Return to Earth For spacecraft that are to be returned to Earth a controlled descent is required. This is initiated by a
"deorbit" maneuver that uses onboard propulsion to slow the vehicle's speed by about 1%. This slight amount is sufficient to lower the orbital path into the upper
atmosphere, where drag will slow it further. Entry into the atmosphere poses special craft-survival problems. Tremendous heat builds up, not on the skin of the vehicle (friction is not involved) but just ahead of it, where a shock wave creates severe air compression. The resulting plasma can reach temperatures as high as on the surface of the Sun. This heat will soak into the vehicle unless it is shielded, either by an ablative covering that carries heat away as it boils off, by an efficient insulator such as the material in the Space Shuttle tiles, or by an active cooling system. Atmospheric resistance slows the vehicle sharply, creating deceleration forces of up to ten times the force of gravity. Ionized, superheated air surrounds the vehicle with a sheath that blocks all radio communications. These effects require a precisely guided
descent profile to enable a safe return.