Which is safer: deep space travel or deep sea travel?

We find out if blasting off or diving down presents the bigger challenges.

Photos courtesy of SpicyTruffel and 3dsam79 via Getty Images

It’s fitting that space travel and deep sea travel literally require you to move in opposite directions, because in the court of public opinion they couldn’t be further apart.

On one hand, every time a SpaceX rocket or Blue Origin vessel takes tourists for a spin or there’s an update on Boeing’s Starliner project, people react with wide-eyed enthusiasm. On the other, we have the recent Titan tragedy that saw undersea tourists lose their lives on a tragic journey, confirming for a lot of people the belief that the sea is just a danger zone to be avoided if at all possible.

So we have to ask the question: why? Why do we see space as a potential joyride but deep sea travel as fraught with endless peril? Hasn’t pop culture shown us enough ways both of these experiences could go horribly wrong?

So we called two experts and asked them. The first: Sean Carroll, a professor at Johns Hopkins University with expertise in astrophysics and space exploration. The second: Robert Stern, a professor at the University of Texas at Dallas with expertise in deep sea research. In conversation, they explain the challenges, and sometimes misconceptions, around deep space and deep sea travel.

Let’s get right into it: Is it safer to travel to space than to travel to the bottom of the ocean?

Carroll: I think that the deep sea comparison is actually very interesting because what it immediately brings to mind for me is—do you remember Star Trek Into Darkness, where the Enterprise is hiding under the ocean and then it sort of rises up? People were like, “That's not what spaceships do!” But the writers tried to explain it by saying something to the effect of, “It’s meant to survive space; of course it can survive the ocean.” But that’s actually not right.

Think about it this way: In space, the atmosphere inside the spaceship is pushing outward. Whereas in the ocean, you're being pushed inward by the water around you, but also just the magnitude of the push is enormously different. No matter how far out you are in space, the difference in pressure between inside and outside is no greater than one atmosphere's worth of pressure. But as soon as you go down even a little bit into the ocean, you can have way more than one atmosphere of pressure pushing on your vessel. So in that sense, deep sea diving is much more dangerous. It requires a lot more technological capability to make sure that you can do it, which is why we've been able to go to the moon, but it's actually hard to go down to the bottom of the Mariana Trench.

Stern: Absolutely. That's what happened with [the Titan] submarine—they didn't design the submarine well enough to withstand those incredible pressures. Every 10 meters of descent in the ocean adds one atmosphere of pressure. So by the time you're at the bottom of the Challenger Deep, you're at 1100 atmospheres pressing on the vessel.

So to Prof. Carroll’s point, you can’t just use spaceship technology in the deep sea, and vice versa.  

Stern: That's why these [undersea research vessels] are usually spherical, whereas what's pressing outward on a spaceship is easier to engineer. The research submarine, in order to deal with the great pressures, they have to have very thick walls, they're very heavy, whereas a spaceship, they can engineer a much thinner wall because it has to maintain the inside pressure. The big concern is that that'll be punctured by micro meteorites, which doesn't seem to be a really big problem yet, but you don't have to worry about micro meteorites puncturing the hull of a research vessel.

Carroll: But in basically all other ways, space travel is harder. And I do think that the fact that no one has died in a catastrophic space tourism accident yet is kind of lucky. I mean, plenty of people have died in space-related accidents—but many have involved either takeoff or landing. I think that numerically—if you count non-tourists—space is way more dangerous. There hasn’t been an accident involving space tourists yet, so we haven’t had that discussion yet or it’s not completely “real” to people yet, but it is going to happen. In the categories of things that can go disastrously wrong, of course, the major one is some giant technological failure—that's common to both deep sea and space missions. There is a lot more danger there in the takeoff and landing phases. In the deep sea, you literally jump into the water. That's not the dangerous part of your trip, right? In space, you literally jump on the top of a multi-ton explosion and you hope that nothing goes bad.

Stern: You also have to design deep sea vessels for specific distances or depths. Most of the ocean is accessible with a very small percentage at 6,500 meters. So yeah, basically, once you get outside of Earth's atmosphere, you're in a vacuum and that vacuum is everywhere and it doesn't change, whereas with a submarine, it depends on the depth. You've got to be engineered to go to a specific depth.

What about the effect both experiences can have on someone’s mental health? In either case, you’re in a dark void for an extended period of time.

Carroll: There are psychological effects—just sort of loneliness and weirdness and not being in the usual environment, being very cramped. And that's probably also true for underwater as well.

Stern: With most research vessels, you're only down there for eight hours or so. You don't stay down very long so with a little bit of thought, you can avoid having to do any bodily functions other than breathing. So in a way, space is more like being on a nuclear military submarine. They stay down for days and weeks. I don't know what the record of submergence on a nuclear submarine is, but I imagine it's probably closer in time to, say, certainly a trip to the moon and back, but maybe not the trip to Mars and back.

Another thing is, on a submersible, you've got the full benefit of gravity, and while gravity is kind of a pain to us, we're kind of used to it. A lot of the big concerns they have about space travel are bones getting weaker because there's no gravity, that's why they have to be always working out just to try and keep their bone density up.

And then same thing with bodily functions. Maybe you take a sandwich and a thermos of coffee down with you in a submarine, but you're not going to cook any food, you're not going to worry about the quality of your diet because you're going to eat dinner on the ship when you come back up.

OK, I have to ask: aliens and sea monsters. What do we have to know?

Carroll: I don't think that aliens are going to bother you in space. That's not a big worry. If you've seen the movie Gravity which has debris colliding with a ship—that's not really a big worry either. Space is very big. It's not yet crowded. Maybe someday it will be, but for the foreseeable future, it's pretty empty.

Stern: I think what a lot of people think about the deep sea is that—do you remember those old Time Life books, where they would have these big foldouts with all of these incredible creatures? I think a lot of people think that you would see those if you went down deep. You don't. There's almost no life at the bottom. It's pretty much a desert. There's a few slow moving creatures. And there are those creatures, they have been seen, but they're really scarce. You're not going to come down on a school of these weird looking fish. They're very much loners. I've never seen any.