The first Earth orbiting
satellite was
Sputnik 1, which was launched 4 October 1957, and remained in orbit for several months.[1] While Sputnik 1 was the first spacecraft to orbit the Earth, other man-made objects had previously reached an altitude of 100 km, which is the height required by the international organization
Fédération Aéronautique Internationale to count as a spaceflight. This altitude is called the
Kármán line. In particular, in the 1940's there were
several test launches of the
V-2rocket, some of which reached altitudes well over 100 km.
The first manned spacecraft was
Vostok 1, which carried Soviet cosmonaut
Yuri Gagarin into space in 1961, and complete a full Earth orbit. There were five other manned missions which used a
Vostok spacecraft.[2] The second manned spacecraft was named
Freedom 7, and it performed a
sub-orbital spaceflight carrying American astronaut
Alan Shepard to an altitude of just over 187 kilometres (116 mi). There were five other manned missions using
Mercury spacecraft.
Some reusable vehicles have been designed only for manned spaceflight, and these are often called
spaceplanes. The first example of such was the
North American X-15 spaceplane, which conducted two manned flights which reached a height over 100 km in the 1960's. 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 SCA 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 Columbiabroke 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 in case of necessity.
Per the
Vision for Space Exploration, the Space Shuttle is due to be retired in 2011 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.
XCOR Aerospace also plans to initiate a suborbital commercial spaceflight service with the
Lynx rocketplane in 2012 through a partnership with
RocketShip Tours. First test flights are planned for 2011.
A spacecraft system comprises various subsystems, dependent upon mission profile. Spacecraft subsystems comprise the spacecraft "bus" and may include: attitude determination and control (variously called ADAC, ADC or ACS), guidance, navigation and control (GNC or GN&C), communications (Comms), command and data handling (CDH or C&DH), power (EPS), thermal control (TCS), propulsion, and structures. Attached to the bus are typically payloads.
Life support
Spacecraft intended for human spaceflight must also include a
life support system for the crew.
Attitude control
A Spacecraft needs an
attitude control subsystem to 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
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
data 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.
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. Electrical power is sent through power conditioning equipment before it passes through a power distribution unit over an electrical bus to other spacecraft components. Batteries are typically connected to the bus via a battery charge regulator, and the batteries are used to provide electrical power during periods when primary power is not available, for example when a
Low Earth Orbit (LEO) spacecraft is
eclipsed by the Earth.
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. Material requirements are such that either high melting temperature, low density materials such as
beryllium and
reinforced carbon-carbon or (possibly due to the lower thickness requirements despite its high density)
tungsten or
ablative carbon/carbon composites are used. Depending on mission profile, spacecraft may also need to operate on the surface of another planetary body. The thermal control subsystem can be passive, dependent on the selection of materials with specific radiative properties. Active thermal control makes use of electrical heaters and certain
actuators such as louvers to control temperature ranges of equipments within specific ranges.
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 (for example
Terra (EOS AM-1) include a propulsion subsystem for altitude adjustments (called drag make-up maneuvers) and
inclination adjustment maneuvers. A propulsion system is also 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
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 (
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.[4]
Launch vehicle
The
launch vehicle propels 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.