5. Landing system
Landings systems can be used when the vehicle's terminal velocity is to high and thus the impact of landing is dangerous to the vehicle or payload. When this is seen in the design, you can opt to include a larger parachute system or a landing system. The trade-off between a modified parachute system or a landing system's inclusion can depend on many factors. There might be a requirement on flight time or drift, thus a large parachute might violate these requirements. Then you can opt for a two-stage or reefed parachute, but this makes the system much more complex. A landing system can be anything that dampens the impact.
The most used systems are airbags, landing legs, or retrorockets. The first two dampen the energy of the impact by absorbing it into springs or pressurised air. Airbags can be seen onboard the Boeing Starliner capsule or the Mars rovers Spirit and Opportunity, whereas the landing legs can be seen on the Falcon 9 first stage and on mock-ups of the Russian Orel capsule. The retrorocket system can be found onboard the Russian Soyuz capsule and means that a set of solid rocket motors is fired just before hitting the ground. This reduces the final velocity to an acceptable level.
The inclusion of a landing system means that the overall system becomes more complex and thus requires more verification and validation before the flight. It is recommended that the team ensures the primary mission objectives can still be met when the vehicle's landing system does not function. This could for instance be the inclusion of a black box system such that the flight data is still recovered.
An alternative to the inclusion of a landing system is to perform a water landing, such that the ocean impact reduces the kinetic energy. However, this often means there is a need for a floatation system and the retrieval operations will become more difficult. Another alternative, which is probably unfeasible for most missions is to catch the capsule in mid-air. This was done by the earlier US Corona spy satellites and currently by Rocket Lab's Electron. The manoeuvre requires a parachute to be inflated and captures by a helicopter or aircraft.