When billionaire Yusaku Maezawa takes his planned trip around the Moon with SpaceX, he’d better be ready to vomit.

Maezawa, who founded online fashion store Zozotown, bought seats for himself and a handful of artists on SpaceX’s Big Falcon Rocket, which combines a massive, 40-story rocket booster and the Big Falcon Spaceship. Musk anticipates the flight will last between four and five days as the tourists trace a figure-eight around the Earth, the Moon, and back again. Those tourists can expect nausea, a small dose of radiation, and — possibly — friction among their group, as a result of their isolation.

“This is dangerous, to be clear.”

The short duration means that the space tourists won’t have to worry about things that plague astronauts on longer stays in space: Maezawa and his artist posse probably won’t develop vision changes or lose bone mass and muscle tone like astronauts do over months in the microgravity of the space station, says Petra Illig, a doctor who specializes in aerospace medicine. “All of those things happen over longer periods of time,” she says. Still, the space tourists’ flight won’t be easy. “It’s dangerous, to be clear,” Musk said during Monday’s announcement. “This is no walk in the park here.” And even if everything goes according to plan, the challenges will start during liftoff.

As the rocket speeds away from Earth, the passengers will be hit by forces of acceleration up to three times the force gravity they’re accustomed to on the ground, or 3 Gs. That can put a strain on the heart as it struggles to pump blood to the head. If the passengers aren’t positioned correctly, the blood could drain from their brains into their feet and they might pass out, Illig says. So it will be important to screen all of the space tourists to confirm their hearts are healthy enough for the trip. “You don’t want people fainting or having heart attacks,” Illig says. “You’re stuck along for the ride.”

“You don’t want people fainting or having heart attacks.”

Once the passengers start feeling weightless, that’s when the vomiting will probably start. “Any participant going into space should be prepared for some motion sickness and being a little confused about where up is and where down is,” says Jennifer Fogarty, chief scientist of NASA’s human research program. “It can be a little disconcerting if you’re not prepared.” Astronauts who’ve been to space a few times have an easier time adjusting, she says. But rookies like Maezawa and the artists who join him won’t know how their bodies will react.

That’s because there’s no earthly analog to space motion sickness. Not seasickness, or roller coasters. “We are terrestrial beings. We walk on flat surfaces. We have a horizon. Our organs of equilibrium are dependent on gravity,” says Illig. “So there’s no way to remove gravity while you’re on Earth and say, ‘Well, let’s see how you feel.’” The closest you can get is a trip aboard a plane nicknamed “the vomit comet,” where passengers can experience brief 20 to 30-second spurts of weightlessness during parabolic flights. But, Illig says, the simulations are so brief that they’re not a perfect predictor.

“We are terrestrial beings. We walk on flat surfaces. We have a horizon.”

The good news is that the nausea tends to pass within the first few days, Fogarty says. But for a four to five day tour around the Moon, that’s most of the trip. “That period of discomfort is going to take a significant chunk out of a space tourist’s initial experience with space,” says Sheryl Bishop, a professor at the University of Texas Medical Branch who investigates human performance in extreme environments. Anti-nausea medications can help with it, Fogarty says — although they can be problematic for astronauts responsible for critical operations because the drugs can make them sleepy. For space tourists just along for the ride, “they would still get to enjoy the view, they could still participate in activities as long as they weren’t critical to the function of the vehicle,” Fogarty says.

One hazard we hear a lot about is space radiation. Astronauts on the ISS are protected from the brunt of it by the Earth’s magnetic field and the structure of the ISS itself, according to Edward Semones, NASA’s radiation health officer. On a loop around the Moon, Maezawa and the artists along for the ride would be exposed to a somewhat higher daily dose of radiation than astronauts on the ISS, but for a shorter period of time. By the end of the trip, they might experience roughly the same amount of radiation they’d get from a CT scan — just distributed across the entire body, Semones says.

“They have to not freak out.”

That dose could skyrocket, however, if the sun belches out charged particle radiation in what’s known as a solar particle event during the space tourists’ flight. The likelihood of that happening during the four to five day trip is small, according to Semones. But if a solar particle event occurred, the passengers on the ship could be potentially exposed to six months’ worth of space radiation in just a couple days. Worst case scenario, that could lead to nausea and fatigue, Semones says: “The very beginnings of what’s called acute radiation syndrome.”

NASA’s strategies for keeping the astronauts on the ISS safe include an early warning system, and places inside the ISS where the crew can seek shelter from the radiation, Semones says. But it’s not clear what precisely Musk has planned for his own passengers. In the past, he’s been blasé about the risks. He said in a Reddit AMA last year that the spacecraft would “need a solar storm shelter, which is a small part of the ship,” but he didn’t go into detail about protective measures in Monday night’s announcement.

“They can’t just say, ‘Hey, I’ve had enough, bring me back.’”

Then there’s the potential psychological toll of being locked into a sealed container with a handful of other people for days on end. “They have to not freak out,” Illig says. “They can’t just say, ‘Hey, I’ve had enough, bring me back.’” That’s why it will be key for the space tourists to go into the flight with clear expectations and understand their roles and responsibilities, Fogarty says. And one way to establish that would be to have the group train together before they leave Earth. “Just to see how people respond to stressors, especially when there might be conflict within the team, or if there were emergency events you have to rally and response to,” she says. It won’t predict everything, she says, but it could “relieve the anxiety of experiencing something totally new when you’re in an extreme environment.”

Musk, too, said that the passengers will need to prepare for the flight. “This will require a lot of training,” he said. “When you’re pushing the frontier, it’s not a sure thing.” The good news is that there will be plenty of prep time. The flight isn’t planned until 2023, and Musk admitted that even he wasn’t sure SpaceX would meet that deadline: “We’re going to do everything humanly possible to bring it to flight as fast as we can, and as safely as we can.”

Update September 19th, 2018 1:45PM ET: Updated to include a previous Musk statement about radiation, and to clarify when experts expect space motion sickness to start during the flight.