Objects gravitationally bound to and orbiting about larger bodies.
Virtually all objects in space are satellites of one body or another. Satellites range in size from galaxies such as the Large and Small Magellanic clouds in orbit about the Milky Way to microscopic flakes of paint in low-Earth orbit that have eroded from artificial spacecraft. In practice, the word satellite is reserved for uncrewed spacecraft in Earth orbit. Crewed spacecraft are usually referred to individually by name, such as the International Space Station. Nonfunctional objects of artificial origin are regarded as orbital debris. Natural satellites of stars are more properly referred to as planets, while natural satellites of planets are more properly referred to as moons.
Satellites travel on elliptical trajectories called orbits, which are freely falling paths determined by the local gravitational field. Although satellites are indeed falling, they are also traveling sideways at extremely high speeds, on the order of 7 kilometers per second (5 miles per second) at 200 kilometers altitude (130 miles). The combination of free fall and high lateral velocity creates a closed trajectory that carries the satellite around Earth repeatedly.
The point on the orbit nearest to the earth is called the perigee; it is also the point at which the satellite has the greatest velocity. The point farthest away is called the apogee. That is also where the satellite velocity is least. If space were a perfect vacuum, satellites would orbit forever, but the atmosphere has no distinct end, and gradually fades away with altitude. Satellites orbiting at altitudes from 200 to 600 kilometers (130 to 400 miles) encounter enough residual atmosphere to create significant aerodynamic drag. Over the months, these low-Earth-orbit satellites lose energy and decrease in apogee until the apogee equals the perigee and the orbit is a circle. The satellites then drop closer to Earth on a spiral path, accelerating as they do so. Eventually, they enter regions where the atmosphere is too thick for them to continue in orbit. Aerodynamic drag becomes so strong that all of the satellite’s energy is converted into heat in a matter of minutes. The air around the satellite becomes hot enough to glow, and exposure to the heat burns up the satellite.
Satellites orbiting below 200 kilometers (130 miles) reenter Earth’s atmosphere in a matter of months. Those orbiting above 600 kilometers (400 miles) seldom reenter.
Satellites are classified according to user (commercial, military, or scientific) and according to mission (communications, remote sensing, or experimentation and measurement). Commercial satellites belong to private businesses. Military satellites support military operations. Scientific satellites perform experiments or make measurements in support of scientific research.
A satellite is only one part of a space mission’s architecture, an assembly which consists of the satellite, the launch system necessary to place it in orbit, the ground support system necessary to control the satellite and communicate with it, and a data analysis and information management system to exploit the data gathered by the satellite.
The satellite design process begins with the delineation of the satellite mission. A mission to photograph Earth from space, for example, might be expressed in terms of the goal that all areas of Earth between 45 degrees north latitude and 45 degrees south latitude be photographed with sufficient clarity that objects as small as 10 meters across can be imaged clearly. This requirement immediately eliminates all orbits of less than 45 degrees inclination and makes the orbital altitude of the satellite heavily dependent on camera quality: high-resolution cameras will be able to fulfill the requirement from greater altitudes than low-resolution cameras. In this way, the mission is expressed in the form of a set of requirements for orbital altitude, inclination, life span, launch date, and other needs which the satellite must fit.
A satellite is composed of the payload and the support bus. The payload consists of those components which perform the primary mission of the satellite. Component choice is driven by the best fit of available hardware to mission requirements. The components chosen will in turn determine payload parameters such as mass and volume, and payload demands such as power consumption, data storage and transmission, and attitude control.
The bus contains various systems to support the payload and provide electric power, thermal control, attitude control and propulsion, communications, and structural support. Bus components must be chosen that are capable of filling all of the payload demands as well as supporting the bus itself.
Total mass and volume are determined once payload and bus design are complete. Total mass and volume together with orbit requirements determine the choice of launch vehicle.
No satellite design process is complete without the development of ground sites. Ground sites monitor the status of the satellite and issue commands as necessary to maintain proper function or to correct anomalies in function. Ground sites receive data sent down by the satellite, process the data into a form intelligible to the user, and deliver it. Ground site personnel continually track the satellite, noting inevitable changes in orbit and issuing predictions for future passes within range of the ground site.
The power system provides the electric power needed to operate electrical and electronic components. Solar cells are usually the primary source of power, converting sunlight to electricity. What is not immediately required for satellite operations is stored in rechargeable batteries for later use. The power requirements of the payload and bus together determine the size of power system components. Solar cells must have enough area to collect all the power needed by the satellite plus more to provide a margin of safety. Because solar cells degrade over time in the harsh space environment, they must be built larger than initially required to guarantee that enough capability remains after years of degradation to continue operating the satellite. The number and size of batteries must be sufficient to meet the voltage and current demands of the payload and bus.
Power consumption must be carefully managed on board satellites. Consumption of electricity inevitably generates heat, which cannot easily escape in the vacuum of space and becomes a challenge for the thermal control system. Batteries build up internal pressure when charging and are in danger of bursting and destroying the satellite if overcharged. On the other hand, batteries that discharge too deeply are in danger of dying completely. Also, electronic components that lose power or receive too little voltage (an undervoltage condition) may cease operating or undergo an uncommanded reset when normal conditions return. Power system conditions such as voltage, current, and temperature are monitored at critical locations with the results transmitted to satellite operators on the ground.
The thermal control system maintains proper temperature throughout the satellite. It removes heat from components in danger of overheating from electric power consumption or exposure to the Sun, and provides heat to components in danger of freezing from exposure to the cold vacuum.
The attitude control system maintains the satellite in the proper orientation required for the satellite to fulfill its mission. Communications satellites must have antennas permanently pointed toward Earth’s surface, for example, while the Hubble Space Telescope must be constantly looking at the object being photographed.
The simplest type of attitude control system is none at all; the satellite is allowed to tumble uncontrollably. This requires the use of antennas that broadcast in all directions at once, so that communication with the ground is never interrupted. This also means that most of the broadcast power is wasted on transmissions into empty space and that only a small fraction of the power reaches the ground. This is acceptable only for the simplest types of low-Earth-orbit satellites.
Oblong satellites can be oriented so that the long axis points toward Earth and couples to tidal gravitational forces to provide gravity gradient stabilization. Once gravity gradient stabilization is achieved, the satellite will permanently present one face toward Earth, where cameras, remote sensing instruments, and communications antennas may be advantageously mounted. Gravity gradient stabilization is usually achieved by building a telescoping boom into the satellite structure, which deploys when the proper orientation is obtained. When extended, the end of the boom closest to Earth feels the strongest gravitational field and is continually pulled downward. That continuous downward pull keeps that end pointed toward Earth.
Active attitude control systems include momentum wheels and control moment gyroscopes. Momentum wheels are spun up in one direction so that the satellite will spin in the opposite direction in reaction. Three momentum wheels mounted in three perpendicular directions provide attitude control about any rotation axis. When the spin axis of a control moment gyroscope is altered, complicated reaction forces are created that may be used to rotate the spacecraft. Both of these systems have the virtue of reorienting the satellite without consuming propellant.
Active attitude control requires the satellite to have some knowledge of its orientation with respect to the outside world. The location of the Sun can be determined through the use of sensors that respond to visible light to indicate which side of the spacecraft is facing the Sun and which is in shade. Earth sensors respond to infrared radiation from the comparatively warm Earth. Star sensors look for the light from very bright stars. Stable platforms controlled by gyroscopes maintain a constant orientation regardless of the rotation of the spacecraft.
The communications system keeps the satellite in contact with the ground support system and moves data and commands to and from the satellite. The communications system includes transmitters and receivers, data encoders and decoders, data storage and retrieval elements (memory), and antennas. High-gain directional antennas carry the maximum amount of data with the minimum amount of power, but must be accurately pointed toward the reception site. This requires additional equipment to control the pointing of the antenna and maintain communications lock. The antenna may move itself, or the attitude control system may be tasked to reorient the entire satellite.
Orbital speeds of the order of 7 kilometers per second (5 miles per second) create significant shifts in the frequency of radio waves transmitted or received by satellites. Frequency goes up as the satellite approaches a ground site and falls as the satellite recedes, a phenomenon known as the Doppler shift. The ground site must continuously adjust frequency of both transmission and reception so that communication is continuous and no information is lost.
Most satellites are in range of a ground site for only ten minutes or less at a time and only during the infrequent occasions when their orbit takes them over the ground site location. Data collected at other times must be stored on board for relay to the ground during the next pass.
The structural system holds the parts of the satellite together and protects the components of the satellite from the high accelerations and intense vibrations experienced during launch. Structures range from simple frames to hold the components of the satellite in place to complicated mechanical systems folded and stowed during launch that must unfold and extend instruments upon deployment. The structure must not respond resonantly to vibrations generated by the launch vehicle or the satellite will shake itself to destruction. Special composite materials and honeycomb construction keep structural members lightweight without sacrificing strength.
The high costs of launch and the inability to make repairs on malfunctioning satellites demand high reliability and long operational lifetimes. Both are expensive and difficult to achieve. Altogether, these requirements force satellite designers and builders to make every attempt to make the satellite perfect the first time and every time. Components are extensively tested individually, and each system is tested and retested as new components are added. Complete systems are tested individually, and then tested and retested as they are linked into the final satellite assembly. Finally, the complete satellite is tested and retested under conditions simulating spaceflight as closely as possible.
The quest for perfection begins at the component level. Items for use in satellites must meet rigorous requirements. Materials cannot emit water vapor or volatile organic compounds in a vacuum. They must not chemically break down, degrade, or darken under exposure to ultraviolet light or atomic oxygen. Electronic parts and components must not be susceptible to ionizing radiation. Electrical systems must not be susceptible to the build-up and discharge of static electricity.
Complete satellite assemblies must survive a harsh launch environment. Launch vehicle accelerations can produce the equivalent of eight to ten times normal weight in the satellite. Rocket exhaust plumes generate strong vibrations and intense noise that can vibrate poorly constructed assemblies to destruction. Satellites therefore undergo vibration testing on massive shake tables that realistically simulate the launch vibration environment. After vibration testing, the satellite is placed in a vacuum chamber and run through heating and cooling cycles that mimic what the satellite will encounter in space.
All stages of satellite construction are extensively documented. Even after all this testing, satellites fail on orbit. Since a failed satellite cannot be retrieved for study, the only way to analyze what went wrong is to review the documentation and deduce the cause of the failure. A complete and thorough record of the design and construction process is essential.
The U.S. Space Command (USSPACECOM) catalogs and tracks every object in Earth orbit greater than 10 centimeters (4 inches) in length with ground-based radar and electro-optically enhanced telescopes. Continuous space surveillance allows U.S. Space Command to predict when and where a decaying space object will reenter Earth’s atmosphere in order to prevent an innocent satellite or inert piece of debris from triggering missile-attack warning sensors of the United States or other countries upon reentry. It also charts the present position and anticipated motion of space objects, detects new manmade objects in space, and determines their country of origin. An extremely important function of space surveillance is to inform the National Aeronautics and Space Administration (NASA) of the identity and path of objects that may endanger the space shuttle.
Space is becoming crowded. The Soviet Union launched Sputnik 1, the first artificial Earth satellite, in October, 1957. The United States launched its first satellite, Explorer 1, in January, 1958. Both have long since decayed and burned during reentry. The oldest satellite still in orbit is Vanguard 1, launched in March, 1958. As of June 6, 2001, U.S. Space Command reported 2,728 satellites in orbit, while 2,569 other satellites had undergone orbital decay and burned on reentry since 1957.
Satellites still in orbit degrade in the harsh space environment, shedding small particles of debris, such as paint flecks and pieces of thermal blanket. In extreme cases, old satellites are completely destroyed when aging batteries burst or leftover propellant spontaneously explodes. As of June 6, 2001, U.S. Space Command reported 6,150 pieces of debris in orbit that were 10 centimeters or greater in length. Satellites in low-Earth orbit run a significant risk of collision with a piece of orbiting debris. At collision velocities on the order of 10 kilometers per second (about 7 miles per second) even a tiny fleck of paint can do significant damage.
In an effort to slow the rate at which new debris is being created, satellite designers routinely include end-of-life planning in the satellite design process. At end-of-life, batteries are disconnected from solar panels to prevent destructive overcharging, and any unused pressurized liquids or gases are vented into the vacuum. The last few gallons (or pounds) of propellant are consumed in an orbital adjustment burn which either forces low-Earth-orbit satellites to reenter the atmosphere and burn up, or moves higher-altitude satellites to disposal orbits where they do not present a hazard to other spacecraft.
Satellites shine by reflected light and are visible to the naked eye for a short time just before sunrise and just after sunset. During these periods, the background sky is dark enough for dim objects to be seen by observers on the ground, but satellites passing overhead are still illuminated by the sun. There are so many satellites in orbit that every morning and evening, several pass over virtually every location on Earth. Satellites of the Iridium group of communications satellites have large, highly polished solar panels that can be extremely bright when the sun is reflected in them. Sightings of so-called Iridium flares are extremely common.
Heavens Above. (www.heavens-above.com) Provides easy-to-use information about satellite passes, both morning and evening, for almost every location on Earth. The user inputs either a place name or latitude and longitude information, and the Web page returns pass predictions for all visible satellites for the coming days. Star maps showing the start, stop, and path of the pass are also available. High-visibility objects, such as the International Space Station and Iridium flares, are specifically noted. Maral, Gerald, and Michel Bousquet. Satellite Communications Systems: Systems, Techniques, and Technology. 3d ed. New York: John Wiley & Sons, 1998. Offers a detailed analysis of satellite communication system construction and operation. Montenbruck, Oliver, and Eberhard Gill. Satellite Orbits: Models, Methods, Applications. New York: Springer Verlag, 2000. A textbook on orbital mechanics covering all aspects of satellite orbit prediction and determination. U.S. Space Command. (www.peterson.af.mil/usspace/index.htm) The U.S. Space Command Web site provides links to the current satellite box score and satellite space catalog.
National Aeronautics and Space Administration
The space shuttle deploys a satellite by means of a robotic arm.