By decelerating, the temporary vertical speed of the spacecraft in contact with Earth or other planets is minimized (ideally zero) to achieve safe landing technology. The purpose of the soft landing is to ensure the safety of the astronauts and the intact instruments and equipment of the spacecraft.
To achieve a smooth landing, the spacecraft must first be decelerated. When a spacecraft enters a planet with an atmosphere, it can use atmospheric drag to slow down. For ballistic spacecraft, parachutes are the most effective aerodynamic deceleration device, supplemented by rockets or protective structures to land safely.
A winged spacecraft can use lift to control its orbit in a dense atmosphere, gradually decelerate, and finally land level with the plane. After achieving a soft landing on Earth (see Spacecraft Recovery System), people achieved soft landings on Venus and Mars (see Landing on Venus, Landing on Mars).
Soft landing of a spacecraft on a planet with no atmosphere must use a braking rocket as decelerating power and must be supplemented by a landing cushion structure (see Moon landing). On the other hand, the way the spacecraft is not slowed down by a special decelerating device, but hits directly and lands faster, is called a crash landing.
Due to excessive landing speed, the spacecraft will be completely or almost completely damaged. Therefore, a forced landing of a spacecraft is a destructive landing, which is different from the concept of “landing” in aircraft in general. The Soviet Union’s “Moon” probes 2, 5, 7 and 8 made a forced landing on the moon; the “Venus” 3 probe made a forced landing on Venus.