Artificial Satellite

A

Power

A satellite provides its own power for the duration of its mission, which can extend to ten years or more. The most common source of power for Earth-orbiting satellites is a combination of solar cells (see Solar Energy) with a battery backup. Solar cells need to be large enough to provide the power that the satellite requires. For example, the solar array of the complex Hubble Space Telescope is about 290 sq m (about 3,120 sq ft) in area and generates about 5,500 watts of electricity, while the solar array of a smaller Global Positioning System satellite is about 4.6 sq m (about 50 sq ft) in area and generates about 700 watts of electricity. Solar cells are often mounted on winglike panels that unfold from the body of the satellite after it reaches its final orbit. Batteries provide power before the solar panels are deployed and when sunlight does not reach the solar panels.

B

Orientation

A satellite’s orientation is the direction each of its sides faces. The satellite keeps the solar panels pointed toward the Sun. In addition, the satellite’s antennas and sensors point toward Earth or toward the object the satellite is observing. For example, communications and weather satellites have antennas and cameras pointed earthward, while space telescopes are pointed toward the astronomical objects that scientists wish to study. Methods of maintaining orientation include small rocket engines, known as attitude thrusters; large spinning wheels that turn the satellite; and magnets that interact with Earth’s magnetic field to correctly orient the satellite. Attitude thrusters can make large changes to orientation quickly, but they are not the best solution when the stability of the turn is critical. Attitude thrusters also require fuel, so the lifetime of the satellite depends on a limited supply of fuel for the thrusters. A spinning wheel on a satellite acts as a gyroscope. The rotational motion of the wheel makes the satellite stay in one orientation, and changing the rotational motion will cause the satellite to turn. Spinning wheels and magnets are slower than thrusters but are excellent for attitude stability and require only electric power.

C

Heat Dissipation

As it orbits Earth, a satellite encounters intense heat and intense cold as it alternately faces or is hidden from the Sun. The electronic equipment on the satellite also creates heat that can cause damage. On Earth, convection, conduction, or radiation of heat can transfer heat (see Heat Transfer). With no air flowing over the satellite to transfer heat by convection and no body to which the satellite can conduct heat, the satellite must radiate heat to control temperature. Often satellites use radiators in the form of louvered panels, including panels that open and close to adjust the amount of radiating surface area. To prevent the direct rays of the Sun from causing hot spots, the satellite may spin or rotate to distribute the Sun’s heat more evenly.

D

Cosmic Radiation and Micrometeoroid Protection

Satellites have to endure the effects of radiation and of continuous, damaging micrometeoroid hits, especially during long-term missions. Earth’s atmosphere blocks most cosmic radiation from affecting microprocessors in computers on the ground. A satellite, however, needs shielding for its computers. Radiation from space also causes some materials to become brittle, so parts of satellites break more easily after long exposure to the electromagnetic radiation of space. Solar panels gradually produce less and less power because of damage from radiation effects and from the impact of micrometeoroids.

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V

Reentry and Satellite Disposal

Satellites reach the end of their useful lives when they reenter Earth’s atmosphere or their instruments fail. Many satellites eventually fall out of orbit and burn up as they reenter the atmosphere. Others continue to orbit as “space junk” long after their instruments have ceased working. Sometimes the onboard rockets are purposely fired to slow a satellite and cause it to reenter Earth’s atmosphere. This technique is usually limited to satellites with equipment packages intended for recovery. Such satellites have shields that enable them to withstand the intense heat of reentry.