Spacecraft.

A spacecraft is a vehicle or device designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters outer space but then returns to the planetary surface (such as Earth) without making a complete orbit. For an orbital spaceflight, a spacecraft enters a closed orbit around the planetary body. Spacecraft used for human spaceflights carry people on board as crew or passengers. Spacecraft used for robotic space missions operate either autonomously or telerobotically. Robotic spacecraft that leave the vicinity of the planetary body are space probes. Robotic spacecraft that remain in orbit around the planetary body are artificial satellites. Starships, which are built for interstellar travel, are so far a theoretical concept only.

Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration, space tourism and space warfare. Spacecraft and space travel are common themes in works of science fiction.

Spacecraft Subsystems.

A spacecraft system comprises various subsystems, dependent upon mission profile. Spacecraft subsystems may include: attitude determination and control (variously called ADAC, ADC or ACS), guidance, navigation, and control (GNC or GN&C), communications (COMS), command and data handling (CDH or C&DH), power (EPS), thermal control (TCS), propulsion, structures, and payload.
Attitude control:
The attitude control subsystem permits proper pointing for the science objective, sun pointing for power to the solar arrays and earth-pointing for communications.

GNC: Guidance refers to the calculation of the commands (usually done by the CDH subsystem) needed to steer the spacecraft where it is desired to be. Navigation means determining a spacecraft's orbital elements or position. Control means adjusting the path of the spacecraft to meet mission requirements. On some missions, GNC and Attitude Control are combined into one subsystem of the spacecraft.

Command and data handling: The CDH subsystem receives commands from the communications subsystem, performs validation and decoding of the commands, and distributes the commands to the appropriate spacecraft subsystems and components. The CDH also receives housekeeping data and science data from the other spacecraft subsystems and components, and packages the data for storage on a solid state recorder or transmission to the ground via the communications subsystem. Other functions of the CDH include maintaining the spacecraft clock and state-of-health monitoring.

Power: Spacecraft need an electrical power generation and distribution subsystem for powering the various spacecraft subsystems. For spacecraft near the Sun, solar panels are frequently used to generate electrical power. Spacecraft designed to operate in more distant locations, for example Jupiter, might employ a Radioisotope Thermoelectric Generator (RTG) to generate electrical power.

Thermal control: Spacecraft must be engineered to withstand transit through the Earth's atmosphere and the space environment. They must operate in a vacuum with temperatures potentially ranging across hundreds of degrees Celsius as well as (if subject to reentry) in the presence of plasmas. The thermal control subsystem makes use of electrical heaters and certain actuators such as louvers to control temperature ranges of equipments within specific ranges.

Propulsion: A propulsion system is needed for spacecraft that perform momentum management maneuvers. Components of a conventional propulsion subsystem include fuel, tankage, valves, pipes, and thrusters. The TCS interfaces with the propulsion subsystem by monitoring the temperature of those components, and by preheating tanks and thrusters in preparation for a spacecraft maneuver.

Structures: Depending upon mission profile, the structural subsystem might need to withstand loads imparted by entry into the atmosphere of another planetary body, and landing on the surface of another planetary body.

Payload: The payloads include scientific instruments (cameras, telescopes, or particle detectors, for example), cargo, or a human crew.

Ground segment: The ground segment, though not technically part of the spacecraft, is vital to the operation of the spacecraft. Typical components of a ground segment in use during normal operations include a mission operations facility where the flight operations team conducts the operations of the spacecraft, a data processing and storage facility, ground stations to radiate signals to and receive signals from the spacecraft, and a voice and data communications network to connect all mission elements.

Launch vehicle
The launch vehicle is used to propel the spacecraft from the Earth's surface, through the atmosphere, and into an orbit, the exact orbit being dependent upon mission configuration. The launch vehicle may be expendable or reusable.

Reusable Spacecraft.

A reusable spacecraft is one which can be used again after its one mission. The first reusable spacecraft, the X-15, was air-launched on a suborbital trajectory on July 19, 1963. The first partially reusable orbital spacecraft, the Space Shuttle, was launched by the USA on the 20th anniversary of Yuri Gagarin's flight, on April 12, 1981. During the Shuttle era, six orbiters were built, all of which have flown in the atmosphere and five of which have flown in space. The Enterprise was used only for approach and landing tests, launching from the back of a Boeing 747 and gliding to deadstick landings at Edwards AFB, California. The first Space Shuttle to fly into space was the Columbia, followed by the Challenger, Discovery, Atlantis, and Endeavour. The Endeavour was built to replace the Challenger when it was lost in January 1986. The Columbia broke up during reentry in February 2003.

The first automatic partially reusable spacecraft was the Buran (Snowstorm), launched by the USSR on November 15, 1988, although it made only one flight. This spaceplane was designed for a crew and strongly resembled the U.S. Space Shuttle, although its drop-off boosters used liquid propellants and its main engines were located at the base of what would be the external tank in the American Shuttle. Lack of funding, complicated by the dissolution of the USSR, prevented any further flights of Buran. The Space Shuttle has since been modified to allow for autonomous re-entry via the addition of a control cable running from the control cabin to the mid-deck which would allow for the automated deployment of the landing gear in the event a un-crewed re-entry was required following abandonment due to damage at the ISS.

Per the Vision for Space Exploration, the Space Shuttle is due to be retired in 2010 due mainly to its old age and high cost of program reaching over a billion dollars per flight. The Shuttle's human transport role is to be replaced by the partially reusable Crew Exploration Vehicle (CEV) no later than 2014. The Shuttle's heavy cargo transport role is to be replaced by expendable rockets such as the Evolved Expendable Launch Vehicle (EELV) or a Shuttle Derived Launch Vehicle.

Scaled Composites SpaceShipOne was a reusable suborbital spaceplane that carried pilots Mike Melvill and Brian Binnie on consecutive flights in 2004 to win the Ansari X Prize. The Spaceship Company will build its successor SpaceShipTwo. A fleet of SpaceShipTwos operated by Virgin Galactic should begin reusable private spaceflight carrying paying passengers in early 2009.

Fictional Spacecraft.

The term spacecraft is mainly used to refer to spacecraft that are real or conceived using present technology. The terms spaceship and starship are generally applied only to fictional spacecraft, usually those capable of transporting people. The spaceship is one of the prime elements in science fiction. Numerous short stories and novels are built up around various ideas for spacecraft, and spacecraft have often been featured in movies. Some hard science fiction books focus on the technical details of the craft, while others treat the spacecraft as a given and delve little into its actual implementation.

Notable spacecraft:

• The Death Star, Millennium Falcon, Star Destroyer, X-Wing and TIE Fighter from the Star Wars science-fiction universe.
• USS Enterprise and Klingon Bird of Prey from the Star Trek franchise.
• The Liberator and Scorpio from the British science fiction television series Blake's 7.
• The TARDIS from the British science fiction television series Doctor Who.
• Discovery One from 2001: A Space Odyssey
• The Battlestar Galactica from the original and re-imagined series of the same name.
• Daedalus class battlecruiser from the television series Stargate SG-1 and Stargate Atlantis.
• USS Saratoga (SCVN-2812) from the television series Space: Above and Beyond.
• Serenity from the television series Firefly
• The UNSC In Amber Clad, Forward Unto Dawn and Pillar of Autumn from the Halo video game series.

Unidentified flying objects:

Some people believe that Unidentified Flying Objects (UFOs) may be alien spacecraft (that is, not of human construction and not originating from Earth), sometimes referred to as flying saucers. But the term UFO used here in this context refers to observed flying objects for which no identification has been made, though other meanings for the word UFO exist. To date, no known, independently verifiable examples of alien spacecraft are known to exist.

Examples of Spacecraft.

Manned spacecraft

Spacecrafts listed below are examples of manned spacecrafts :

• Apollo Spacecraft
• Gemini Spacecraft
• International Space Station
• Mercury Spacecraft
• Manned Maneuvering Unit - world's smallest manned spacecraft
• Mir
• Salyut
• Shuttle Buran
• Shenzhou Spacecraft
• Skylab
• Soyuz Spacecraft
• Space Shuttle
• Voskhod Spacecraft
• Vostok Spacecraft