V0.1 - 22-06-2020

Planetary landing

The Earth is by far the planetary body with the most re-entry and landing attempts of spacecraft, but not the only one. For decades scientists and engineers have attempted to land spacecraft on other planets, moons or comets. With such a different set of environments, different engineering solutions have to be found to re-enter and land on other planets.

Without atmosphere

There are many, mostly smaller bodies, that don’t have any atmosphere at all. Bodies like this include comets, asteroids and our Moon. This environment has a large impact on the re-entry and landing system as parachutes are useless to slow the vehicle down. The only way to prevent a hard landing is by using a propulsive landing system. The size of the body to land on will determine the size and power of the landing system. A benefit with the lack of atmosphere is the lack of aerothermal heating during re-entry. This means a heatshield is not required to safely land.

Dense atmosphere

Certain planets and moons have a very large, dense atmosphere. Examples of these include Venus and Saturn’s moon Titan and Earth itself. This poses interesting challenges to the system as the vehicle will experience very high loads during reentry. A benefit of such an atmosphere is that a parachute will be very effective and can be kept small to still allow for a safe landing velocity. For missions to Venus, the aggressive atmosphere has led to structural failures of parachutes while still landing safely, simply due to the body drag of the satellite itself.

The first probe to land on Venus was Venera 4 in October 1967. The surface composition and precise atmosphere weren't known at the time and this satellite provided key data for future missions to Venus. Because of the unknown environment, the spherical landing capsule was pressurized to 25 bar and was designed to be able to float in case of a 'water' landing. In the event of a water landing, the capsule had a lock made of sugar. This would dissolve allowing the transmitting antenna to be freed and able to send data back to Earth. The probe continued to transit up to 25km altitude and it, therefore, can't be confirmed it had a soft touchdown.

Lean atmosphere

Perhaps the most challenging environment to reenter and land in is a body with a low-density atmosphere. The most notable example of this is Mars. This has the problem that a parachute alone will likely not be enough to slow the vehicle down enough and yet the aerodynamic heating becomes substantial that a heat shield is required. To land in such a difficult environment a combination of several systems are needed. On Mars the earlier NASA’s probes used a parachute and airbag system to safely land a vehicle. Nowadays they are combining parachutes with a propulsive system to land, with the latest example being Perseverance which landed in 2021.