NASA

Controllers at NASA's Kennedy Space Center spent Sunday preparing for the first commercial cargo flight to the International Space Station, readying a SpaceX rocket for an evening launch.

The Falcon 9 rocket with its unmanned Dragon capsule is scheduled for liftoff at 8:35 p.m. if the weather holds. It will be the first of a dozen NASA-contracted flights to resupply the International Space Station, at a total cost of $1.6 billion.

The scheduled launch comes nearly five months after a demonstration mission in which a Dragon capsule successfully berthed at the station and returned to Earth. SpaceX President Gwynne Shotwell said the mission slated to start Sunday evening isn't "substantially different" from that flight, "with the exception that we got there once."

"We demonstrated we could do it, so there might be a teeny, teeny bit of relaxation," Shotwell told reporters Saturday night. "Not a lot, though."

The unmanned capsule will be packed with about 1,000 pounds of cargo -- everything from low-sodium food kits to clothing and computer hard drives. It's slated to return in late October with about 2,000 pounds, including scientific experiments and failed equipment that can be repaired and sent back, ISS Program Manager Mike Suffredini said.

"These flights are critical to the space station's sustainment and to begin full utilization of the space station for research and technology development," he said.

If the launch goes off on time Sunday, the Dragon spacecraft will catch up with the Space Station early Wednesday morning. Station Commander Sunita Williams and Aki Hoshide from the Japanese Space Agency will use the robotic arm to grab Dragon and berth it to the station.

Much of Dragon's cargo is material to support extensive experimentation aboard the space station. One deals with plant growth. Plants on Earth use about 50% of their energy for support to overcome gravity. Researchers want to understand how the genes that control that process would operate in microgravity -- when objects are in free-fall in space. Down the road, that could benefit food supplies here on the planet.

The spacecraft is also carrying nearly two dozen microgravity experiments designed and being flown through the Student Experiment Spaceflight Program. More than 100 students and teachers and family members will be at Cape Canaveral for the launch.

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First commercial flight to ISS set to take off