But preparing for the historic flight has been challenging, SpaceX's billionaire CEO Elon Musk said.

The unmanned Dragon capsule is slated to launch atop the company's own Falcon 9 rocket from the Cape Canaveral Air Force Station in Florida on April 30. It will be the second flight of the Dragon spacecraft, but the first time the California-based company attempts to rendezvous and dock with the space station.

"There are a huge number of things that we had to address," Musk told SPACE.com. "The biggest thing is to pass all of NASA's safety requirements for going and docking, or berthing, with the space station, because the space station is a $100 billion asset. It has astronauts from different countries onboard, and Dragon is approaching it as a robotic spacecraft, and if it were to go haywire, in theory, it could damage or destroy the space station. So, it has to go through a very rigorous set of safety and reliability requirements and pass all those, which we just got our approval last week. That was the toughest thing."