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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.

Structural Subsystem

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.

Telecommunications

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.

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