Billionaire entrepreneur Elon Musk called the July 1969 Apollo 11 mission to land a man on the moon "probably the most inspiring thing in history. It certainly inspired me. I'm not sure that SpaceX would exist if not for Apollo 11."

SpaceX CEO Elon Musk with "Sunday Morning" contributor Jeffrey Kluger. CBS News

Still, Musk, the CEO and lead designer of SpaceX (which he founded in 2002), said he expected the United States to have a base on the moon by now, and to have sent people to Mars. But no astronaut has set foot on the lunar surface since 1972, and the extent of Mars exploration to date has been some unmanned rovers and orbiters.

In an interview to be broadcast on "CBS Sunday Morning" July 21, Musk talks with contributor Jeffrey Kluger, the co-author of "Apollo 13," about the future of space exploration.

The first private company to launch, orbit and recover a spacecraft, SpaceX (which is short for Space Exploration Technologies) is one of the companies that delivers cargo to the International Space Station. In addition to Musk, fellow billionaires Richard Branson and Amazon's Jeff Bezos are in the space race. Branson hopes to offer commercial space flights to tourists through his Virgin Galactic, while Bezos, through his company Blue Origin, wants to create colonies on the moon.

Musk is thinking bigger, and perhaps faster: "Well, this is gonna sound pretty crazy … with an un-crewed vehicle, I believe we could land on the moon in two years," he told Kluger.

"Sending crews to Mars in four years, I think that, that sounds pretty doable," Musk said. "Like, internally, we would aim for two years, and then reality might be four."

To watch last month's launch of the SpaceX Falcon Heavy launch vehicle, click on the video player below:

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