SpaceX's next unmanned cargo trip to restock supplies at the International Space Station is scheduled for September 20, the US space agency said Friday.

The Dragon spacecraft will launch atop a Falcon 9 rocket at 2:16 am (0616 GMT) on September 20 from Cape Canaveral Air Force Station in Florida, NASA said.

"If for any reason the launch is postponed, the next launch opportunity is Sunday, Sept. 21 at approximately 1:53 am," it said in a statement.

The cargo ship will be loaded with more than 5,000 pounds (2,300 kilograms) of supplies and scientific experiments, including an ocean surface wind speed monitor, equipment for studying rodents in microgravity and cabbage-like plants to see how their growth adapts in space.

The mission, called SpaceX CRS-4, "is the fourth of 12 SpaceX flights NASA contracted with the company to resupply the space station," the space agency said.

The trip marks the fifth Dragon journey to the orbiting outpost, including its initial demonstration mission.

SpaceX and Orbital Sciences Corporation each have billion-dollar contracts with NASA to supply the space station over the coming years.

The retirement of the space shuttle program in 2011 has left the United States without a government spacecraft to transport cargo or crew to the space station.