A space capsule is a spacecraft designed to transport cargo, scientific experiments, and/or astronauts to and from space. Reentry capsules are distinguished from other spacecraft by the ability to survive atmospheric reentry and return a payload to the Earth's surface from orbit or sub-orbit, and are distinguished from other types of recoverable spacecraft (eg. spaceplanes) by their blunt shape, not having wings. They often contain little fuel other than what is necessary for a safe return. Capsule-based crewed spacecraft such as Soyuz or Orion are often supported by a service or adapter module, and sometimes augmented with an extra module for extended space operations. A crewed space capsule must be able to sustain life in an often demanding thermal and radiation environment in the vacuum of space. It may be expendable (used once, like Soyuz) or reusable (like Dragon 2). Capsules make up the majority of crewed spacecraft designs, although one crewed spaceplane, the Space Shuttle, has flown in orbit. Current examples of crewed space capsules include the orbital Soyuz, Shenzhou, Dragon 2 and NASA's Orion. Sub-orbital capsules, such as Blue Origin's New Shepard are also in operation. Examples of new crew capsules currently in development include Boeing's Starliner, Russia's Orel, India's Gaganyaan, and China's Mengzhou. Historic examples of crewed capsules include Vostok, Mercury, Voskhod, Gemini, and Apollo.
History of crewed space capsules
Vostok
The Vostok was the Soviet Union's first crewed space capsule. The first human spaceflight was Vostok 1, accomplished on April 12, 1961 by cosmonaut Yuri Gagarin. The capsule was originally designed for use both as a camera platform for the Soviet Union's first spy satellite program, Zenit and as a crewed spacecraft. This dual-use design was crucial in gaining Communist Party support for the program. The design used a spherical reentry module, with a biconic descent module containing attitude control thrusters, on-orbit consumables, and the retro rocket for orbit termination. The basic design has remained in use for some 40 years, gradually adapted for a range of other uncrewed satellites. It was a single-seat capsule that was 4.4 meters long and 2.4 meters in diameter, weighing 4.73 tonnes at launch. The reentry module was completely covered in ablative heat shield material, 2.3 meters (7.5 ft) in diameter, weighing 2,460 kilograms (5,420 lb). The capsule was covered with a nose cone to maintain a low-drag profile for launch, with a cylindrical interior cabin approximately 1 meter (3.3 ft) in diameter nearly perpendicular to the capsule's longitudinal axis. The cosmonaut sat in an ejection seat with a separate parachute for escape during a launch emergency and landing during a normal flight. The capsule had its own parachute for landing on the ground. Although official sources stated that Gagarin had landed inside his capsule, a requirement for qualifying as a first crewed spaceflight under International Aeronautical Federation (IAF) rules, it was later revealed that all Vostok cosmonauts ejected and landed separately from the capsule. The capsule was serviced by an aft-facing conical equipment module 2.25 meters (7.4 ft) long by 2.43 meters (8.0 ft), weighing 2,270 kilograms (5,000 lb) containing nitrogen and oxygen breathing gasses, batteries, fuel, attitude control thrusters, and the retrorocket. It could support flights as long as ten days. Six Vostok launches were successfully conducted, the last two pairs in concurrent flights. The longest flight was just short of five days, on Vostok 5 on June 14–19, 1963. Since the attitude control thrusters were located in the instrument module which was discarded immediately prior to reentry, the reentry module's path and orientation could not be actively controlled. This meant that the capsule had to be protected from reentry heat on all sides, determining the spherical design (as opposed to Project Mercury's conical design, which allowed for maximum volume while minimizing the heat shield diameter). During reentry, the heat of atmospheric friction is so great that air molecules around the capsule are ionized, creating a layer of plasma around the capsule which blocks radio communication with the ground. However, ionized gases in the plasma layer can also be used to create an artificial radio window, allowing communication signals to be transmitted and received despite the interference. Some control of the capsule's reentry orientation was possible by offsetting its center of gravity. Proper orientation with the cosmonaut's back to the direction of flight was necessary in order to best sustain the which also maximized the 8 to 9 g-force.
Voskhod
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