When the Dragon capsule, made by SpaceX of Hawthorne, California, splashes down in the Pacific Ocean on Sunday after a sojourn at the International Space Station (ISS), it will mark the end of the company's first critical supply mission for NASA.
But the return will also restore a capability lost since the space shuttle retired in July 2011 ? the means to bring science experiments and other cargo back to Earth.
"We're now turning to the next step in space exploration, which is that commercial vehicles are bringing this stuff home," says Scott Smith, a nutritionist at NASA's Johnson Spaceflight Centre in Houston, Texas, who is relying on SpaceX to bring back tubes of blood and urine used in monitoring astronauts' health.
Until SpaceX's trip, the only way for astronauts to get to and from the station was aboard Russia's Soyuz capsules, but these don't have enough room for equipment as well.
Uncrewed supply vehicles run by Russia, Japan and Europe also travel to the ISS, but they are designed to remove rubbish and then burn up in the atmosphere. So once the shuttle stopped operating in July 2011, no other craft could safely bring research materials back to Earth.
"We've been anxiously awaiting a vehicle to bring this stuff home," Smith says.
He and his colleagues have been analysing the biochemistry of astronauts' blood and urine for several years. These contain biomarkers that can reveal changes in bone density and eye tissue, and so could allow astronauts to be monitored remotely and continuously, while in space.
Frozen samples
NASA prepared for the shuttle's retirement by reserving freezer space on the ISS so they could continue collecting blood and urine samples from participating astronauts.
The Dragon capsule first journeyed to the space station in May, but that was a trial run. Its first official mission, in which it carried equipment for experiments aboard the space station, plus food, clothes and life-support gear, began on 7 October.
Dragon is due to be released on 28 October and splash down in the Pacific Ocean the same day, where it will be retrieved.
It will be carrying 900 kilograms of scientific experiments, biological samples and failed equipment.
Space cucumbers
The cargo will include experiments exploring how the anti-fungal properties of the bacteria Bacillus subtilis and the predation techniques of jumping spider change in space ? both part of the educational YouTube Space Lab; results from the European BioLab, which studies how single cells and complex organisms develop in low gravity, and a Japanese experiment called CsPINs, which studies how cucumber plants grow in space, a step towards the cultivation of plants there.
SpaceX's capsule will also bring back used or damaged equipment, like expended life-support and crew healthcare systems, and spacewalk gloves that belonged to previous crew members.
Does it make any difference that a private company is doing this? Not to Smith. "We've been anxious for sample return, period," he says. "We're not picky about how it comes back." He adds: "It isn't a plus or a minus, just the way things are."
Chewed fingernails
But Smith is nervous about his samples staying cold. The blood and urine samples will be packed in special freezers or cold bags to make sure they stay below -30?C from the time they leave the space station's freezers on Friday to the time they get to Houston on Wednesday morning. If they thaw, the researchers will lose important information, he says.
"We have a very carefully orchestrated plan," he says. "But my heart rate will be up from the minute they take them out of the freezers on the station to the minute we put them in the freezers in my lab."
That's a long time to bite your fingernails ? but no longer than it was during the shuttle era. "I'm used to it," he says. "My fingernails are pretty short."
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