An unmanned cargo capsule built by SpaceX under a $1.6 billion commercial contract with NASA was prepared for launch Sunday on its first operational flight to the International Space Station, a milestone mission intended to restore the agency's ability to deliver critical components and supplies to the lab complex and to bring hardware and experiment samples back to Earth.
The Dragon capsule, perched atop a SpaceX Falcon 9 rocket, was scheduled for liftoff from launch complex 40 at the Cape Canaveral Air Force Station at 8:35:07 p.m. EDT (GMT-4), roughly the moment Earth's rotation carries the pad into the plane of the space station's orbit.
Forecasters predicted a 60 percent chance of acceptable weather, improving to 80 percent "go" Monday and Tuesday.
Launched into an initially elliptical orbit with a high point of 202 miles and a low point of around 124 miles, the solar-powered spacecraft will carry out a complex computer-orchestrated series of rendezvous rocket firings to catch up with the space station early Wednesday.
If all goes well, station commander Sunita Williams and Japanese astronaut Akihiko Hoshide will use the lab's robot arm to grapple the Dragon capsule around 7:22 a.m. Wednesday, maneuvering it to a berthing at the Earth-facing port of the forward Harmony module.
Over the next three weeks or so, the station crew will unload a half-ton of equipment and supplies, including experiment hardware, a freezer, spare parts, clothing and food. Taking advantage of the freezer, ice cream was included, a rare treat for space crews.
As the capsule is unloaded, the astronauts plan to stow nearly a ton of no-longer needed gear, failed components and experiment samples that, until now, have had no way to get back to Earth. Again using the robot arm, Williams and Hoshide plan to unberth the capsule Oct. 28 for re-entry and splashdown in the Pacific Ocean off the southern coast of California.
The SpaceX Dragon capsule is the only space station cargo craft designed to safely return to Earth, a critical capability that was lost when NASA's space shuttle fleet was retired in 2011. The Russian Soyuz spacecraft that ferry crews to and from the space station can only carry a few hundred pounds of small items back to Earth. All other station vehicles - unmanned Russian Progress supply ships and European and Japanese cargo craft - burn up during re-entry.
"The SpaceX Dragon is a really important vehicle for us because it supports the laboratory use of ISS, both in bringing cargo up to the space station and in bringing research samples home," said Julie Robinson, the space station program scientist at NASA Headquarters.
"It has a great return capability, it essentially replaces that capacity that we lost when the shuttle retired so that now we'll be able to bring home a wide variety of biological samples, physical sciences samples and we'll be able to bring home research equipment that we need to refurbish and then relaunch again."
Scott Smith, a nutritionist at the Johnson Space Center in Houston, will be eagerly awaiting crew urine samples for on-going medical research on how the body adapts to weightlessness.
"We have not brought any samples back since the last shuttle flight," he told reporters Saturday. "When NASA knew the shuttle was going to retire, we actually flew extra freezers to the space station to hold those samples so the crews could continue to collect samples on orbit knowing we would bring them back when we had the chance.
"The novelty at this point of SpaceX is, this is the first real return vehicle for these type of samples. Obviously, we can get the crew home on the Soyuz, but the cargo capability of the Soyuz is extremely limited. So this is our first set of samples that will come back."