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 In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...

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, the final descent is via

parachute A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, w ...

, 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 spaceplane ...

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 sys ...

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 Soyuz () is a series of spacecraft which has been in service since the 1960s, having made more than 140 flights. It was designed for the Soviet space program by the Korolev Design Bureau (now Energia). The Soyuz succeeded the Voskhod spacecr ...

is an example. Most capsules have used an ablative heat shield for reentry and been non-reusable. The

Orion Multi-Purpose Crew Vehicle Orion (officially Orion Multi-Purpose Crew Vehicle or Orion MPCV) is a partially reusable crewed spacecraft used in NASA's Artemis program. The spacecraft consists of a Crew Module (CM) space capsule designed by Lockheed Martin and the Eur ...

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. 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 ablative heat shield 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 ablative heat shield that melts and then vaporizes, removing the heat. The Apollo command module reentered with the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

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 con ...

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 n ...

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 m ...

, 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, 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

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, 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 A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

, 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. Mathematicall ...

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 Shooting star refers to a meteor. Shooting star may also refer to: Film, television, and theater * ''Shooting Star'' (2015 film), a 2015 Bulgarian short film * ''Shooting Star'' (2020 film), a 2020 Canadian short film * ''Shooting Stars'' ...

. 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", 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 design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...

flights, had its couches modified to allow for taller crewmembers to fly, and features "

glass cockpit A glass cockpit is an aircraft cockpit that features electronic (digital) flight instrument displays, typically large LCD screens, rather than the traditional style of analog dials and gauges. While a traditional cockpit relies on numerous ...

" 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 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. Subsequent flights, from Soyuz 12 to Soyuz 40, utilized a two-man crew because the third seat had to be removed for the pressure suit controls. The

Soyuz-T The Soyuz-T (russian: Союз-T, ''Union-T'') spacecraft was the third generation Soyuz spacecraft, in service for seven years from 1979 to 1986. The ''T'' stood for transport (, ). The revised spacecraft incorporated lessons learned from the ...

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 * McDonnell Mercury capsule * KH-6 Lanyard * Zenit (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 * 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 * VA TKS reentry capsule * Orbital Re-entry Experiment * Stardust Sample Return Capsule * Genesis Sample Return Capsule *

Atmospheric Reentry Demonstrator The Advanced Reentry Demonstrator (ARD) was a European Space Agency (ESA) suborbital reentry vehicle. It was developed and operated for experimental purposes, specifically to validate the multiple reentry technologies integrated upon it and the veh ...

* 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 w ...

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 Orion (officially Orion Multi-Purpose Crew Vehicle or Orion MPCV) is a partially reusable crewed spacecraft used in NASA's Artemis program. The spacecraft consists of a Crew Module (CM) space capsule designed by Lockheed Martin and the Eur ...

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 *

IXV The Intermediate eXperimental Vehicle (IXV) is a European Space Agency (ESA) experimental suborbital re-entry vehicle. It was developed to serve as a prototype lifting body orbital return vehicle to validate the ESA's work in the field of reus ...

(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's re-entry capsule. *

Martian Moons eXploration (MMX) The Martian Moons eXploration (MMX) is a robotic space probe set for launch in 2024 to bring back the first samples from Mars' largest moon Phobos. Developed by the Japanese Aerospace Exploration Agency (JAXA) and announced on 9 June 2015, MM ...

* Earth Entry vehicle (Mars sample-return) * Kavoshgar E

References

* – Describes Soyuz-TMA descent.