EDITORIAL 2: Why spacecraft prefer splashdowns over landing on ground
Context
The spacecraft carrying Indian astronaut Shubhanshu Shukla and his three colleagues on the Axiom-4 mission splashed down in the Pacific Ocean after a 20-hour journey from the International Space Station.
Landing in the ocean
- Landing in the ocean is the preferred way to bring back astronauts from space, though some spacecraft also make a landing on a runway like an aircraft.
- But the latter has special requirements, needs more sophisticated systems, and is usually costlier.
- 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 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.
Conclusion
After splashdown, Shukla and his crew mates will be removed from the spacecraft using a SpaceX recovery vehicle. It is on this vehicle that the crew will undergo their first medical checks. Afterwards, they will be flown to land on a helicopter that will land on the recovery vehicle.