A spacecraft is any piloted or unpiloted vehicle designed for travel in space. The systems and instruments a spacecraft must carry depends upon the data it will gather and the functions it will carry out. Their complexity varies greatly but all must endure the hostile environment of space. Spacecraft may be broadly categorized according to the missions they are intended to fly.
Manned spacecraft: A piloted spacecraft designed to carry astronauts into space. Unlike an unmanned probe, it requires a crew compartment and life support systems. Manned spacecraft are either reusable, such as the Space Shuttle, or designed for one time use, such as Soyuz. The latter type is generally modular, such as consisting of a reentry module which houses the crew and a service module which contains propulsion, power supply and life support. Only the reentry module returns to Earth.
Flyby spacecraft: A spacecraft which follows a continuous trajectory past a target object, never to be captured into an orbit. It must carry instruments that are capable of observing passing targets by compensating for the target's apparent motion. The spacecraft must be able to survive long periods of interplanetary cruise and must be able to downlink data at high rates. Examples of flyby spacecraft include Pioneers 10 and 11, and Voyagers 1 and 2.
Orbiter spacecraft: A spacecraft designed to travel to a distant planet or moon and enter orbit. It must carry a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion. An orbiter spacecraft must endure periods during which it is shaded from sunlight, thus it must be resistant to extreme thermal variation and will require power storage capacity if equipped with solar panels. Examples of orbiter spacecraft include Magellan, Galileo and Mars Global Surveyor.
Atmospheric probe: A small instrumented craft which separates from the main spacecraft prior to closest approach to a planet to study the gaseous atmosphere of the body as it drops through it. It is equipped with an aeroshell to protect it during atmospheric entry and a parachute to slow its decent. Data is typically telemetered to the mother craft where it is recorded for later transmission to Earth.
Atmospheric balloon package: An instrumented package suspended from a buoyant gas bag, deployed in a planet's atmosphere to study wind circulation patterns. It has a limited complement of spacecraft systems aboard. The minimum requirements include a power supply and telecommunications equipment to permit tracking.
Lander spacecraft: A spacecraft designed to reach the surface of a planet or moon and survive long enough to telemeter data back to Earth. It may perform a powered descent and landing or may descend to the surface suspended from a parachute. Examples of lander spacecraft include Surveyor, Viking, Mars Pathfinder and the Soviet Venera.
Surface penetrator: A probe designed to penetrate the surface of a body, surviving an impact of hundreds of g's, measuring and telemetering the properties of the penetrated surface. Data is typically telemetered to the mother craft for re-transmission to Earth. The Mars Polar Lander will deploy two penetrators upon its arrival at Mars.
Surface rover: A semi-autonomous roving vehicle, steerable from Earth, deployed on the surface of a planet or other body, taking images and soil analyses for telemetering back to Earth. The Mars Pathfinder mission included a solar-powered, wheeled microrover named Sojourner.
The spacecraft bus is a major part of the structural subsystem of a spacecraft which provides a place to attach components internally and externally, and to house delicate modules requiring a measure of thermal and mechanical stability. It is an integral card chassis for supporting the circuit boards of radio equipment, data recorders, computers, gyroscopes and other components. The bus also establishes the basic geometry of the spacecraft, and it provides the attachment points for appendages such as booms, antennas and scan platforms.
Power Supply and Distribution
Power supply systems are those that produce electricity for use by other onboard systems and instruments. The type of power system used on a spacecraft depends on factors such as the duration of the mission and location at where the spacecraft must operate.
A spacecraft's electrical components can be switched on or off via command by using relays that connect or disconnect the component from the common distribution circuit, called a main bus. On some spacecraft it is necessary to power off some components before switching others on to keep the electrical load within the limits of the supply. Typically a shunt-type regulator maintains a constant voltage from the power source. The voltage applied as input to the regulator is generally variable but higher than the spacecraft's required constant bus voltage. The regulator converts excess electrical energy into heat which is radiated away into space.
Batteries: Devices with two or more connected cells that produce a direct current by converting chemical energy into electrical energy. Due to their short lifetime, batteries are only used when a very short operating life is required. They are often used in combination with solar panels to store power for use during times when the panels are shaded from sunlight.
Photovoltaic cells: Crystalline wafers, called solar cells, which convert sunlight directly into electricity without moving parts. These cells are grouped into an array and cemented onto a substrate. The resulting assemblies are called solar panels or solar arrays. Solar power is practical for spacecraft operating no farther from the sun than about the orbit of Mars.
Radioisotope thermoelectric generator: A device that converts the heat produced by the radioactive decay of plutonium-238 into electricity by an array of thermocouples made of silicon-germanium junctions. They are used when a spacecraft must operate at significant distances from to sun (usually beyond the orbit of Mars). The Pu-238 is contained within a crash resistant housing.
Fuel cells: Cells in which chemical reaction is used directly to produce electricity. The reactants are typically hydrogen and oxygen which results in water as a by-product. The water can then be used for cooling and human consumption. Fuel cells are generally used on manned spacecraft.
Telecommunication components for a particular spacecraft are chosen in response to the requirements of the mission it will perform. Anticipated distances, planned frequency bands, data rates and available on-board transmitter power are all taken into account.
High-gain antenna: A dish-shaped spacecraft antenna principally used for high rate communication with Earth. This type of antenna is highly directionally and must be pointed to within a fraction of a degree of Earth. It may be either steerable or fixed to the spacecraft bus. High-gain antennas are typically used on interplanetary spacecraft.
Low-gain antenna: An omnidirectional spacecraft antenna that provides relatively low data rates at close range, several astronomical units for example. Many spacecraft include both a low-gain antenna and a high-gain antenna.
Medium-gain antenna: A spacecraft antenna that provides greater data rates than a low-gain antenna, with wider angles of coverage than a high gain antenna, about 20-30 degrees.
Transmitter: An electronic device that generates and amplifies a tone at a single designated radio frequency, called a carrier wave. The carrier wave can be sent from the spacecraft to Earth as it is, or it can be modulated with a data-carrying subcarrier. The transmitter radiates the resulting signal, called downlink, from an antenna.
Receiver: An electronic device that receives incoming radio signals, called uplink, and converts them to perceptible forms. The received uplink is stripped of its command-data-carrying subcarrier which is converted into binary code, which is then typically passed to the spacecraft's command and data subsystem.
Frequently, transmitters and receivers are combined into one electronic device which is called a transponder.
The onboard computer responsible for overall management of a spacecraft's activity is generally the same one which maintains timing, interprets commands from Earth, collects, processes and formats the telemetry data which is to be returned to Earth, and manages high-level fault protection and safing routines. This computer is sometimes referred to as the command and data subsystem. For convenience that term will be used here, recognizing that other names may apply to similar subsystems or sets of subsystems which accomplish some or all of the same tasks.
Spacecraft clock: A counter maintained by the command and data subsystem. It meters the passing of time during the life of the spacecraft, and regulates nearly all activity within the spacecraft systems. Many types of commands uplinked to the spacecraft are set to begin execution at specific spacecraft clock counts.
Telemetry: The system for radioing information from a spacecraft to the ground. Telemetry is typically a mixture of science data from the experiments and spacecraft engineering or health data. Engineering or health data is composed of a wide range of measurements, from switch positions and subsystem states to voltages, temperatures and pressures. Telemetry may be transmitted in real time, or it may be written to a data storage device until transmission is feasible.
Tape recorder: A mechanical device for recording digital information on magnetic tape and for playing back the recorded material. The stored data can be played back for downlink when receiving station resources are available.
RAM: Random access memory, the solid-state equivalent of a tape recorder. Banks of RAM can store large quantities of digital information without any moving parts.
Fault protection: Algorithms, which normally reside in more than one of a spacecraft's subsystems, that insure the ability of the spacecraft to both prevent a mishap and to reestablish contact with Earth if a mishap occurs and contact is interrupted.
Attitude and Articulation Control
A spacecraft's attitude, its orientation in space, must be stabilized and controlled so that its high-gain antenna may be accurately pointed to Earth, so that onboard experiments may accomplish precise pointing for accurate collection and subsequent interpretation of data, so that the heating and cooling effects of sunlight and shadow may be used intelligently for thermal control, and so that propulsive maneuvers may be executed in the right direction.
Spin stabilization: Stabilization accomplished by rotating the spacecraft mass, thus using gyroscopic action as the stabilizing mechanism. Thrusters are fired to make desired changes in the spin-stabilized attitude. Spin-stabilized spacecraft provide a continuous sweeping desirable for fields and particle instruments, but they may require complicated systems to de-spin antennas or optical instruments which must be pointed at targets.
Three-axis stabilization: Stabilization accomplished by nudging a spacecraft back and forth within a deadband of allowed attitude error, using small thrusters or reaction wheels. Three-axis controlled spacecraft can point optical instruments and antennas without having to de-spin them, but they may have to carry out rotation maneuvers to best utilized their fields and particle instruments.
Reaction wheels: Electrically-powered wheels mounted in three orthogonal axes aboard a spacecraft. To rotate the vehicle in one direction, you spin up the proper wheel in the opposite direction. To rotate the vehicle back, you slow down the wheel. Excess momentum that builds up in the system due to external torques must be occasionally removed from the system via propulsive maneuvers.
Attitude and articulation control subsystem: The onboard computer that manages the tasks involved in spacecraft stabilization via its interface equipment. For attitude reference, star trackers, star scanners, solar trackers, sun sensors, and planetary limb trackers are used. Gyroscopes are carried for attitude reference for those periods when celestial references are not being used. The AACS also controls the articulation of a spacecraft's moveable appendages such as solar panels, high-gain antennas, and optical instrument scan platforms.
In order to maintain or restore three-axis stability, to control spin, to execute maneuvers and make minor adjustments in trajectory, spacecraft are provided with sets of propulsive devices. These devices are typically of either the hypergolic bi-propellant type or the mono-propellant type. Many if the activities of the propulsion subsystems are routinely initiated by the attitude and articulation control subsystem. Some or all may be directly controlled by or through the command and data subsystem.
Engines: The larger of a spacecraft's propulsive devices, perhaps producing a force of several hundred Newtons, used to provide the large torques necessary to maintain stability during a solid rocket motor burn, or they may be the rockets used for orbit insertion.
Thrusters: A set of small propulsive devices, typically generating between less than 1 N and 10 N, used to provide the delta-V required for interplanetary trajectory correction maneuvers, orbit trim maneuvers, reaction wheel desaturation maneuvers, or routine three-axis stabilization or spin control.
Propellant: The fuel and oxidizer burned to produce thrust in a rocket engine. Propellant subsystems include propellant tanks, plumbing, valves, and helium tanks to supply pressurization for the propellant tanks.
Pyrotechnics are electrically initiated explosive devices used to operate valves, ignite solid rocket motors, and explode bolts to separate from or jettison hardware, or to deploy appendages. They obtain their electrical power from a bank of capacitors which are charged from the main bus several minutes prior to the planned detonation of a device.
A spacecraft must operate and survive in the hostile environment of space and, in most cases, it must have a usable life of many years. Delicate Spacecraft components must be protected from this harsh environment. Environmental subsystems are those designed to protect spacecraft components from extreme thermal variations, micrometeoroid bombardment, and other space hazards.
Passive cooling: The use of painting, shading, reflectors and other techniques to cool a spacecraft. Internal components are typically painted black to radiate heat more efficiently. White thermal blankets are used to reflect infrared radiation, helping to protect a spacecraft from excess solar heating. Critical components are generally shaded using gold or optical solar reflectors. Mechanical louvers are frequently used to control thermal radiation from within parts of a spacecraft.
Active heating: The use of resistive electric heaters or radioisotope heaters to keep spacecraft components above their minimum allowable temperatures. Electric heaters can be controlled either autonomously or via command, while radioisotope heaters are used where it is necessary to provide components with a permanent supply of heat.
Micrometeoroid protection: Shielding used to protect spacecraft components from micrometeoroid impacts. Interplanetary spacecraft typically use tough blankets of Kevlar or other strong fabrics to absorb the energy from high-velocity particles.
Life support: The subsystems aboard a manned spacecraft or space station responsible for maintaining a livable environment within a pressurized crew compartment. Included are subsystems for providing oxygen, drinking water, waste processing, temperature control, ventilation and CO2 removal.
A spacecraft designed to land on a planet or moon must be provided with subsystems to slow its descent and guide it to a soft landing. The craft may be equipped with propulsion for making a powered descent, or a parachute to retard free-fall. A piloted spacecraft, such as the Space Shuttle, may be equipped with wings for a glided landing. If the craft is to pass through an atmosphere it must be equipped with shielding to protect it from the high temperatures generated during atmospheric entry.
Heat shield: A device that protects crew and equipment from heat, such as an ablative shield in front of a reentry capsule or atmospheric probe.
Parachute: An apparatus used to retard free fall, consisting of a light, usually hemispherical canopy attached by cords and stored folded until deployed in descent.
Drogue: A small parachute used to slow and stabilize a spacecraft, usually preceding deployment of a main landing parachute.
Descent engine: The rocket used to power a spacecraft as it makes a controlled landing on the surface of a planet or moon.
Spacecraft designed to carry out scientific missions are typically equipped with a large complement of science instruments, covering large portions of the electromagnetic spectrum, and including both remote and direct-sensing components.
Direct-sensing instruments interact with phenomena in their immediate vicinity, and register characteristics of them. Remote-sensing instruments record characteristics of objects at a distance, sometimes forming an image by gathering, focusing, and recording reflected light from the Sun, or reflected radio waves emitted by the spacecraft. When an instrument provides the illumination, as does radar, it is referred to as an active remote-sensing instrument.
High-energy particle detector: A device for measuring the energy spectra of trapped energetic electrons, and the energy and composition of atomic nuclei.
Low-energy charged particle detector: A device designed to characterize the composition, energies, and angular distributions of charged particles in interplanetary space and within planetary systems.
Plasma detector: A device for measuring the density, composition, temperature, velocity and three-dimensional distribution of plasmas that exist in interplanetary regions and within planetary magnetospheres.
Dust detector: A device for measuring the velocity, mass, charge, flight direction and number of dust particles striking the instrument.
Magnetometer: A device for measuring the strength and direction of the interplanetary and solar magnetic fields. They typically detect the strength of magnetic fields in three planes.
Plasma wave detector: A device for measuring the electrostatic and electromagnetic components of local plasma waves in three dimensions.
Imaging instruments: Optical imaging is performed by two families of detectors: vidicons and the newer charged coupled devices. Although the detector technology differs, in each case an image in focused by a telescope onto the detector where it is converted to digital data.
Vidicon: An imaging device consisting of a vacuum tube, which produces a video signal based on the varying electrical potential measured as an electron beam is swept across a phosphor coating on the glass where the image is focused.
Charged coupled device: An imaging device consisting of a large-scale integrated circuit which has a two-dimensional array of hundreds of thousands of charge-isolated wells, each representing a pixel.
Infrared radiometer: An optical instrument that measures the intensity of infrared energy radiated by its targets.
Polarimeter: An optical instrument that measures the direction and extent of the polarization of light reflected from its targets.
Photometer: An optical instrument that measures the intensity of light from a source.
Spectrometer: An optical instrument that splits the light received from an object into its component wavelengths by means of a diffraction grating; then measuring the amplitudes of the individual wavelengths.
Scan platform: An articulated, powered appendage to the spacecraft bus which points in commanded directions, allowing optical observations to be taken independently of the spacecraft's attitude.
Active Sensing Instruments
Synthetic aperture radar: A radar imaging instrument which provides a penetrating illumination of radio waves, and is capable of imaging surfaces covered by clouds and haze. SAR images are constructed of a matrix where lines of constant distance or range intersect with lines of constant Doppler shift.
Altimeter: A device that measures altitude above the surface of a planet or moon. Spacecraft altimeters work by timing the round trip of radio signals bounced off the surface.
Compiled and edited by Robert A. Braeunig, 1999.