringing back research equipment and other station components so they can be refurbished and flown again.
SpaceX didn't specially design Dragon as a round-trip cargo hauler. The capsule is intended to double as a passenger ship for astronauts.
In addition to its cargo missions for NASA, SpaceX has a separate $440-million partnership agreement to upgrade the Dragon the capsule and its Falcon 9 launcher to carry humans.
"The cargo is just a steppingstone," said Garrett Reisman, a former NASA astronaut who now oversees SpaceX's commercial crew programs.
"The next steppingstone is working on carrying people in the Dragon and the Falcon 9. They were designed with human-rating standards in mind," he said.
The key part of the upgrade is an emergency escape system so astronauts can be sprinted away from the rocket in case of an accident during launch.
SpaceX plans a launch pad abort test next year and an inflight abort test in 2014.
"We're going to launch a Falcon 9 with a Dragon on top and go up to around max Q (the point of maximum aerodynamic pressure on a vehicle) and show that we can punch it off the top and safely bring the crew away even in that critical flight regime," Reisman said last week at the International Symposium of Personal and Commercial Spaceflight in New Mexico.
If all goes as planned, SpaceX plans to test fly a Dragon capsule with its own astronauts aboard in May 2015.
"We're going great guns, we're working real hard and we hope to have people flying very soon inside the Dragon," Reisman said.
NASA also has partnership agreements with Boeing and Sierra Nevada Corp for alternative human spaceships. Boeing is working on a capsule called the CST-100 that would be launched on an Atlas 5 rocket, while Sierra Nevada is designing a winged vehicle called Dream Chaser that also would fly aboard an Atlas 5.
Unlike previous NASA programs, the companies retain ownership and intellectual property rights of their spaceships, positioning them to offer flights and/or vehicles on the commercial market.
"Once we have this up and running for NASA, we are free to go and use it for other purposes," Reisman said.
Dragon is due to be released from the station's robot arm at 9:26 a.m. EDT on Sunday and splash down in the Pacific Ocean off the coast of Baja California at 3:20 p.m. that same day. Its cargo includes 866 pounds of science gear and experiment samples -- and 400 bags of crew urine.