The crew of the International Space Station welcomed U.S. astronauts Douglas Hurley and Bob Behnken after their privately built SpaceX capsule docked Sunday.

NASA astronaut Christopher Cassidy and Russian cosmonauts Anatoly Ivanishin and Ivan Vagner shook hands with their new station-mates as they climbed aboard.

"Welcome to Bob and Doug. I will tell you, the whole world saw this mission and we are so, so proud of everything you have done for our country and, in fact, to inspire the world," NASA Administrator Jim Bridenstine said back on Earth as his personal welcome.

"We sure appreciate that, sir," Hurley said. "It's obviously been our honor to be just a small part of this. We have to give credit to SpaceX, the commercial crew program, and, of course, NASA. It's great to get the United States back in the crewed launch business. And we're just really glad to be onboard this magnificent complex."

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Hurley said he "couldn't be happier" about the performance of the Crew Dragon space capsule, which brought them to the space station less than a day after liftoff from Cape Canaveral, Florida.

"The Dragon was a slick vehicle, and we had good airflow, so we had an excellent, excellent evening," Hurley said.

The Crew Dragon was flown into orbit aboard a SpaceX rocket - a commercially built spacecraft. It is the first time astronauts or cosmonauts have flown into space aboard a ship not built by a government agency.

SpaceX owner Elon Musk said he was "quite overcome with emotion" when he saw his dream come true with a perfect liftoff Saturday.

"It's been 18 years working towards this goal," he said. "This is hopefully the first step on a journey towards civilization on Mars."

Hurley and Behnken will participate in a number of scientific experiments during what is expected to be a three-month stay on the space station.