Encyclopedia
A
spacecraft is a vehicle designed to operate beyond the surface of the Earth in
outer space. Spacecraft may either be
unmanned or
manned. Spacecraft are designed for a variety of missions which may include
communications, earth observation,
meteorology,
navigation, planetary exploration,
space tourism and
space warfare. The term spacecraft is also used to describe artificial
satellites.
Overview
A spacecraft is a system made up of various subsystems, dependent upon mission profile. Spacecraft subsystems may include: attitude determination and control , guidance, navigation, and control , communications , command and data handling , power , thermal control , propulsion, structures, and payload. Manned spacecraft have the additional requirement of providing life support to the crew. Though not part of the spacecraft itself, the launch vehicle used to place a spacecraft in orbit, and the ground system used to send telecommands to the spacecraft and receive telemetry from the spacecraft, are frequently also considered subsystems. A
space suit has at times been called a miniature spacecraft.
Typical spacecraft subsystems
Attitude control
Spacecraft need an attitude control subsystem in order that they may be correctly oriented in space and respond to external
torques and forces properly. The attitude control subsystem consists of sensors and
actuators, together with controlling algorithms. 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
The communications subsystem, sometimes called the Telemetry, Tracking, and Control subsystem serves as an interface between the spacecraft and the ground system, or between the spacecraft and other spacecraft. The communication subsystem receives telecommands from the ground subsystem, and transmits telemetry from the spacecraft.
Communications
Guidance refers to the calculation of the commands 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 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 function of the CDH include maintaining the spacecraft clock and state-of-health monitoring .
Power
The launch vehicle is used 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.
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. Depending on mission profile, spacecraft may also need to operate on the surface of another planetary body.
Propulsion
Spacecraft may or may not have a
propulsion subsystem, depending upon whether or not the mission profile calls for propulsion. The
Swift spacecraft is an example of a spacecraft that does not have a propulsion subsystem. Typically though, LEO spacecraft
Structures
Spacecraft must be engineered to withstand launch loads imparted by the launch vehicle, and must have a point of attachment for all the other subsystems. 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 payload is dependent upon the mission of the spacecraft, and is typically regarded as the part of the spacecraft "that pays the bills". Typical payloads could include scientific instruments , cargo, or a
human crew.
Ground system
The ground system, though not technically part of the spacecraft, is vital to the operation of the spacecraft. Typical components of a ground system 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
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 to generate electrical power. Electrical power is sent through power conditioning equipment before it passes through a power distribution unit over an electrical bus to other spacecraft components. A battery are typically connected to the bus via a battery charge regulator, and the battery is used to provide electrical power during periods when primary power is not available, for example when a Low Earth Orbit spacecraft is eclipsed by the Earth.
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 . 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. Some of the most famous fictional spacecraft include the USS Enterprise from
Star Trek, and the
Millennium Falcon from
Star Wars.
Some people believe that
Unidentified Flying Objects may be
alien spacecraft , 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...
...
External links
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Heaviest spacecraftUnmanned spacecraftFarthest spacecraftFastest spacecraft- Helios I & II Solar Probes - 158,000 mph or 43.9 miles per second
Spacecraft under developmentCommercial spacecraft- Genesis-1 inflated orbiting habitat
Commercial Spacecraft under developmentCancelled spacecraft programsCancelled SSTO spacecraft- RR/British Aerospace HOTOL
- ESA Hopper Orbiter
- McDonnell Douglas Clipper DC-X
- Roton Rotored-Hybrid
- Lockheed-Martin VentureStar
See also
References
External articles
- Will Knight, "". January 2006, NewScientist.com.
