Which is more dangerous: Outer space or the deep sea?

Explorers of space and the deep sea face similar dangers, but some differences make one realm safer than the other.

By Syris Valentine for 'The Scientific American'

You, your crew and your craft float in hostile environs miles away from civilization. As you set about your mission, only a few inches of metal separate you from an environment that could kill you in moments. Your life depends on the engineers who designed your vessel. All it takes is one crack or puncture to rupture your shell and extinguish your life. These conditions apply equally to a submersible in the deep sea and a spacecraft beyond Earth. Given the similar risk of sudden death, it’s natural to ask: Which is safer?

Recent news shows the question is more relevant than ever. In June 2023 a submersible craft called the Titan operated by OceanGate Expeditions was crushed while descending to the wreck of the Titanic on the seafloor of the Atlantic Ocean, killing all five men inside. The incident occurred only two months after SpaceX’s Starship, the largest rocket built to date, exploded less than four minutes after surging toward space in its first test flight—luckily, no one was onboard.

Whether you’re going 20,000 leagues under the sea or from the Earth to the moon, spacecraft and submersibles follow many of the same engineering principles. Their passenger compartments are pressure vessels: containers designed, like an overachiever raised by demanding parents, to withstand high pressures inside and out.

The risks of space

When engineers develop a spacecraft, they ask a few key questions that guide their process, says David Klaus, a professor of aerospace engineering at the University of Colorado Boulder, who studies risk assessment and human spaceflight. They are, he explains, “What do we want to do? What does it take to do it? What can go wrong? And what can we do to reduce the chance of the bad things going wrong?” These questions seem simple enough, but the answers are essential.

When NASA, SpaceX, Blue Origin or anyone else considers sending humans into space, the vehicle they build must do three things. First, it has to accommodate the crew members by meeting their basic needs such as oxygen, food and water. Second, it should allow them to accomplish mission objectives. Lastly, it must protect the people onboard and on the ground from the risks of spaceflight and reentry. Sometimes meeting one need introduces new risks. The crew needs to breathe, of course, but it was high-pressure tanks of liquid oxygen that caused an explosion on Apollo 13, and it was the fuel powering an oxygen generator that sparked a fire onboard the Mir space station in 1997.

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The space environment itself piles on hazards. Not only do astronauts float through a near vacuum filled with radiation that no practical amount of shielding can stop, but in low-Earth orbit, where the International Space Station (ISS) operates, crews have to contend with a growing cloud of high-tech shrapnel circling Earth. These pieces of orbital detritus are often broken-up bits of old satellites. The ISS’s debris shields can absorb impacts from particles smaller than half an inch, but between 1999 and 2022, the station had to fire thrusters 32 times to avoid collisions with larger objects. A single hit would be disastrous; at speeds of 17,500 miles per hour, even something the size of a softball can cause severe damage.

Thankfully, neither the Apollo 13 explosion nor the Mir fire nor the ISS’s many near misses have resulted in any fatalities. Astronauts have not always been so fortunate, however. In 1986, after a frigid night and only 73 seconds into its final flight, the space shuttle Challenger broke apart, killing all seven people onboard. The disaster caused a three-year pause on all space shuttle operations, but it led NASA to create the Office of Safety, Reliability, and Quality Assurance—later renamed the Office of Safety and Mission Assurance. This office couldn’t prevent all problems, though, and 17 years later the space shuttle Columbia disintegrated during reentry.

Still, besides Challenger and Columbia, NASA has had only one other fatal mid-flight accident in its six and a half decades of human spaceflight: in 1967 a pilot died in a jet-powered plane crash after reaching an altitude of just more than 50 miles. That tally, however, leaves out the 1967 capsule fire that killed the Apollo 1 crew during a ground test on the launchpad. And Soyuz 1 and Soyuz 11, two Soviet spacecraft, suffered accidents while attempting to return to Earth in 1967 and 1971 respectively, killing a total of four cosmonauts. “Congress has said since the very beginning, space [travel] is inherently an ultrahazardous activity,” says Sara Langston, an assistant professor of aerospace law at Embry-Riddle Aeronautical University. “And with any ultrahazardous activity, that means, no matter how much you try to make it safe, it is not safe.” Klaus’s research backs this up. In a 2018 paper, he and his former graduate student Robert Ocampo found that going to space has historically had a similar level of risk as climbing Mount Everest.

The US government is so aware of the dangers that it currently refuses to certify any commercial vehicle as safe for ferrying humans to space. Under Federal Aviation Administration (FAA) regulations, space companies are required to inform would-be spacefarers of this lack of certification before payments are made, and all passengers must sign liability waivers with the government. To ensure informed consent can be given, space companies also have to walk space tourists and commercial crew members through the history of spaceflight safety prior to the launch.

Deep-sea risks

But what about diving into the dark, watery depths that span two thirds of our world? Given the gruesome deaths suffered by the five men onboard the Titan, it’s easy to believe that the ultrahigh pressures of the deep sea are more dangerous than any ride to space. Ocean exploration is, however, often safer than it seems.

Aside from OceanGate, the submersible industry overall has a strong track record of safety. Prior to Titan, there had not been a fatal accident involving commercial submersibles since 1974, despite thousands of submersible dives every year since the late 1980s, according to a fact sheet prepared by the Marine Technology Society’s Submarine Committee.

The standard of safety for submarine operations results in part from the robust rules and regulations maintained by third-party certifiers such as the American Bureau of Shipping. For any ship or submersible to be certified, the agency will review the design and calculations prior to construction. During subsequent assembly, inspectors will verify builders are using the stated materials and correct techniques, and they’ll be on hand when the finished vehicle undergoes testing to confirm that appropriate procedures are followed. Only then will the vessel be formally classed for a specific activity.

Out of the 11 active commercial vessels designed to plunge to depths of 4,000 meters (nearly 2.5 miles) or more, only Titan was not inspected and certified by a third party. The craft was able to avoid this in part because certification isn’t legally required—once you enter international waters, you leave the law behind. According to Diane Desierto, a professor of international law at Notre Dame University, there are no laws, regulations or conventions that govern the design and operation of deep-sea submersibles on the high seas. Plus, she says, under current US Coast Guard requirements, “a submersible that has less than six people is not subject to inspection.” Titan held five. “That’s largely why they were able to deploy without any certification,” Desierto says. With one more passenger, a Coast Guard certificate of inspection would have been required for any passenger vessel built or launched in the US—and Titan was constructed in Everett, Wash.

Even with a third-party certification, operators still need to be ready for the inherent risks involved in what they’re doing. “The best practice is: when you're doing deep-ocean diving, you have to have an emergency operations plan,” says William Kohnen, CEO of Hydrospace Group, an engineering firm with more than 25 years of experience in the submersible industry. One of the standard protocols is to have a second submersible on hand that can reach the first one if necessary, which is a tactic film director and deep-sea explorer James Cameron used during his 33 dives to the Titanic.

These protocols and certifications have helped to make submersibles safer, statistically, than going to space. Another safety boon for deep-sea explorers is the fact that a submersible doesn’t have to endure the violent forces of launch and reentry that spacecraft currently undergo. Furthermore, subs have a longer record of operation, which makes them even safer. In their 2018 paper, Klaus and Ocampo found a clear correlation between the safety of an activity and the number of times it’s performed. “The more you do something, the more lessons you learn,” Klaus says. “You encounter problems. You fix them. You move forward. So most things tend to get safer just by virtue of lessons learned and applied.”

Given that there have been orders of magnitude more sub dives than rocket launches, submersibles may always have a safety advantage over spacecraft. But as SpaceX, Blue Origin and others perfect their reusable vehicles and strive for an increased launch frequency, we can look forward to an ever safer environment for space travel as well.

(The author is a freelance science journalist with a bachelor's degree in Earth and space sciences from the University of Washington)