When the Dragon spacecraft returned from 199 days in space with an unusual nighttime splashdown off the Gulf Coast of Florida, it obliquely reinstated a parachute-braked descent into a water landing as the standard method for bringing back spacecraft.

Why have spaceflight operations like Dragon gone back to splashdowns decades after abandoning the technique? Why did NASA select the option of splashing down in the first place? And why do some missions opt for dry landings instead?

When Every Ounce Matters

It all comes down to the confluence of two factors, both profoundly central to human spaceflight: weight and safety. Weight (or formally speaking, mass) plays a decisive role in spaceflight because of the enormous speed, and even more enormous energy requirement, needed for orbital spaceflight.

For every ounce of a spacecraft that returns from orbit, including its crew, several pounds have to go up from the launch pad in the form of rocket stages and their load of propellant.

Air and Space notes that this critical fact drives every aspect of spacecraft design. Flying back to a runway, as the space shuttle did, has numerous advantages but one massive disadvantage: the weight of wings and aircraft-style landing gear. Parachutes are far lighter, a key reason for their adoption at the dawn of the space age by both American and Russian spacecraft.

But why not simply land on land? On dry land there is no risk of a spacecraft sinking, as one Mercury capsule did after splashing down, nearly drowning an astronaut.

A Boat Can Float Anywhere

Landing on dry land turns out to have some complications. Landing on uneven ground could lead to a capsule tipping over or even rolling down a slope. And “dry land” can’t be counted on to be dry. As Air and Space also reports, one Soviet-era Russian space mission happened to come down on a freezing lake, and the cosmonauts aboard were lucky to survive.

Moreover, even with parachute braking, space capsules hit the ground pretty hard — hard enough to risk injury to the crew. Russian spacecraft use a short final blast of retrorockets to slow enough for a gentler touchdown, but the landing is still pretty bumpy. The rockets also add weight. As Space.com reported at the time, NASA originally favored landing on land for the Orion moonship, but later, per Air and Space, went back to water landing to avoid the extra weight of retrorockets.

Landing on water is also fairly abrupt, but the splash provides a final stretch of relatively smooth braking.

One factor that further affected the decision — then and now — to splash down at sea is that US launch sites are coastal, and their launch paths stretch out over the ocean. In the event of an emergency abort, an American space capsule would thus almost certainly come down over water, so the design needed to float and be suited to water rescue.

Parameters of Return

A modern-day splashdown is far less chancy than the return of early American spacecraft in the Apollo era and before, or the mid-1970s Russian mission that came down on a frozen lake. But they can still have their surprises. As Ars Technica reported, the Dragon spacecraft’s four main parachutes was noticeably slower to fully open than the other three — enough to get the attention of the recovery team and onlookers, though within design parameters.

Parameters — formalized definitions of conditions that are considered safe — are the key guidelines ensuring the safe return of spacecraft, and are set forth in great detail by NASA. Per Space.com, the Dragon’s night landing was selected because of ideal weather conditions at the primary landing site, but the great flexibility of splashdown landings meant that multiple alternative zones were available for the Dragon splashdown.

Parameters of Recovery

Once the spacecraft is in the water, the final stage of the mission begins: extracting the crew and recovering the spacecraft.

The same guidelines that specify parameters for recovery site suitability also specify subtle but critical details about readiness for the recovery operation. For example, the Dragon return could not commence until the recovery helicopters successfully started up and performed a hover operation.

Another NASA document outlines the way these procedures and parameters have been determined for the Orion moonship. While decades have passed since the last of the Apollo-era landings, the recovery operations for the shuttle’s reusable solid boosters (and of future SLS boosters) have kept NASA thoroughly current in handling complex recovery tasks at sea.

Mastering splashdown and recovery, with a focus on safety and reliability, could open new doors in the future of commercial space travel.

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