Donned with protective masks and goggles, the crew members opened the hatch door at 5:53 a.m. (0953 GMT) and moved inside of the Dragon, to begin preparations for unloading cargo over the next four days.

The capsule is carrying about 1,200 pounds (544 kilograms) of food, water, clothing and supplies for the station crew. It will be repacked with equipment to come back to Earth, and depart the station on May 31.

Splashdown in the Pacific Ocean off the coast of southern California would occur later that day.

The capsule, built and operated by Space Exploration Technologies, or SpaceX, is the first of two new commercial freighters NASA will use to fly cargo to the $100 billion outpost following the retirement of the space shuttles last year.

SpaceX is the first private company to successfully launch their own spacecraft to the orbital outpost.

The United States plans to buy commercial flight services for its astronauts as well, breaking Russia's monopoly on flying crews to the station.