After two successful test flights, SpaceX is set to launch an unmanned Dragon cargo ship loaded with a half-ton of the supplies -- including ice cream -- to the International Space Station.

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 p.m. EDT, 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.

During a test flight in May, a SpaceX Dragon capsule successfully rendezvoused with the International Space Station. The first operational cargo flight of a SpaceX capsule is scheduled for launch Sunday.

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 on October 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.

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SpaceX cargo ship primed for first operational station flight