Elon Musk tamped down expectations about Space Exploration Technologies Corp.'s new rocket designed to carry private citizens into space, saying whoever chooses to be among the first passengers will need to be "brave."

The SpaceX Falcon Heavy, a rocket with two extra boosters attached and a total of 27 engines that must fire simultaneously, will have enormous stresses and has been difficult to test on the ground, Musk said Wednesday in Washington.

He jokingly urged attendees of a conference on the International Space Station to watch the first attempted launch. 

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"It's guaranteed to be exciting," he said. When asked whether the risks would make potential customers pause before signing up for a flight, he said: "I want to make sure we set expectations accordingly."

SpaceX has an ambitious agenda for the cosmos in coming years. The company began taking deposits from private citizens for a trip around the moon on the Falcon Heavy rocket. And it is working with NASA to carry astronauts to the International Space Station. But the company has only transported cargo so far, and Musk said shifting to carrying passengers is "a huge step up."

Getting certified to carry NASA astronauts has been a challenge for SpaceX, as there is a much higher bar than transporting hardware for the agency, Musk said. He called NASA's oversight for "really tough" but justified because of the potential risks to humans.

"It's the right motivation," he said.