Indeed, Dragon's ability to return cargo to Earth is unique among the unmanned cargo craft the station's international partners provide. NASA's space shuttles were the only other vehicles able to do this. But NASA flew its final shuttle mission in July 2011, and the orbiters now are museum pieces.
All of the other cargo craft operating, as well as the capsule a second US company is building to supply commercial cargo service for NASA, become trash incinerators once they leave the station. They and the refuse they carry burn up on reentry.
Dragon is slated to return to Earth with 1,673 pounds of cargo, including components from the station's life-support system that failed and were replaced with on-orbit spares.
"It's good to be able to have that capability back," says Michael Suffredini, NASA’s space-station program manager. It saves money when hardware needs to be repaired and sent back to the station, and it allows engineers to examine the components to figure out why they failed.
And, he added during a pre-launch briefing, it's nice to have a US-based carrier back in the mix. It gives the US more flexibility in getting replacement parts to the space station in a timely fashion. Moreover, "shipping and customs can kill you when you're trying to get [parts] overseas" to ride on Russian, Japanese, or European cargo craft.
"We're excited to have the vehicle coming up to visit the ISS," he says.
The launch is set for 8:35 p.m. Eastern Daylight Time Sunday, although the weather forecast gives the mission a 40 percent chance of a scrub because conditions could violate launch requirements. Once off the pad, Dragon should take about 53 hours to reach the station, said Gwynne Shotwell, SpaceX's president. The capsule is expected to remain hitched to the space station for about three weeks.