Elon Musk’s space exploration company SpaceX—which has already revolutionized the spaceflight industry by launching and landing reusable rockets—has a bigger goal: to take people to the moon, and to eventually establish human colonies on Mars. Musk is known for his optimistic timelines for SpaceX projects, last year announcing that he planned to send an unmanned rocket to the Red Planet “as soon as 2018.” But during a talk at the International Space Station Research and Development Conference on Wednesday, Musk was uncharacteristically realistic about SpaceX’s inaugural trip into outer space.

Giving an update on SpaceX’s Falcon Heavy rocket and the potential for an eventual manned mission to Mars, Musk downplayed expectations to the audience of scientists. “[Falcon Heavy] requires the simultaneous ignition of 27 orbit-class rockets,” he said, and added that there’s a “lot of risk associated with Falcon Heavy.” He continued: “There’s a real good chance that it does not make it to orbit . . . I hope it gets far enough away from the launch pad that it does not cause pad damage—I would consider that a win.” Developing the rocket has proven to be “way, way more difficult” than Musk had initially thought. “No question, whoever’s on the first flight . . . brave. Brave,” he said.

Those aren’t necessarily the most reassuring words for whoever eventually pilots Musk’s first lunar mission, let alone flies onward to Mars. Still, Musk didn’t rule out eventually joining a SpaceX crew for a trip to the International Space Station. “I would like to at some point,” he said. “Assuming things work out, yeah, maybe in three or four years.” He also mentioned the importance of putting a base for astronauts on the moon. “If you want to get the public fired up, you've got to put a base on the moon,” he explained. “Having some permanent presence on another heavenly body . . . that’s the continuance of the dream of Apollo.”

SpaceX has a way to go before it can consider an interplanetary mission, manned or unmanned. The company, which is working with NASA to carry astronauts to the International Space Station, has already taken significant deposits from private citizens for a trip around the moon on the Falcon Heavy. Musk must also contend with more terrestrial matters. Last fall, days before a SpaceX Falcon 9 rocket was scheduled to bring Facebook’s first satellite to orbit, the rocket exploded during a pre-launch test on a launch pad in Cape Canaveral. The risks, unlike the moon base, remain very real.