Simpler, safer: Why spacecraft prefer splashdowns over landing on ground

For its Gaganyaan programme, Indian Space Research Organisation (ISRO) has designed a crew module that will land on sea.

Why water is preferred

Splashdown is generally a simpler and safer option. When a spacecraft begins to re-enter the Earth’s atmosphere, at an altitude of 110-120 km, it is usually travelling at speeds up to 27,359 km per hour. There isn’t enough time for it to decelerate to speeds that are suitable for a vertical landing on the ground. Additional braking systems, and structures like landing legs would be required.

It is safer to land a spacecraft in water even when it is travelling at 25-30 km per hour — the speed of the Axiom-4 spacecraft at the time of splashdown. The low viscosity and high density of water provides much greater cushion. While it does not ensure a completely smooth landing, it absorbs enough shock from the impact to avoid damage to the spacecraft’s structure, payload, or the crew inside.

The other reason is the availability of vast open spaces. On the ground, the landing location has to be precisely defined; in the ocean, there is no danger of hitting something else even if the spacecraft is dragged a bit off-course by air or water currents.

The capsules are designed to float on water. They are conical in shape, and either their top or bottom (depending on how they land) is rounded metal, which works like the hull of a ship, and keeps them bobbing on the surface.

How landing is executed

When a spacecraft re-enters the Earth’s atmosphere, it experiences deceleration due to friction. But additional measures are required to slow it to safe speeds.

One such method is the use of parachutes. Spacecraft usually deploy two drag (or drogue) parachutes first, which stabilise the vehicle following re-entry, and reduce speed. The Dragon spacecraft deploys these parachutes at around 18,000 feet.

At about 6,500 feet, the drogue parachutes are detached, and the four main parachutes are released, which continue to slow the spacecraft further.

The spacecraft does not travel vertically to Earth, but glides down at an angle. From the point of re-entry to touchdown, it traverses a distance of 5,000-7,000 km. During this time, its speed comes down to about 25-30 km per hour, which is safe for a splashdown in the ocean.