Elon Musk isn’t too sure that stepping foot on Mars will go as smoothly as we may hope. 

The SpaceX founder said in a recent interview with Peter Diamandis, founder of the X Prize Foundation, that “a bunch of people” will probably die in early voyages to the red planet. 

Responding to the idea Mars is “an escape hatch for rich people,” Musk said at the 12:35 mark in the video above, “Going to Mars reads like that ad for Shackleton going to the Antarctic, you know: it’s dangerous, it’s uncomfortable, it’s a long journey, you might not, you know, come back alive, um—but it’s a glorious adventure and it’ll be amazing, an amazing experience.” 

Then, with a laugh, at the 13:25 mark above: “Yeah, I mean, honestly, a bunch of people probably will die in the beginning. It’s tough sledding over there.”

Musk added that it’s “not for everyone,” but rather a trip for “volunteers only.”

SpaceX has launched more than 100 rockets in the last 10 years, hoping to test out the possibilities for those looking to travel to the moon and Mars, but many have ended unsuccessfully. Either way, the Tesla Technoking is “highly confident” in predicting he’ll have humans on the planet by 2026, after he previously mentioned putting a city on Mars back in 2015. 

“If we get lucky, maybe four years,” Musk said at the end of 2020. “We want to send an uncrewed vehicle there in two years.”