On July 21, 1961, American astronaut Gus Grissom experienced the pinnacle of the world – and he truly was there. Grissom was a member of the Liberty Bell 7 mission, a ballistic test flight that launched him through the atmosphere with a rocket. During the test, he sat in a small capsule that reached an altitude of over 100 miles before landing in the Atlantic Ocean. A Navy ship, the USS Randolph, monitored the successful end of the mission from a safe distance. Everything went according to plan, controllers at Cape Canaveral were delighted, and Grissom knew he had just entered the VIP club as the second American astronaut in history. Grissom remained in the capsule, swaying on the gentle ocean waves. While waiting for a helicopter to transfer him to the dry deck of the USS Randolph, he completed recording flight data. But then things unexpectedly changed. An incorrect command in the capsule's explosive system caused the hatch to blow off, allowing water to enter the small space. Grissom also forgot to close the valve on his spacesuit, so water started entering his suit as he struggled to stay afloat. After a dramatic escape from the capsule, he fought to keep his head above water while signaling to the helicopter pilot that something had gone wrong. The helicopter managed to rescue him at the last moment. Grissom's close encounter with death remains one of the most dramatic splashdowns in history. However, splashdowns are still one of the most common methods of returning astronauts to Earth. I am an aerospace engineering professor studying the mechanisms involved in these phenomena. Fortunately, most splashdowns are not as stressful, at least on paper.

Explanation of Splashdowns
Before it can land safely, a spacecraft returning to Earth must slow down. As it re-enters the Earth's atmosphere, the spacecraft has a lot of kinetic energy. Friction with the atmosphere introduces resistance, slowing down the spacecraft. Friction converts the spacecraft's kinetic energy into thermal energy or heat. All this heat radiates into the surrounding air, which becomes very, very hot. Since re-entry speeds are several times faster than the speed of sound, the force of the air returning on the spacecraft turns the environment around it into a glowing stream of about 1,500 degrees Celsius. In the case of SpaceX's massive Starship rocket, this temperature reaches up to 1,700 degrees Celsius. Unfortunately, no matter how quickly this transfer occurs, there isn't enough time during re-entry for the spacecraft to slow down to a safe speed without crashing. So engineers turn to other methods to slow the spacecraft during splashdown.

Parachutes
Parachutes are the first option. NASA typically uses brightly colored designs, such as orange, making them easily visible. They are also huge, with diameters over 30 meters, and each spacecraft usually uses more than one for the best stability. The first parachutes to deploy, called drogue chutes, are released when the spacecraft's speed drops below about 700 meters per second. Even then, the rocket cannot land on a hard surface. It must land somewhere that will cushion the impact. Researchers realized early on that water is an excellent shock absorber. Thus, splashdown was born.

Why Water?
Water has a relatively low viscosity – that is, it deforms quickly under stress – and it has a much lower density than hard rock. These two properties make it ideal for landing spacecraft. But another major reason why water works so well is that it covers 70% of the planet's surface, so the chances of hitting it are high when falling from space. The science behind splashdowns is complex, as evidenced by its long history. In 1961, the US conducted the first crewed splashdowns in history. They used Mercury re-entry capsules. These capsules were roughly conical in shape and fell base-first into the water. The astronaut inside sat facing upward. The base absorbed most of the heat, so researchers designed a heat shield that ablated as the capsule passed through the atmosphere. As the capsule slowed and friction decreased, the air cooled, allowing it to absorb the excess heat from the spacecraft, thereby cooling it. At a sufficiently low speed, the parachutes would deploy.

The Splashdown Process
Splashdown occurs at a speed of about 24 meters per second. It's not exactly a smooth impact, but it's slow enough for the capsule to hit the ocean and absorb the impact without damaging its structure, cargo, or the astronaut inside. After the loss of Challenger in 1986, when the space shuttle Challenger disintegrated shortly after launch, engineers began focusing their spacecraft designs on what is called crashworthiness – or the degree of damage a spacecraft sustains after hitting a surface. Now, all spacecraft must prove they can offer a chance of survival in water after returning from space. Researchers build complex models and then test them in laboratory experiments to prove that the structure is strong enough to meet this requirement.

The Future of Splashdowns
Between 2021 and June 2024, seven SpaceX Dragon capsules performed flawless splashdowns upon returning from the International Space Station. On June 6, the most powerful rocket to date, SpaceX's Starship, performed a phenomenal vertical splashdown in the Indian Ocean. Its rocket boosters continued to work upon approach to the surface, creating an extraordinary cloud of hissing steam around the nozzles. SpaceX has used splashdowns to recover Dragon capsules after launch, without significant damage to their critical parts, allowing them to be recycled for future missions. Unlocking this reuse capability will save private companies millions of dollars in infrastructure and reduce mission costs. Splashdowns remain the most common tactic for re-entering spacecraft, and with more space agencies and private companies aiming for the stars, we will likely see many more splashdowns in the future. This article has been updated to correct that SpaceX recovers its Dragon capsules during splashdowns.

Original:
Marcos Fernandez Tous
Assistant Professor of Space Studies, University of North Dakota