Artificial Satellite - astronomy. I INTRODUCTION Artificial Satellite, any object purposely placed into orbit around Earth, other planets, or the Sun. Since the launching of the first artificial satellite in 1957, thousands of these "man-made moons" have been rocketed into Earth orbit. Today, artificial satellites play key roles in the communications industry, in military intelligence, and in the scientific study of both Earth and outer space. See also Space Exploration. II TYPES OF SATELLITES Engineers have developed many kinds of satellites, each designed to serve a specific purpose or mission. For instance the telecommunications and broadcasting industries use communications satellites to carry radio, television, and telephone signals over long distances without the need for cables or microwave relays. Navigational satellites pinpoint the location of objects on Earth, while weather satellites help meteorologists forecast the weather (see Meteorology). The United States government uses surveillance satellites to monitor military activities (see Remote Sensing). Scientific satellites serve as space-based platforms for observation of Earth, the other planets, the Sun, comets, and galaxies, and are useful in a wide variety of other applications. A Communications Satellites Almost all of the earliest satellites included some communications equipment. The National Aeronautics and Space Administration (NASA) launched the first telephone and television satellite, AT&T's Telstar 1, in 1962. The U.S. Department of Defense launched Syncom 3 in 1964. Syncom 3 was the first communication satellite to use a geostationary orbit--that is, an orbit that keeps the satellite over the same spot above Earth's equator. Over 300 communications satellites have been launched since 1957. Today satellites in geostationary orbit provide voice, data, and television communications, including the direct broadcast of television to homes around the world. B Navigation Satellites Navigation satellites can help locate the position of ships, aircraft, and even automobiles that are equipped with special radio receivers. A navigation satellite sends continuous radio signals to Earth. These signals contain data that a special radio receiver on Earth translates into information about the satellite's position. The receiver further analyzes the signal to find out how fast and in what direction the satellite is moving and how long the signal took to reach the receiver. From this data, the receiver can calculate its own location. Some navigation satellite systems use signals from several satellites at once to provide even more exact location information. The U.S. Navy launched the first navigation satellite, Transit 1B, in 1960. The United States ended its support of the Transit system in 1996. The U.S. Air Force operates a system, called the NAVSTAR Global Positioning System (GPS), that consists of 24 satellites. Depending on the type of receiver and the method used, GPS can provide position information with an accuracy from 100 m (about 300 ft) to less than 1 cm (less than about 0.4 in). The Global Orbiting Navigation Satellite System (GLONASS) of the Russian Federation consists of 24 satellites and provides accuracy similar to GPS. In December 2005 the European Union (EU) launched the first of 30 satellites that will make up a civilian satellite navigation system called Galileo. The system will have an accuracy of about 1 m (3.3 ft) and will become operational in 2009. Also known as the Global Navigation Satellite System (GNSS), the European system is designed to be compatible with those of the United States and Russia, allowing receivers around the world to communicate with satellites in any of the three systems. The United States also negotiated an agreement with the EU that will allow it to scramble signals over a battlefield or military target without shutting down the entire system. The two systems will compete economically, however, for commercial applications. C Weather Satellites Weather satellites carry cameras and other instruments pointed toward Earth's atmosphere. They can provide advance warning of severe weather and are a great aid to weather forecasting. NASA launched the first weather satellite, Television Infrared Observation Satellite (TIROS) 1, in 1960. TIROS 1 transmitted almost 23,000 photographs of Earth and its atmosphere. NASA operates the Geostationary Operational Environmental Satellite (GOES) series, which are in geostationary orbit. GOES provides information for weather forecasting, including the tracking of storms. GOES is augmented by Meteosat 3, a European weather satellite also in geostationary orbit. The National Oceanic and Atmospheric Administration (NOAA) operates three satellites that collect data for long-term weather forecasting. These three satellites are not in geostationary orbit; rather, their orbits carry them across the poles at a relatively low altitude. D Military Satellites Many military satellites are similar to commercial ones, but they send encrypted data that only a special receiver can decipher. Military surveillance satellites take pictures just as other earth-imaging satellites do, but cameras on military satellites usually have a higher resolution. The U.S. military operates a variety of satellite systems. The Defense Satellite Communications System (DSCS) consists of five spacecraft in geostationary orbit that transmit voice, data, and television signals between military sites. The Defense Support Program (DSP) uses satellites that are intended to give early warning of missile launches. DSP was used during the Persian Gulf War (1991) to warn of Iraqi Scud missile launches. Some military satellites provide data that is available to the public. For instance, the satellites of the Defense Meteorological Satellite Program (DMSP) collect and disseminate global weather information. The military also maintains the Global Positioning System (GPS), described earlier, which provides navigation information that anyone with a GPS receiver can use. E Scientific Satellites Earth-orbiting satellites can provide data to map Earth, determine the size and shape of Earth, and study the dynamics of the oceans and the atmosphere. Scientists also use satellites to observe the Sun, the Moon, other planets and their moons, comets, stars, and galaxies. The Hubble Space Telescope is a general-purpose observatory launched in 1990. Some scientific satellites orbit bodies other than Earth. The Mars Global Surveyor, for example, orbits the planet Mars. III SATELLITE LAUNCHES Placing a satellite into orbit requires a tremendous amount of energy, which must come from the launch vehicle, or device that launches the satellite. The satellite needs to reach an altitude of at least 200 km (120 mi) and a speed of over 29,000 km/h (18,000 mph) to lift into orbit successfully. Satellites receive this combination of potential energy (altitude) and kinetic energy (speed) from multistage rockets burning chemical fuels. The first stage of a multistage rocket consists of rocket engines that provide a huge amount of force, or thrust. The first stage lifts the entire launch vehicle--with its load of fuel, the rocket body, and the satellite--off the launch pad and into the first part of the flight. After its engines use all their fuel, the first stage portion of the rocket separates from the rest of the launch vehicle and falls to Earth. The second stage then ignites, providing the energy necessary to lift the satellite into orbit. It, too, then separates from the satellite and any remaining rocket stages. The rest of the launch depends on the satellite's mission. For example, if the mission requires a geostationary orbit, which can be achieved only at a distance of about 35,000 km (22,000 mi) above Earth, a third rocket stage provides the thrust to lift the satellite to its final orbital altitude. After the satellite has reached the final altitude, another rocket engine fires and gives the satellite a circular orbit. All rocket-engine burns occur at a precise moment and last for a precise amount of time so that the satellite achieves its proper position in space. In 1990 the United States began launching some satellites from aircraft flying at high altitudes. This method still requires a rocket-powered launch vehicle, but because the vehicle does not have to overcome friction with the thick atmosphere found at low altitudes, much less fuel is needed. However, the size of the rocket is limited by the size and strength of the aircraft, so only smaller satellites can be launched this way. Another method of launching satellites is to have astronauts launch them from the U.S. space shuttle. The space shuttle can carry large satellites and, because the shuttle is already in orbit when the satellite is launched, the astronauts can verify that the satellite has survived the rigors of launch. The space shuttle can also bring satellites back to Earth for repair, or astronauts can repair satellites in space. The Single Stage to Orbit (SSTO) is a launch vehicle that may lower the cost of launching satellites by decreasing the number of launch stages needed and increasing the reusability of launch vehicles. The SSTO would be a piloted vehicle like the space shuttle, but it would be designed to launch satellites more inexpensively and efficiently than the space shuttle can. IV OPERATIONS IN SPACE Because satellites must survive the launch and must operate in the harsh environment of space, they require unique and durable technologies. Satellites have to carry their own power source because they cannot receive power from Earth. They must remain pointed in a specific direction, or orientation, to accomplish their mission. Satellites need to maintain proper temperature in the face of direct rays from the Sun and in the cold blackness of space. They must also survive high levels of radiation and collisions with micrometeoroids (see Meteoroid). Most satellites have onboard computers that help with satellite operations and with the satellite's mission. 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. 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. A Orbit Decay and Reentry A satellite that orbits within a few hundred miles of Earth's surface experiences friction from the thin atmosphere that exists at those altitudes. Eventually the satellite's altitude will decrease until atmospheric friction causes the satellite to plunge earthward out of orbit. The lifetime of a satellite depends on its orbit, the satellite's orientation in its orbit, and the size, shape, and weight of the satellite. A large, light satellite will probably reenter Earth's atmosphere sooner than a small, heavy satellite that orbited at the same altitude, because the large satellite has more surface area and experiences more atmospheric friction. At an orbital altitude of 200 km (120 mi), a satellite will likely last from a week to three months. At 300 km (190 mi), a satellite may stay in orbit for two years or more. Satellites that orbit above 1,000 km (620 mi) will stay aloft for thousands of years. B Disposal of Satellites The space around Earth seems boundless, but space operations tend to take place in a limited number of preferred types of orbits. The U.S. Air Force tracks satellites and other objects within these orbits so that other satellites and piloted vehicles can avoid collisions with the objects. Radio interference between satellites can also present spacing problems. Many satellites share a limited area, called the geostationary corridor, where a satellite's orbit takes it around Earth at the same rate that Earth rotates. Satellites in this area have to maintain certain separation distances, so that the radio signals sent to one satellite do not interfere with the signals sent to nearby satellites. A final rocket thrust is sometimes used to put old satellites into less-desirable orbits to make room for newer satellites. VI SATELLITE ORBITS The defining characteristics of an orbit are its shape, its altitude, and the angle it makes with Earth's equator. A satellite's controllers choose an orbit with a particular combination of shape, altitude, and angle that will best serve the satellite's mission. Most orbits are circular, but some satellites use elliptical orbits--that is, orbits in which the satellite's distance from Earth is not constant. The altitude of an orbit determines how long the satellite takes to circle Earth and how much of the planet is visible to the satellite at one time. Satellites pass over different ranges of Earth's latitude depending on the angle of their orbits with respect to the equator. Some satellites orbit along the equator. Satellites that pass over high northern and southern latitudes have orbits that form a large angle to the equator. Some satellites move clockwise around Earth as seen from the North Pole, but most satellites move counterclockwise around Earth. A Geostationary Equatorial Orbit Satellites in geostationary equatorial orbit (GEO) orbit Earth around the equator at a very specific altitude that allows them to complete one orbit in the same amount of time that it takes Earth to rotate once. As a result, these satellites stay above one point on Earth's equator at all times. The altitude of GEO is about 5.6 times the radius of Earth, or about 35,800 km (about 22,200 mi). Direct-broadcast television satellites are in GEO. A few satellites in GEO can provide coverage for the entire Earth, and antennas do not need to track the satellite to receive a signal. Earth-surveillance missions, including military surveillance and weather tracking missions, also use GEO. B Low Earth Orbit A satellite in low Earth orbit (LEO) orbits at an altitude of 2,000 km (1,200 mi) or less. Almost every satellite enters a LEO after it is launched. If a satellite's mission requires an orbit other than LEO, it uses rockets to move into its final orbit. A low Earth orbit minimizes the amount of fuel needed. In addition, a satellite in LEO can obtain clearer surveillance images and can avoid the Van Allen radiation belts, which contain harmful high-energy particles. It needs less powerful signals to communicate with Earth than satellites with higher orbits. A signal to or from a low Earth orbit also reaches its destination more quickly, making LEO satellites especially good for transmitting data. C Medium Earth Orbit Medium Earth orbit (MEO) satellites orbit at an altitude about 10,000 km (about 6,000 mi) and balance the benefits and problems between LEO and GEO. The most common uses of MEO are by navigation and communication satellites. The U.S. navigation system NAVSTAR Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), and Odyssey, a private U.S. communications satellite program, all use MEO. D Polar Orbits Satellites in polar orbits orbit around Earth at right angles to the equator over both the North and South poles. Polar orbits can occur at any altitude, but most satellites in polar orbits use LEOs. Two polar satellites belonging to the U.S. National Oceanic and Atmospheric Administration provide weather information for all areas of the world every six hours. The satellites also map ozone levels (see Ozone Layer) in the atmosphere, including the level over the poles. Landsat is a U.S. government remote-sensing satellite system that operates in polar orbit. Scientists often use Landsat to view agricultural phenomena such as deforestation and crop blight. Transit, the first satellite-based navigation system, used polar orbits in order to support navigation around the world, especially for submarines in the polar regions. E Sun-Synchronous Orbits A satellite in a Sun-synchronous orbit always passes over a certain point of Earth when the Sun is at the same position in Earth's sky. A Sun-synchronous satellite has a retrograde orbit (it moves clockwise around Earth), orbits in a low Earth orbit, and orbits at a specific angle with respect to Earth's equator (about 98°). The satellite crosses each latitude about 1° east of where it crossed the latitude the previous day. Thus, the satellite stays synchronized with the location of the Sun relative to Earth. Sun-synchronous orbits are useful for satellites photographing Earth, because the Sun will be at the same angle each time the satellite passes over a point on Earth. VII THE FIRST SATELLITES The first artificial satellite to orbit Earth was Sputnik 1. Built by the Soviet Union and launched on October 4, 1957, Sputnik had an elliptical orbit, ranging in altitude from 225 to 950 km (140 to 590 mi). Sputnik broadcast a steady signal of beeps for 21 days and burned up in Earth's atmosphere upon reentry on January 4, 1958. The Soviet Union also launched the first living creature, a dog named Laika, into space on November 3, 1957. Laika flew inside a pressurized chamber aboard the satellite Sputnik 2. She died from overheating and panic after a few hours in orbit. Sputnik 2 reentered Earth's atmosphere and burned up on April 14, 1958. The United States launched its first satellite, Explorer 1, on January 31, 1958. Explorer 1 had a highly elliptical orbit, ranging in altitude from 360 to 2,500 km (220 to 1,600 mi). Scientists discovered the Van Allen radiation belts using data transmitted back to Earth from Explorer 1. On August 10, 1960, the United States launched a surveillance satellite, Discoverer 13, that carried the first artificial object ever retrieved from space. While Discoverer 13 remained in orbit it ejected a capsule earthward, which was then recovered by a team from the U.S. Navy. Later satellites carried cameras that photographed parts of Earth and then ejected recoverable containers of the exposed film toward Earth. Contributed By: Leonard R. Kruczynski Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
