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A module is a self-contained unit of a launch vehicle that serves as a building block for the overall structure. It is commonly referred to by its primary functionfor example the "command module" used in the Apollo lunar missions. More recently, the term has been used to describe a distinct pressurized, crewed section of an orbiting spacecraft, suitable for conducting science, applications, and technology activities. An example of this would be the Spacelab module in the Space Transportation System.

Early Use of Modules

Modular construction was used in many early piloted spacecraft to minimize the size and weight of the re-entry vehicle and to ease assembly of the spacecraft. Modules can be constructed and tested independently of other sections and then integrated into the rest of the spacecraft at a later stage. Completion of the International Space Station (ISS) depends on this technique of modular construction, as no single rocket could lift the entire station into orbit.

The first human in space was also the first to ride aboard a modular spacecraft. Yuri Gagarin's Vostok 1 was composed of two modules, the spherical descent module and the cone-shaped service module. The service module contained various consumables for life support (such as food, water, and oxygen), the attitude control system, batteries, telemetry systems, and a retrorocket at its base. The 5,100 kilogram (11,243 pound) service module was jettisoned before the 5,300 kg (11,684 pound) descent module returned to Earth.

Later Spacecraft

The Soviets modified the Vostok spacecraft for use in their Voskhod and Soyuz programs. Voskhod craft retained the two-module organization. The more advanced Soyuz spacecraft added an orbital module where the cosmonauts ate and slept, but it and the instrument module (which contained the thrusters and power supply) were jettisoned before the descent vehicle returned.

For the Gemini program, NASA modified its Mercury capsule to hold two astronauts and added an adapter module to its base. The adapter module's increased capacity to carry oxygen and other supplies permitted astronauts to stay in orbit for up to two weeks. (Mercury astronauts could only stay aloft for a day at most.) The adapter module also had an attitude control system that gave the astronauts full control over their spacecraft, allowing them to practice docking techniques for Apollo missions.

Apollo spacecraft comprised three modules, one of which was a separate spacecraft. The command module (CM) served as the crew's quarters and flight control section. The Service Module (SM), which held the rocket motors and supplies, remained attached to the CM until re-entry. Together, they were called the Command-and-Service Module, or CSM. The Lunar Module, or LM, transported two crew members to the lunar surface and back to the waiting CSM.

The International Space Station

The International Space Station requires far more specialized modular construction than any previous spacecraft. Approximately forty-three rocket and space shuttle launches will be necessary to ferry the components into orbit. Sections will include a habitation module, a docking module, laboratory modules, four modules containing the eight solar power arrays, and the Multipurpose Logistics Module, a reusable section that will deliver and return any cargo requiring a pressurized environment via the space shuttle.

see also Capsules (volume 3); International Space Station (volumes 1 and 3); Mercury Program (volume 3).

Chad Boutin


Angelo, Joseph J., Jr. The Dictionary of Space Technology, 2nd ed. New York: Facts on File, 1999.

Williamson, Mark. A Dictionary of Space Technology. Bristol, UK: Adam Hilger, 1990.

Internet Resources

"SoyuzDevelopment of the Space Station; ApolloVoyage to the Moon." NASA web site. <>.

"Space Station Gallery." NASA web site. <>.

"Structural Description." Apollo Saturn Reference Page. <>.

Monkeys See Primates, Non-Human (Volume 3).