A reentry capsule is the portion of a

space capsule A space capsule is an often-crewed spacecraft that uses a blunt-body reentry capsule to reenter the Earth's atmosphere without wings. Capsules are distinguished from other satellites primarily by the ability to survive reentry and return a payl ...

which returns to Earth following a spaceflight. The shape is determined partly by

aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dy ...

; a capsule is aerodynamically stable falling blunt end first, which allows only the blunt end to require a heat shield for

atmospheric entry Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the en ...

. A crewed capsule contains the spacecraft's instrument panel, limited storage space, and seats for crew members. Because a capsule shape has little

aerodynamic lift A fluid flowing around an object exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direc ...

, the final descent is via parachute, either coming to rest on land, at sea, or by active capture by an aircraft. In contrast, the development of

spaceplane A spaceplane is a vehicle that can fly and glide like an aircraft in Earth's atmosphere and maneuver like a spacecraft in outer space. To do so, spaceplanes must incorporate features of both aircraft and spacecraft. Orbital spaceplanes te ...

reentry vehicles attempts to provide a more flexible reentry profile.

Structure

Reentry capsules have typically been smaller than in diameter due to

launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload ( spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pads, supported by a launch control center and ...

aerodynamic requirements. The capsule design is both volumetrically efficient and structurally strong, so it is typically possible to construct small capsules of performance comparable to

lifting body A lifting body is a fixed-wing aircraft or spacecraft configuration in which the body itself produces lift. In contrast to a flying wing, which is a wing with minimal or no conventional fuselage, a lifting body can be thought of as a fuselage ...

designs in all but lift-to-drag ratio for less cost. The Soyuz spacecraft is an example. Most capsules have used an

ablative heat shield Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the ent ...

for reentry and been non-reusable. The Orion Multi-Purpose Crew Vehicle appears likely, as of December 2005, to use a ten-times reusable capsule with a replaceable heat shield. There is no limit, save for lack of engineering experience, on using high-temperature ceramic tiles or ultra-high temperature

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

sheets on reentry capsules. Materials for the capsule are designed in different ways, like the

Apollo command module The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother shi ...

’s

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

honeycomb structure Honeycomb structures are natural or man-made structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal material cost. The geometry of honeycomb structures ca ...

. Aluminum is very light, and the structure gives the capsule extra strength. The early spacecraft had a coating of glass embedded with synthetic resin and put in very high temperatures.

Carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...

, reinforced plastics and ceramic are new materials that are constantly being made better for use in space exploration.

Reentry

Most reentry capsules have used an

for reentry and been non-reusable. The early spacecraft had a coating of glass embedded with synthetic resin and put in very high temperatures. Reentry capsules are well-suited to high energy reentries. Capsules reenter aft-end first with the occupants lying down, as this is the optimum position for the human body to withstand the g-forces induced as the capsule impacts the atmosphere. The rounded shape (blunt body) of a capsule forms a shock wave that keeps most of the heat away from the heat shield, but a

thermal protection system Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the entr ...

is still necessary. The space capsule must be strong enough to withstand reentry forces such as drag, and must reenter at a precise angle of attack to prevent a skip off the surface of the atmosphere or destructively high accelerations. When the reentry capsule comes through the atmosphere the capsule compresses the air in front of it, which heats up to very high temperatures. The surface temperature of a capsule can reach as it descends through the Earth's atmosphere. To prevent this heat from reaching interior structures, capsules are typically equipped with an

that melts and then vaporizes, removing the heat. The Apollo command module reentered with the center of mass offset from the center line; this caused the capsule to assume an angled attitude through the air, providing lift that could be used for directional control.

Reaction control system A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control and translation. Alternatively, reaction wheels are used for attitude control. Use of diverted engine thrust to provide stable attitude cont ...

thrusters were used to steer the capsule by rotating the lift vector. Parachutes are used for the final descent, sometimes augmented by braking rockets if the capsule is designed to land on the Earth's surface. Examples of land landing capsules include Vostok, Voskhod, Soyuz, Shenzhou and the

Boeing CST-100 Starliner The Boeing CST-100 Starliner
is a class of two partially

. Other capsules, such as Mercury, Gemini, Apollo, Orion, and Dragon, splash down in the ocean.

Aerodynamic heating

Capsules are well-suited to high-temperature and dynamic loading reentries. Whereas delta-wing gliders such as the

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program ...

can reenter from

Low Earth Orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never mor ...

, and lifting bodies are capable of entry from as far away as the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

, it is rare to find designs for reentry vehicles from

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

that are not capsules. The current

RKK Energia PAO S. P. Korolev Rocket and Space Corporation Energia (russian: Ракетно-космическая корпорация «Энергия» им. С. П. Королёва, Raketno-kosmicheskaya korporatsiya "Energiya" im. S. P. Korolyov ...

design for the

Kliper Kliper (Клипер, English: Clipper) was an early-2000s proposed partially- reusable crewed spacecraft concept by RSC Energia. Due to lack of funding from the ESA and RSA, the project was indefinitely postponed by 2006. Designed primarily to ...

, being capable of flights to Mars, is an exception. Engineers building a reentry capsule must take forces such as

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

and drag into consideration. The capsule must be strong enough to slow down quickly, must endure extremely high or low temperatures, and must survive the landing. When the capsule comes close to a planet's or moon's surface, it has to slow down at a very exact rate. If it slows down too quickly, everything in the capsule will be crushed. If it does not slow down quickly enough, it will crash into the surface and be destroyed. There are additional requirements for atmospheric reentry. If the angle of attack is too shallow, the capsule may skip off the surface of the atmosphere. If the angle of attack is too steep, the deceleration forces may be too high or the heat of reentry may exceed the tolerances of the heat shield. Capsules reenter aft-end first with the occupants lying down, as this is the optimum position for the human body to withstand the decelerative g-force. The aft end is formed in a rounded shape (blunt body), as this forms a shock wave that doesn't touch the capsule, and the heat is deflected away rather than melting the vehicle. The Apollo Command Module reentered with the center of mass offset from the center line; this caused the capsule to assume an angled attitude through the air, providing a sideways lift to be used for directional control. Rotational thrusters were used to steer the capsule under either automatic or manual control by changing the lift vector. At lower altitudes and speeds parachutes are used to slow the capsule down by making more drag. Capsules also have to be able to withstand the impact when they reach the Earth's surface. All US crewed capsules (Mercury, Gemini, Apollo) landed on water; the Soviet/Russian Soyuz and Chinese Shenzhou (and planned US, Russian, and Indian) crewed capsules use small

retrorockets A retrorocket (short for ''retrograde rocket'') is a rocket engine providing thrust opposing the motion of a vehicle, thereby causing it to decelerate. They have mostly been used in spacecraft, with more limited use in short-runway aircraft land ...

to touch down on land. In the lighter

gravity of Mars The gravity of Mars is a natural phenomenon, due to the law of gravity, or gravitation, by which all things with mass around the planet Mars are brought towards it. It is weaker than Earth's gravity due to the planet's smaller mass. The average ...

, airbags are sufficient to land some of the robotic missions safely.

Gravity, drag, and lift

Two of the biggest external forces that a reentry capsule experiences are

and drag. Drag is the capsule's resistance to it moving through

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

. Air is a mixture of different molecules, including nitrogen, oxygen and carbon dioxide. Anything falling through air hits these molecules and therefore slows down. The amount of drag on a capsule depends on many things, including the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

of the air, and the shape, mass, diameter and roughness of the capsule. The speed of a spacecraft highly depends on the combined effect of the two forces — gravity, which can speed up a rocket, and drag, which will slow down the rocket. Capsules entering Earth's atmosphere will be considerably slowed because our atmosphere is so thick. When the capsule comes through the atmosphere, it compresses the air in front of it which heats up to very high temperatures (contrary to popular belief friction is not significant). A good example of this is a shooting star. A shooting star, which is usually tiny, creates so much heat coming through the atmosphere that the air around the meteorite glows white hot. So when a huge object like a capsule comes through, even more heat is created. As the capsule slows down, the compression of the air molecules hitting the capsules surface creates a lot of heat. The surface of a capsule can get to as it descends through the Earth's atmosphere. All this heat has to be directed away. Reentry capsules are typically coated with a material that melts and then vaporizes ("ablation"). It may seem counterproductive, but the vaporization takes heat away from the capsule. This keeps the reentry heat from getting inside the capsule. Capsules see a more intense heating regime than spaceplanes and ceramics such as used on the Space Shuttle are usually less suitable, and all capsules have used ablation. In practice, capsules do create a significant and useful amount of lift. This lift is used to control the trajectory of the capsule, allowing reduced g-forces on the crew, as well as reducing the peak heat transfer into the capsule. The longer the vehicle spends at high altitude, the thinner the air is and the less heat is conducted. For example, the Apollo CM had a lift to drag ratio of about 0.35. In the absence of any lift the Apollo capsule would have been subjected to about 20g deceleration (8g for low-Earth-orbiting spacecraft), but by using lift the trajectory was kept to around 4g.

Current designs

Shenzhou

The reentry capsule is the "middle" module of the three-part

Soyuz Soyuz is a transliteration of the Cyrillic text Союз ( Russian and Ukrainian, 'Union'). It can refer to any union, such as a trade union (''profsoyuz'') or the Union of Soviet Socialist Republics (Сою́з Сове́тских Социалис ...

or Shenzhou spacecraft – the orbital module is located at the front of the spacecraft, with the service or equipment module attached to the rear. A feature in the landing system allows the use of a single parachute and "

braking rocket A retrorocket (short for ''retrograde rocket'') is a rocket engine providing thrust opposing the motion of a vehicle, thereby causing it to decelerate. They have mostly been used in spacecraft, with more limited use in short-runway aircraft land ...

", thus the heatshield is dropped from the spacecraft similar to the landing bag deployment on the U.S. Mercury spacecraft. Like the Command Module of the

Apollo spacecraft The Apollo spacecraft was composed of three parts designed to accomplish the American Apollo program's goal of landing astronauts on the Moon by the end of the 1960s and returning them safely to Earth. The expendable (single-use) spacecraft ...

, the Shenzhou reentry capsule has no reusable capabilities; each spacecraft is flown once and then "thrown away" (usually sent to museums). Few details are known about the Shenzhou reentry capsule, except that it uses some technology from the Soyuz TM design. The new Soyuz TMA spacecraft, now used solely for

International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...

flights, had its couches modified to allow for taller crewmembers to fly, and features " glass cockpit" technology similar to that found on the

and newer commercial and military aircraft.

Soyuz

The former Soviet Union suffered two disasters, and one near-disaster, all three involving the capsule during the de-orbit and reentry.

Soyuz 1 Soyuz 1 (russian: Союз 1, ''Union 1'') was a crewed spaceflight of the Soviet space program. Launched into orbit on 23 April 1967 carrying cosmonaut colonel Vladimir Komarov, Soyuz 1 was the first crewed flight of the Soyuz spacecraft. Th ...

ended in disaster when the parachutes failed to deploy and the capsule smashed into the earth at speeds over , killing cosmonaut

Vladimir Komarov Vladimir Mikhaylovich Komarov ( rus, Влади́мир Миха́йлович Комаро́в, p=vlɐˈdʲimʲɪr mʲɪˈxajləvʲɪtɕ kəmɐˈrof; 16 March 1927 – 24 April 1967) was a Soviet test pilot, aerospace engineer, and cosmonaut. ...

Soyuz 5 Soyuz 5 (russian: Союз 5, ''Union 5'') was a Soyuz mission using the Soyuz 7K-OK spacecraft launched by the Soviet Union on 15 January 1969, which docked with Soyuz 4 in orbit. It was the first docking of two crewed spacecraft of any nat ...

almost ended in disaster, when the reentry capsule entered the atmosphere nose first – attributed to a failure of the service module to separate similar to that on the

Vostok 1 Vostok 1 (russian: link=no, Восток, ''East'' or ''Orient'' 1) was the first spaceflight of the Vostok programme and the first human orbital spaceflight in history. The Vostok 3KA space capsule was launched from Baikonur Cosmodrome on Apr ...

flight. Luckily, the service module was burned off and the capsule righted itself.

Soyuz 11 Soyuz 11 (russian: link=no, Союз 11, lit=Union 11) was the only crewed mission to board the world's first space station, Salyut 1 ( Soyuz 10 had soft-docked, but had not been able to enter due to latching problems). The crew, Georgy Dob ...

ended in disaster in 1971 when an equalization valve, used to equalize air pressure during the Soyuz final descent, prematurely opened in the vacuum of space, killing the three crew members, who were not wearing

spacesuits A space suit or spacesuit is a garment worn to keep a human alive in the harsh environment of outer space, vacuum and temperature extremes. Space suits are often worn inside spacecraft as a safety precaution in case of loss of cabin pressure, ...

. Subsequent flights, from

Soyuz 12 Soyuz 12 (russian: Союз 12, ''Union 12'') was a September, 1973, crewed test flight by the Soviet Union of the newly redesigned Soyuz 7K-T spacecraft that was intended to provide greater crew safety in the wake of the Soyuz 11 tragedy. Th ...

Soyuz 40 The Soyuz 40 mission was a 1981 Soviet crewed spaceflight and the final flight of the Soyuz 7K-T spacecraft. It was a collaboration between the Soviet Union and Romania.The mission report is available here: http://www.spacefacts.de/mission/english ...

, utilized a two-man crew because the third seat had to be removed for the pressure suit controls. The Soyuz-T version restored the third seat.

List of reentry capsules

Flight-proven: * KH-1 to KH-4

CORONA Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...

* Vostok reentry module *

KH-5 Argon KH-5 ARGON was a series of reconnaissance satellites produced by the United States from February 1961 to August 1964. The KH-5 operated similarly to the CORONA series of satellites, as it ejected a canister of photographic film. At least 12 ...

* McDonnell Mercury capsule *

KH-6 Lanyard BYEMAN codenamed LANYARD, the KH-6 was the unsuccessful first attempt to develop and deploy a very high-resolution optical reconnaissance satellite by the United States National Reconnaissance Office. Launches and launch attempts spanned the per ...

Zenit Zenit, meaning "zenith", may refer to: Spaceflight and rocketry * Zenit (rocket family), a Soviet family of space launch vehicles * Zenit (satellite), a type of Soviet spy satellite * Zenit sounding rocket, a Swiss rocket Sports * Zenit (sports ...

(Vostok-derived) *

KH-7 Gambit BYEMAN codenamed GAMBIT, the KH-7 (Air Force Program 206) was a reconnaissance satellite used by the United States from July 1963 to June 1967. Like the older CORONA system, it acquired imagery intelligence by taking photographs and returning ...

(Two capsules per launch) * McDonnell Gemini capsule * Voskhod reentry modules *

Apollo Command Module The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother shi ...

KH-8 Gambit 3 The KH-8 (BYEMAN codename Gambit-3) was a long-lived series of reconnaissance satellites of the "Key Hole" (KH) series used by the United States from July 1966 to April 1984, and also known as Low Altitude Surveillance Platform. The satellite ej ...

* Soyuz reentry module *

Luna 16 ''Luna 16'' was an uncrewed space mission, part of the Soviet Luna program. It was the first robotic probe to land on the Moon and return a sample of lunar soil to Earth. The 101 grams (3.56 ounces) sample was returned from Mare Fecunditatis. ...

KH-9 Hexagon KH-9 (BYEMAN codename HEXAGON), commonly known as Big Bird or KeyHole-9, p.32 Big Bird was a series of photographic reconnaissance satellites launched by the United States between 1971 and 1986. Of twenty launch attempts by the National Recon ...

(4 capsules per launch) *

Luna 20 ''Luna 20'' was the second of three successful Soviet lunar sample return missions. It was flown as part of the Luna program as a robotic competitor to the six successful Apollo lunar sample return missions. Overview ''Luna 20'' was placed ...

* Yantar *

Fanhui Shi Weixing The Fanhui Shi Weixing () series of satellites was China's first reconnaissance satellite program. The satellites were used for military reconnaissance and civilian imagery tasks and completed 23 missions between November 1974 and April 2016. The ...

Luna 24 ''Luna 24'' was a robotic probe of the Soviet Union's Luna programme. The last of the Luna series of spacecraft, the mission of the ''Luna 24'' probe was the third Soviet mission to return lunar soil samples from the Moon (the first two sampl ...

* VA TKS reentry capsule * Orbital Re-entry Experiment * Stardust Sample Return Capsule * Genesis Sample Return Capsule * Atmospheric Reentry Demonstrator * Shenzhou reentry module *

Space Capsule Recovery Experiment The Space Capsule Recovery Experiment (SCRE or more commonly SRE or SRE-1) is an Indian experimental spacecraft which was launched at 03:53 UTC on January 10, 2007, from Sriharikota by the Indian Space Research Organisation (ISRO). The launch ...

SpaceX Dragon American private space transportation company SpaceX has developed and produced several spacecraft named Dragon. The first family member, now referred to as Dragon 1, flew 23 cargo missions to the ISS between 2010 and 2020 before being retired. ...

* Hayabusa Reentry Capsule * Orion MPCV *

OSIRIS-REx OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is a NASA asteroid-study and sample-return mission. The mission's primary goal is to obtain a sample of at least from 101955 Bennu, a carbona ...

return capsule *

SpaceX Dragon 2 Dragon 2 is a class of partially reusable spacecraft developed and manufactured by American aerospace manufacturer SpaceX, primarily for flights to the International Space Station (ISS). SpaceX has also launched private missions such as Ins ...

Boeing CST-100 The Boeing CST-100 Starliner
is a class of two partially IXV (lifting body) *

Next-generation crewed spacecraft The next-generation crewed spacecraft () is a type of reusable spacecraft developed and manufactured by China Aerospace Science and Technology Corporation (CASC). The prototype of the spacecraft underwent its first uncrewed test flight on 5 May ...

Chang'e 5 Chang'e 5 () was the fifth lunar exploration mission of the Chinese Lunar Exploration Program, and China's first lunar sample-return mission. Like its predecessors, the spacecraft is named after the Chinese moon goddess Chang'e. It launched ...

's re-entry module In development: *

SpaceX Starship Starship is a fully reusable, super heavy-lift launch vehicle under development by SpaceX, an American aerospace company. With more than twice the thrust of the Saturn V, it is designed to be the most powerful launch vehicle ever built and the ...

Gaganyaan Gaganyaan (Sanskrit IAST: ''gagan-yāna'', ) is an Indian crewed orbital spacecraft intended to be the formative spacecraft of the Indian Human Spaceflight Programme. The spacecraft is being designed to carry three people, and a planned upgrad ...

* Oryol/Orel *

Chang'e 6 Chang'e 6 () is a planned robotic Chinese lunar exploration mission that is expected to launch in 2025 and perform China's second sample return mission. Like its predecessors, the spacecraft is named after the Chinese moon goddess Chang'e. Ov ...

's re-entry capsule. * Martian Moons eXploration (MMX) * Earth Entry vehicle (Mars sample-return) * Kavoshgar E

References

* – Describes Soyuz-TMA descent.