The science behind the splashdown – an aerospace engineer explains how NASA and SpaceX return spacecraft safely to Earth

For about 15 minutes on July 21, 1961, American astronaut Gus Grissom felt on top of the world – and indeed he was.

Grissom crewed the Liberty Bell 7 mission, a ballistic test flight that launched it through the atmosphere on a rocket. During the test, it landed inside a small capsule and reached a peak of over 100 miles up before splashing into the Atlantic Ocean.

A Navy ship, the USS Randolph, observed the successful completion of the mission from a safe distance. Everything had gone according to plan, the controllers at Cape Canaveral were happy, and Grissom knew he had just entered a VIP club as the second American astronaut in history.

Grissom remained inside his capsule and rocked on the gentle waves of the ocean. While he waited for a helicopter to take him to the dry deck of the USS Randolph, he finished recording some flight data. But then, things took an unexpected turn.

A faulty command in the capsule’s explosives system caused the hatch to come off, which allowed water to flow into the small space. Grissom had also forgotten to close a valve in his spacesuit, so water began seeping into his suit as he struggled to stay afloat.

After a dramatic escape from the capsule, he struggled to keep his head above the surface while signaling to the helicopter pilot that something had gone wrong. The helicopter managed to save him at the last moment.

Grissom’s near-death escape remains one of the most dramatic splashes in history. But splashdown remains one of the most common ways astronauts return to Earth. I am a professor of aerospace engineering who studies the mechanisms involved in these phenomena. Fortunately, most splashes aren’t that annoying, at least on paper.

Splashdown explained

Before it can make a safe landing, a spacecraft returning to Earth must slow down. While returning to Earth, a spacecraft has a lot of kinetic energy. Friction with the atmosphere creates drag, which slows the spacecraft down. Friction converts the kinetic energy of the ship into thermal energy or heat.

All this heat radiates into the surrounding air, which gets really, really hot. Since re-entry speeds can be several times the speed of sound, the force of the air pushing back against the vehicle turns the vehicle’s surroundings into a scorching flow that is about 2,700 degrees Fahrenheit (1,500 degrees Celsius). In the case of SpaceX’s massive Starship rocket, this temperature even reaches 3,000 degrees Fahrenheit (nearly 1,700 degrees Celsius).

Unfortunately, no matter how quickly this transfer occurs, there is still not enough time during reentry for the vehicle to slow down to a safe enough speed to avoid a crash. So engineers use other methods that can slow down a spacecraft during splashdown.

Parachutes are the first option. NASA usually uses bright colored designs, such as orange, which make them easy to spot. They are also large, over 100 feet in diameter, and each reentry vehicle usually uses more than one for better stability.

The first deployed parachutes, called drag parachutes, deploy when the vehicle’s speed drops below about 2,300 feet per second (700 meters per second).

Even then, the rocket cannot collide with a solid surface. It should sit somewhere that will cushion the impact. Researchers realized early on that water is an excellent shock absorber. Thus, the splashdown was born.

Why water?

Water has a relatively low viscosity—that is, it deforms quickly under stress—and has a much lower density than solid rock. These two qualities make it ideal for landing spacecraft. But the other main reason water works so well is because it covers 70% of the planet’s surface, so the chances of hitting it are high when you’re falling from space.

The science behind splashdown is complex, as a long history attests.

In 1961, the US carried out the first manned spraying in history. These used Mercury reentry capsules.

These capsules had a roughly conical shape and fell with the base towards the water. The astronaut inside sat face up. The base absorbed most of the heat, so the researchers designed a heat shield that boiled as the capsule passed through the atmosphere.

As the capsule slowed and friction decreased, the air became cooler, which made it able to absorb excess heat in the vehicle, cooling it as well. At a low enough speed, the parachutes would deploy.

Splashdown occurs at a speed of about 80 feet per second (24 meters per second). It’s not exactly a smooth impact, but it’s slow enough for the capsule to drop into the ocean and absorb the shock of the crash without damaging its structure, its cargo, or any astronauts inside.

After the loss of Challenger in 1986, when the space shuttle Challenger disintegrated shortly after liftoff, engineers began focusing their vehicle designs on what’s called the phenomenon of crashworthiness—or the degree of damage a spacecraft takes after hitting a surface.

Now, all vehicles must prove they can stand a chance of surviving in water after returning from space. Researchers build complex models, then test them with laboratory experiments to prove that the structure is strong enough to meet this requirement.

In the future

Between 2021 and June 2024, seven of SpaceX’s Dragon capsules performed perfect splashdowns on their return from the International Space Station.

On June 6, SpaceX’s most powerful rocket to date, Starship, made a phenomenal vertical splash into the Indian Ocean. Her rocket boosters continued to fire as they neared the surface, creating a tremendous cloud of hissing steam that surrounded the nozzles.

SpaceX has used splashdowns to recover Dragon capsules after launch without significant damage to their critical parts in order to recycle them for future missions. Unlocking this reuse will allow private companies to save millions of dollars in infrastructure and reduce mission costs.

Splashdown continues to be the most common spacecraft reentry tactic, and with more space agencies and private companies hunting for the stars, we’re likely to see many more in the future.

This article has been updated to correct that SpaceX recovered their Dragon capsules during launch.

This article was republished by The Conversation, a not-for-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Marcos Fernandez Tous, University of North Dakota

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Marcos Fernandez Tous does not work for, consult with, own stock in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliation beyond their academic appointment.