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space technology[′spās tek‚näl·ə·jē]
The systematic application of engineering and scientific disciplines to the exploration and utilization of outer space. Space technology developed so that spacecraft and humans could function in this environment that is so different from the Earth's surface. Conditions that humans take for granted do not exist in outer space. Objects do not fall. There is no atmosphere to breathe, to keep people warm in the shade, to transport heat by convection, or to enable the burning of fuels. Stars do not twinkle. Liquids evaporate very quickly and are deposited on nearby surfaces. The solar wind sends electrons to charge the spacecraft, with lightninglike discharges that may damage the craft. Cosmic rays and solar protons damage electronic circuits and human flesh. The vast distances require reliable structures, electronics, mechanisms, and software to enable the craft to perform when it gets to its goal—and all of this with the design requirement that the spacecraft be the smallest and lightest it can be while still operating as reliably as possible.
All spacecraft designs have some common features: structure and materials, electrical power and storage, tracking and guidance, thermal control, and propulsion. The spacecraft structure is designed to survive the forces of launching and ground handling. The structure is made of metals (aluminum, beryllium, magnesium, titanium) or a composite (boron/epoxy, graphite/epoxy). It must also fit the envelope of the launcher.
To maintain temperatures at acceptable limits, various active and passive devices are used: coatings or surfaces with special absorptivities and emissivities, numerous types of thermal insulation, such as multilayer insulation and aerogel, mechanical louvers to vary the heat radiated to space, heat pipes, electrical resistive heaters, or radioisotope heating units.
The location of a spacecraft can be measured by determining its distance from the transit time of radio signals or by measuring the direction of received radio signals, or by both. The direction of a spacecraft can be determined by turning the Earth station antenna to obtain the maximum signal, or by other equivalent and more accurate methods.
The velocity of a spacecraft is changed by firing thrusters. Solid propellant thrusters are rarely used. Liquid propellant thrusters are either monopropellant or bipropellant. Electric thrusters, such as mercury or cesium ion thrusters, have also been used. Electric thrusters have the highest efficiency (specific impulse) but the lowest thrust. See Ion propulsion
Most spacecraft are spin-stabilized or are three-axis body-stabilized. The former uses the principles of a gyroscope; the latter uses sensors and thrusters to maintain orientation. Some body-stabilized spacecraft (such as astronomical observatories) are fixed in inertial space, while others (such as Earth observatories) have an axis pointed at the Earth and rotate once per orbit. A body-stabilized spacecraft is simpler than a spinner but requires more hardware. The orientation of a spacecraft is measured with Sun sensors (the simplest method), star trackers (the most accurate), and horizon (Earth or other body) or radio-frequency (rf) sensors (usually to determine the direction toward the Earth). Attitude corrections are made by small thrusters or by reaction or momentum wheels; as the motor applies a torque to accelerate or decelerate the rotation, an equal and opposite torque is imparted to the spacecraft.
Primary electrical power is most often provided by solar cells made from a thin section of crystalline silicon protected by a thin glass cover. Excess power from the solar cells is stored in rechargeable batteries so that when power is interrupted during an eclipse, it can be drawn from the batteries. Other sources of power generation include fuel cells, radio isotope thermoelectric generators (RTGs), tethers, and solar dynamic power. Fuel cells have been used on the Apollo and space shuttle programs and produce a considerable amount of power, with drinkable water as a by-product. See Solar cell
The status and condition of a spacecraft are determined by telemetry. Temperatures, voltages, switch status, pressures, sensor data, and many other measurements are transformed into voltages, encoded into pulses, and transmitted to Earth. This information is received and decoded at the spacecraft control center. Desired commands are encoded and transmitted from the control center, received by the satellite, and distributed to the appropriate subsystem. Commands are often used to turn equipment on or off, switch to redundant equipment, make necessary adjustments, and fire thrusters and pyrotechnic devices. See Space communications, Telemetering
Many spacecraft missions have special requirements and hence necessitate special equipment. Satellites that leave the Earth's gravitational field to travel around the Sun and visit other planets have special requirements due to the greater distances, longer mission times, and variable solar radiation involved.
Spacecraft that return to Earth require special protection for reentry into Earth's atmosphere. In some missions one spacecraft must find, approach, and make contact with another spacecraft.
Space is distant not only in kilometers but also in difficulty of approach. Large velocity changes are needed to place objects in space, which are then difficult to repair and expensive to replace. Therefore spacecraft must function when they are launched, and continue to function for days, months, or years. The task is similar to that of building a car that will go 125,000 mi (200,000 km) without requiring mechanical repair or refueling. Not only must space technology build a variety of parts for many missions, but it must achieve a reliability far greater than the average. This is accomplished by building inherent reliability into components and adding redundant subsystems, supported by a rigorous test schedule before launch. Efforts are made to reduce the number of single points of failure, that is, components that are essential to mission success and cannot be bypassed or made redundant.