Elon Musk wants humans to travel to Mars. He just doesn't want to be the first to go. Because, uh, there's a very good chance of dying.

"The risk of fatality will be high," Musk conceded in the course of describing SpaceX's absurdly ambitious (and still preliminary) plan to establish a human colony on Mars. "There's no way around it." Much like early voyages to the South Pole or the initial moon launches were incredibly dangerous, the first would-be Martian explorers will face huge risks. It doesn't mean we shouldn't try. That's just the price of conquering new worlds and exploring hostile environments.

So what exactly makes a journey to Mars so perilous? Chris McKay, a senior scientist at NASA Ames Research Center involved in planning future Mars missions, walked us through some of the hazards. Some, like exploding rockets, are hair-raising; others, like radiation exposure, could prove more tolerable.

But it's a good bet that something may go terribly wrong in a mission this audacious and this complex. "Going to Mars is a big, big thing. It's going to involve risks," McKay says. "We'll always try for perfect safety. But we all realize that part of the deal … is that there could be fatalities."

1) Your rocket could blow up before leaving Earth

 (Javier Zarracina/Vox)

Elon Musk's plan to go to Mars involves strapping a giant spaceship atop the biggest rocket that humanity has ever built. Because any rocket launch basically involves a long, controlled explosion, it's inherently precarious — no matter how many safety tests are done beforehand. If anything goes wrong, if the explosion gets out of control, the people strapped to that big container of fuel don't stand a chance.

For context, NASA's space shuttle program carried 833 passengers between 1981 and 2011. Of those, 14 people died in explosions on two high-profile accidents (Challenger and Columbia), a fatality rate of 1.6 percent. That's vastly more dangerous than driving and a bit riskier than climbing Mount Everest. (The fatality rate for the Apollo program to the moon was even higher, at 9 percent.)

But, of course, SpaceX would be using newer, more complex, and yet-untested rockets to get to Mars. So it's tough to say what the actual odds of death would be. Possibly much higher! Note that a couple of SpaceX's smaller Falcon 9 rockets have either exploded on the launchpad or blown up mid-flight. Engineers and rocket scientists can improve that, but it's unlikely that the risk will be zero.

2) You could suffer serious radiation exposure from a solar flare

 (Javier Zarracina/Vox)

For the entirety of the journey to Mars and back, astronauts are likely to endure much higher exposure to radiation than they would on Earth.

There's the flight to and from Mars. Outer space isn't empty; it's teeming with high-energy particles blasted out by the sun and other stars. Here on Earth, we're protected by our planet's magnetic field, which deflects them away. But once our Martian voyagers leave Earth, they'll be exposed.

For the most part, this is a mid-sized problem. When NASA sent the Curiosity Rover to Mars, it found that the one-way trip alone would expose unshielded astronauts to an extra 0.3 sieverts of radiation, equivalent to 24 CAT scans. That's 15 times the annual radiation limit for workers at nuclear power plants, but not fatal. (For context, one sievert is associated with a 5.5 percent increase in cancer risk. Eight sieverts can kill.)

The more worrisome part is what happens if the sun erupts and sends a major solar flare hurtling toward the spaceship mid-flight. In that case, the astronauts could be exposed to much higher, potentially fatal doses. These flares are unpredictable, although with a proper monitoring system in place we could in theory warn the astronauts beforehand. No one wants to be caught out on a spacewalk.

So any journey will have to think about how to protect its passengers. The spaceship that transports the astronauts, for instance, might store its water and fuel such that it could be used as a barrier in case of a flare. "Water is an excellent shield," McKay says. "I don't want to trivialize the problem, but I think its solvable."

Meanwhile, the radiation problem persists once astronauts get to Mars. Because the planet has no ozone layer and a weak magnetic field, any humans on the surface would be pelted with both cosmic rays and UV rays from the sun — at far higher doses than they'd get on Earth. (Curiosity found that a 500-day stint on Mars gets you another 0.3 sieverts.) Odds are, Martian explorers would have to spend a good chunk of time indoors, perhaps even underground, shielding their habitat with layers of dirt.

3) You could crash on Mars' surface while trying to land

 Javier Zarracina/Vox

Okay, you've successfully launched out of Earth, traveled six months across the solar system, and the red planet is in your sights. Now comes the really scary part. "Flying a spaceship is mostly long stretches of tedium," says McKay, "punctuated by a few moments of terror — takeoff and landing."

Under Musk's plan, your spaceship would be approaching Mars at 62,000 miles per hour. As it descends into the Martian atmosphere, the ship would then use "supersonic retro propulsion" — i.e., lots of little rockets firing at once — in order to land on the surface of Mars.

This landing maneuver could prove dicey. Because Mars' atmosphere is so much thinner than Earth's, there's less friction to slow you down (though still enough to heat the ship up to temperatures of 1,700 degrees Celsius). NASA has struggled to land even small robots on the surface: The system used to decelerate and then parachute the Curiosity rover onto Mars was mind-boggling. And Curiosity only weighed 1 ton. Musk is envisioning a payload of 450 tons. Plenty could go wrong. Buckle up.

4) Mars' low gravity might wreak havoc on your bones and muscles

 (Javier Zarracina/Vox)

"This is something you never see in any movies," says McKay. "But the lower gravity on Mars is potentially one of the biggest concerns."

Here's what he means. We know that humans do just fine in Earth's gravity. On the other hand, we know that human bones and muscles can atrophy terribly in zero gravity — as seen in experiments on the International Space Station. (Astronauts on the ISS have to exercise for two hours a day to prevent muscle loss, but their bones still lose calcium over time, and they don't recover until they come back to Earth.)

What we don't know is how the human body will fare on Mars, where gravity is only about 0.38 the strength of Earth. Many people have long assumed that humans can survive just fine in Martian gravity — it'll be just like Earth, only we can jump a little higher. "But we don't actually know this for a fact," McKay says. It might turn out that low Martian gravity is just as bad for human health as zero gravity.

If that's the case, that's really bad news for astronauts. Even with exercise, their bones will grow weaker, making injury in an unforgiving environment far more likely. Mars' low gravity might also erode other capabilities that are crucial for everyday functioning. For instance, NASA has found that eyesight can mysteriously deteriorate in zero gravity: Astronaut John Phillips went from a perfect 20/20 to 20/100 after just six months aboard the ISS. Astronauts also find it harder to sleep in zero gravity, and many passengers in space suffer insomnia.

The trickiest part is that we won't really know what to expect until we actually send humans to Mars. "If 0.38 g is bad, there is not much we can do about it," McKay notes. "Let's all hope it's not bad."

5) Your spacesuit or habitat could leak — and you can't breathe Martian air

 (Javier Zarracina/Vox)

Anyone planning a trip to Mars will obviously have to prepare for the fact that there aren't ready supplies of food, water, and oxygen. These should be surmountable hurdles. You can pack enough food and water. And humans could invent machines to break apart the carbon dioxide on Mars and yield oxygen. (NASA plans to test this process out in an uncrewed rover it's deploying to Mars in 2020.)

The real danger is with accidents. A spacesuit or habitat could tear, causing a loss of air. Mars is filled with dust and dirt that's likely to get everywhere — if any of it clogs a crucial seal and causes leaks, people could well die. "If a door seal or spacesuit gets dirty, that's a much bigger deal than it is here on Earth," McKay notes.

For his part, Musk hasn't given much thought to these mundane life-support questions. (At a Q&A on Tuesday, someone asked him what the bathroom situation would be on Mars; he hadn't really considered it.) He did, however, suggest that we might one day power a Martian civilization with solar panels or geothermal: "If you have energy on Mars, you're going to have water because there's massive amounts of ice."

Musk also hinted at the possibility of terraforming Mars — releasing huge amounts of carbon dioxide into the atmosphere to warm the planet and bring back liquid water, making it habitable for humans. "If we could warm Mars up, we would once again have a thick atmosphere and warm oceans," he said.

Maybe someday. But the early colonists will have to stay alive first.

6) You could get poisoned by the toxins in Mars' soil

 (Javier Zarracina/Vox)

In the movie The Martian, a mighty sandstorm leaves astronaut Mark Watney stranded on Mars after high winds rip out an antenna and destroy most of his camp. That scene was a little exaggerated. Because Mars' atmosphere is so thin, 60 mph winds don't produce nearly as much force as they do on Earth.

But sand and dirt on Mars is definitely a problem. Mars periodically gets massive sandstorms that spread out across the planet and can last for days or weeks at a time. You don't want to be outside in one. All those little particles flying around could conceivably tear a hole in your spacesuit. Or, more prosaically, they could clog door seals, mess up machinery, or even cover up solar panels, depriving astronauts of power for extended periods.

A related concern is the fact that Martian soil is toxic. It contains very high concentrations of perchlorates — salts that can do serious damage to the human thyroid gland. "If your backyard had as much perchlorate as Mars does, it'd be a Superfund site," McKay says.

It's okay to touch Martian dirt with your bare hands. But you really don't want any to get into your drinking water or food when you tramp it into to your habitat. You also don't want to grow plants using Martian soil.

McKay also brought up another related risk: Right now we're pretty sure that there's no life on Mars, no strange micro-organisms lurking in the soil. But we're not absolutely sure. So it might be a good idea to test out any proposed landing site in advance, in case there's anything harmful lurking. And that worry goes both ways: We'll want to be careful about contaminating or killing any Martian life, too. The 1967 Outer Space Treaty forbids the "harmful contamination" of alien worlds with our Earthly microbes.

7) Your fellow travelers could drive you crazy

 (Javier Zarracina/Vox)

One of the great challenges NASA is working through in preparation for a Mars mission is surprisingly mundane: making sure crew members don't grow to hate each other. After all, they'll be together for many months on end, far away from their home planet, isolated from their friends, intensely bored, and lacking much personal space. Better hope no one snaps!

There's good news and bad news here. First, researchers have found that there are definitely people out there with the sort of even-keeled personalities suited for long space missions. Between 2007 and 2011, the MARS-500 project enlisted six-man crews for various lengths of time to simulate a Mars mission at a facility in Moscow. The final mission, lasting 520 days, went smoothly. No conflicts. Everyone got along amiably.

The trouble is that we don't know if psychologists can reliably screen for the right personalities every single time. And it doesn't take much to sow chaos. Because when crew dynamics go awry, they really go awry. Perhaps most notoriously, during the Biosphere II project in 1994, the seven-person crew eventually split into warring factions, and the mission had to end prematurely.

That's an extreme example, but it underscores the fact that the first crew to Mars will need to get along and keep their feelings under control. Because in this situation, roommate drama really could be life or death.

Going to Mars is unbelievably risky. But that's no reason to shy away.

Early expeditions to Mars could prove as dangerous as any in human history. Anyone who signs up will have to know they're venturing to another planet with the real expectation that they may never get there, or potentially, never come back.

Still, it's unlikely this fact will deter every last volunteer. McKay recounts how he used to go down to Antarctica every year with a relatively small team to conduct research. About once a year, someone would die due to an unforeseeable accident — a helicopter crash, or a person falling down a crevasse, or a diver suffering an embolism.

Every death was awful and tragic. But it didn't put a halt to research in Antarctica. "There was always a long line of scientists who wanted to go," McKay says. "We never, ever took safety casually. I certainly did not want to die. But we were working in an environment that was intrinsically hostile and didn't have all the safety infrastructure they have back in civilization. So to do the things we wanted to do, we had to take risks."

So it is with Mars.