Behnken said it was uncertain how long their mission would last after arriving at the ISS on Sunday.

"It could be relatively short or we could potentially double or triple maybe what was originally expected for us just a few short months ago," he said.

"It's a little bit strange...I'm trying to explain it to my son now, just six-years-old.

And from his perspective, he's just excited that we're going to get a dog when I get home." SpaceX, the private rocket company of billionaire entrepreneur Elon Musk, launched the two American astronauts into orbit from Florida on Saturday (May 30) in a landmark mission marking the first spaceflight of NASA astronauts from U.S. soil in nine years.