Billionaire tech entrepreneur Elon Musk says his company SpaceX is close to sending passengers on test flights to Mars, but with a catch: the trip will likely be deadly.

Musk told attendees of the South by Southwest tech and pop culture festival that SpaceX is aiming to launch an interplanetary shuttle to Mars by the first half of 2019. The Tesla founder said the trips would be “up and down flights” using reusable rockets, with flights likely costing less than $6 million. However, he warned that flights to Mars would not simply be a luxury excursion for the rich and famous.

“For the early people that go to Mars, it will be far more dangerous,” Musk said. “It kind of reads like [explorer Ernest] Shackleton’s ad for Antarctic explorers: difficult, dangerous, good chance you will die. Excitement for those who survive.”

While Musk identified the first half of 2019 as a tentative launch date for the first Mars flights, he conceded that his timelines for new innovations have been “optimistic” in the past. SpaceX conducted a successful launch and retrieval of its reusable Falcon Heavy rocket last month.

Arguably the preeminent proponent of interplanetary exploration, Musk says the colonization of Mars is necessary as a check against the dangers of nuclear weapons, artificial intelligence and other threats on Earth.

Musk said he expects Mars to become a hub of entrepreneurial activity once it is successfully colonized, with everything from pizza joints to “Mars bars.”

“Once you get up there, the opportunity is immense,” Musk said. “We will do our best to get you there and create an environment where entrepreneurs can flourish.”