A satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth’s original, natural satellite, and there are many manmade (artificial) satellites, usually closer to Earth.
The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee.
Artificial satellites generally are not mass-produced. Most satellites are custom built to perform their intended functions. Exceptions include the GPS satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).
Approximately 23,000 items of space junk — objects large enough to track with radar that were inadvertently placed in orbit or have outlived their usefulness — are floating above Earth. The actual number varies depending on which agency is counting. Payloads that go into the wrong orbit, satellites with run-down batteries and leftover rocket boosters all contribute to the count. This online catalog of satellites has almost 26,000 entries!
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Although anything that is in orbit around Earth is technically a satellite, the term “satellite” is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. In other words, satellite also refers to an ‘artificial satellite’ also which is a man-made object that orbits the Earth or another body. Scientists may also use the term to refer to ‘natural satellite’.
Natural Satellite
Moon, the common noun, is used to mean any natural satellite. There are at least 140 moons within the solar system and in fact many others orbiting the planets of other stars. There is a standard model of moon formation from the same collapsing region of protoplanetary disk. This give rise to primary.
There are also exceptions or variations in this regard. Several moons are thought to be captured asteroids; others may be fragments of larger moons collapsed by impacts, a portion of the planet itself blasted into orbit by a large impact. As most moons are known only through a few observations via investigations or telescopes, most theories about their origins are still uncertain.
Artificial Satellites
An artificial satellite is a manufactured object that continuously orbits Earth or some other body in space. Most artificial satellites orbit Earth. People use them to study the universe, help forecast the weather, transfer telephone calls over the oceans, assist in the navigation of ships and aircraft, monitor crops and other resources, and support military activities.
Artificial satellites also have orbited the moon, the sun, asteroids, and the planets Venus, Mars, and Jupiter. Such satellites mainly gather information about the bodies they orbit.
Piloted spacecraft in orbit, such as space capsules, space shuttle orbiters, and space stations, are also considered artificial satellites. So, too, are orbiting pieces of “space junk,” such as burned-out rocket boosters and empty fuel tanks that have not fallen to Earth. Artificial satellites differ from natural satellites, natural objects that orbit a planet. Earth’s moon is a natural satellite.
The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Since then, the United States and about 40 other countries have developed, launched, and operated satellites. Today, about 3,000 useful satellites and 6,000 pieces of space junk are orbiting Earth.
Satellite orbits
Satellite orbits have a variety of shapes. Some are circular, while others are highly elliptical (egg-shaped). Orbits also vary in altitude. Some circular orbits, for example, are just above the atmosphere at an altitude of about 155 miles (250 kilometers), while others are more than 20,000 miles (32,200 kilometers) above Earth. The greater the altitude, the longer the orbital period — the time it takes a satellite to complete one orbit.
A satellite remains in orbit because of a balance between the satellite’s velocity (speed at which it would travel in a straight line) and the gravitational force between the satellite and Earth. Were it not for the pull of gravity, a satellite’s velocity would send it flying away from Earth in a straight line. But were it not for velocity, gravity would pull a satellite back to Earth.
To help understand the balance between gravity and velocity, consider what happens when a small weight is attached to a string and swung in a circle. If the string were to break, the weight would fly off in a straight line. However, the string acts like gravity, keeping the weight in its orbit. The weight and string can also show the relationship between a satellite’s altitude and its orbital period. A long string is like a high altitude. The weight takes a relatively long time to complete one circle. A short string is like a low altitude. The weight has a relatively short orbital period.
Many types of orbits exist, but most artificial satellites orbiting Earth travel in one of four types: (1) high altitude, geosynchronous; (2) medium altitude, (3) sun-synchronous, polar; and (4) low altitude. Most orbits of these four types are circular.
A high altitude, geosynchronous orbit lies above the equator at an altitude of about 22,300 miles (35,900 kilometers). A satellite in this orbit travels around Earth’s axis in exactly the same time, and in the same direction, as Earth rotates about its axis. Thus, as seen from Earth, the satellite always appears at the same place in the sky overhead. To boost a satellite into this orbit requires a large, powerful launch vehicle.
A medium altitude orbit has an altitude of about 12,400 miles (20,000 kilometers) and an orbital period of 12 hours. The orbit is outside Earth’s atmosphere and is thus very stable. Radio signals sent from a satellite at medium altitude can be received over a large area of Earth’s surface. The stability and wide coverage of the orbit make it ideal for navigation satellites.
A sun-synchronous, polar orbit has a fairly low altitude and passes almost directly over the North and South poles. A slow drift of the orbit’s position is coordinated with Earth’s movement around the sun in such a way that the satellite always crosses the equator at the same local time on Earth. Because the satellite flies over all latitudes, its instruments can gather information on almost the entire surface of Earth. One example of this type of orbit is that of the TERRA Earth Observing System’s NOAA-H satellite. This satellite studies how natural cycles and human activities affect Earth’s climate. The altitude of its orbit is 438 miles (705 kilometers), and the orbital period is 99 minutes. When the satellite crosses the equator, the local time is always either 10:30 a.m. or 10:30 p.m.
A low altitude orbit is just above Earth’s atmosphere, where there is almost no air to cause drag on the spacecraft and reduce its speed. Less energy is required to launch a satellite into this type of orbit than into any other orbit. Satellites that point toward deep space and provide scientific information generally operate in this type of orbit. The Hubble Space Telescope, for example, operates at an altitude of about 380 miles (610 kilometers), with an orbital period of 97 minutes.
Types of artificial satellites
Artificial satellites are classified according to their mission. There are six main types of artificial satellites: (1) scientific research, (2) weather, (3) communications, (4) navigation, (5) Earth observing, and (6) military.
Scientific research satellites gather data for scientific analysis. These satellites are usually designed to perform one of three kinds of missions. (1) Some gather information about the composition and effects of the space near Earth. They may be placed in any of various orbits, depending on the type of measurements they are to make. (2) Other satellites record changes in Earth and its atmosphere. Many of them travel in sun-synchronous, polar orbits. (3) Still others observe planets, stars, and other distant objects. Most of these satellites operate in low altitude orbits. Scientific research satellites also orbit other planets, the moon, and the sun.
Weather Satellites
Weather satellites help scientists study weather patterns and forecast the weather. Weather satellites observe the atmospheric conditions over large areas.
Some weather satellites travel in a sun-synchronous, polar orbit, from which they make close, detailed observations of weather over the entire Earth. Their instruments measure cloud cover, temperature, air pressure, precipitation, and the chemical composition of the atmosphere. Because these satellites always observe Earth at the same local time of day, scientists can easily compare weather data collected under constant sunlight conditions. The network of weather satellites in these orbits also functions as a search and rescue system. They are equipped to detect distress signals from all commercial, and many private, planes and ships.
Other weather satellites are placed in high altitude, geosynchronous orbits. From these orbits, they can always observe weather activity over nearly half the surface of Earth at the same time. These satellites photograph changing cloud formations. They also produce infrared images, which show the amount of heat coming from Earth and the clouds.
Communication Satellites
Communications satellites serve as relay stations, receiving radio signals from one location and transmitting them to another. A communications satellite can relay several television programs or many thousands of telephone calls at once. Communications satellites are usually put in a high altitude, geosynchronous orbit over a ground station. A ground station has a large dish antenna for transmitting and receiving radio signals. Sometimes, a group of low orbit communications satellites arranged in a network, called a constellation, work together by relaying information to each other and to users on the ground. Countries and commercial organizations, such as television broadcasters and telephone companies, use these satellites continuously.
Navigation Satellites
Navigation satellites enable operators of aircraft, ships, and land vehicles anywhere on Earth to determine their locations with great accuracy. Hikers and other people on foot can also use the satellites for this purpose. The satellites send out radio signals that are picked up by a computerized receiver carried on a vehicle or held in the hand.
Navigation satellites operate in networks, and signals from a network can reach receivers anywhere on Earth. The receiver calculates its distance from at least three satellites whose signals it has received. It uses this information to determine its location.
Earth Observing Satellites
Earth observing satellites are used to map and monitor our planet’s resources and ever-changing chemical life cycles. They follow sun-synchronous, polar orbits. Under constant, consistent illumination from the sun, they take pictures in different colors of visible light and non-visible radiation. Computers on Earth combine and analyze the pictures. Scientists use Earth observing satellites to locate mineral deposits, to determine the location and size of freshwater supplies, to identify sources of pollution and study its effects, and to detect the spread of disease in crops and forests.
Military Satellites
Military satellites include weather, communications, navigation, and Earth observing satellites used for military purposes. Some military satellites — often called “spy satellites” — can detect the launch of missiles, the course of ships at sea, and the movement of military equipment on the ground.
The life and death of a satellite
Building a satellite
Every satellite carries special instruments that enable it to perform its mission. For example, a satellite that studies the universe has a telescope. A satellite that helps forecast the weather carries cameras to track the movement of clouds.
In addition to such mission-specific instruments, all satellites have basic subsystems; groups of devices that help the instruments work together and keep the satellite operating. For example, a power subsystem generates, stores, and distributes a satellite’s electric power. This subsystem may include panels of solar cells that gather energy from the sun. Command and data handling subsystems consist of computers that gather and process data from the instruments and execute commands from Earth.
A satellite’s instruments and subsystems are designed, built, and tested individually. Workers install them on the satellite one at a time until the satellite is complete. Then the satellite is tested under conditions like those that the satellite will encounter during launch and while in space. If the satellite passes all tests, it is ready to be launched.
Launching the satellite
Space shuttles carry some satellites into space, but most satellites are launched by rockets that fall into the ocean after their fuel is spent. Many satellites require minor adjustments of their orbit before they begin to perform their function. Built-in rockets called thrusters make these adjustments. Once a satellite is placed into a stable orbit, it can remain there for a long time without further adjustment.
Performing the mission
Most satellites operate are directed from a control center on Earth. Computers and human operators at the control center monitor the satellite’s position, send instructions to its computers, and retrieve information that the satellite has gathered. The control center communicates with the satellite by radio. Ground stations within the satellite’s range send and receive the radio signals.
A satellite does not usually receive constant direction from its control center. It is like an orbiting robot. It controls its solar panels to keep them pointed toward the sun and keeps its antennas ready to receive commands. Its instruments automatically collect information.
Satellites in a high altitude, geosynchronous orbit are always in contact with Earth. Ground stations can contact satellites in low orbits as often as 12 times a day. During each contact, the satellite transmits information and receives instructions. Each contact must be completed during the time the satellite passes overhead — about 10 minutes.
If some part of a satellite breaks down, but the satellite remains capable of doing useful work, the satellite owner usually will continue to operate it. In some cases, ground controllers can repair or reprogram the satellite. In rare instances, space shuttle crews have retrieved and repaired satellites in space. If the satellite can no longer perform usefully and cannot be repaired or reprogrammed, operators from the control center will send a signal to shut it off.
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Falling from orbit
A satellite remains in orbit until its velocity decreases and gravitational force pulls it down into a relatively dense part of the atmosphere. A satellite slows down due to occasional impact with air molecules in the upper atmosphere and the gentle pressure of the sun’s energy. When the gravitational force pulls the satellite down far enough into the atmosphere, the satellite rapidly compresses the air in front of it. This air becomes so hot that most or all of the satellite burns up.
Importance of Satellite
Satellites were exotic, top-secret devices. They were used primarily in a military capacity, for activities such as navigation and espionage. Now they are an essential part of our daily lives. We see and recognize their use in weather reports, television transmission by DIRECTV and the DISH Network, and everyday telephone calls. In many other instances, satellites play a background role that escapes our notice:
Some newspapers and magazines are more timely because they transmit their text and images to multiple printing sites via satellite to speed local distribution.
Before sending signals down the wire into our houses, cable television depends on satellites to distribute its transmissions.
The most reliable taxi and limousine drivers are sometimes using the satellite-based Global Positioning System (GPS) to take us to the proper destination.
The goods we buy often reach distributors and retailers more efficiently and safely because trucking firms track the progress of their vehicles with the same GPS. Sometimes firms will even tell their drivers that they are driving too fast.
Emergency radio beacons from downed aircraft and distressed ships may reach search-and-rescue teams when satellites relay the signal.
Miniaturized satellite
Classification:
Minisatellite
Microsatellite
Nanosatellite
Picosatellite
Miniaturized satellites are artificial satellites of ordinarily low weights and small sizes, usually under 500 kg (1,100 lb.). While all such satellites can be referred to as small satellites, different classifications are used to categorize them based on mass as given below.
One reason for miniaturizing satellites is to reduce the cost: heavier satellites require larger rockets of greater cost to finance; smaller and lighter satellites require smaller and cheaper launch vehicles and can sometimes be launched in multiples. They can also be launched ‘piggyback’, using excess capacity on larger launch vehicles. Miniaturized satellites allow for cheaper designs as well as ease of mass production, although few satellites of any size other than ‘communications constellations’ where dozens of satellites are used to cover the globe have been mass produced in practice.
Besides the cost issue, the main motivation for the use of miniaturized satellites is the opportunity to enable missions that a larger satellite could not accomplish, such as:
Constellations for low data rate communications.
Using formations to gather data from multiple points.
In-orbit inspection of larger satellites.
Minisatellite
The term “minisatellite” usually refers to an artificial satellite with a “wet mass” (including fuel) between 100 and 500 kg (220 and 1,100 lb.), though these are usually simply called “small satellites”. Minisatellites are usually simpler but use the same technologies as larger satellites.
Microsatellite
Microsatellite or “microsat” is usually applied to the name of an artificial satellite with a wet mass between 10 and 100 kg (22 and 220 lb.). However, this is not an official convention and sometimes microsat can refer to satellites larger than that. Sometimes designs or proposed designs of these types have microsatellites working together or in a formation. The generic term “small satellite” is also sometimes used.
Nanosatellite
The term “nanosatellite” or “nanosat” is usually applied to the name of an artificial satellite with a wet mass between 1 and 10 kg (2.2 and 22 lb.). Again designs and proposed designs of these types usually have multiple nanosatellites working together or in formation (sometimes the term “swarm” is applied). Some designs require a larger “mother” satellite for communication with ground controllers or for launching and docking with nanosatellites.
Picosatellite
Picosatellite or “picosat” (not to be confused with the PICOSat series of microsatellites) is usually applied to the name of an artificial satellite with a wet mass between .1 and 1 kg (0.22 and 2.2 lb.). Again designs and proposed designs of these types usually have multiple Picosatellites working together or in formation (sometimes the term “swarm” is applied). Some designs require a larger “mother” satellite for communication with ground controllers or for launching and docking with Picosatellite. The CubeSat design, with 1 kg maximum mass, is an example of a large Picosatellite .
Cube Sat
A CubeSat is a type of miniaturized satellite for space research that usually has a volume of exactly one liter (10 cm cube), weighs no more than 1.33 kilogram, and typically uses commercial off-the-shelf electronics components.
CubeSat isometric drawing
Since CubeSats are all 10×10 cm (regardless of length) they can all be launched and deployed using a common deployment system. CubeSats are typically launched and deployed from a mechanism called a Poly-Picosatellite Orbital Deployer (P-POD), also developed and built by Cal Poly. The P-POD is a rectangular box with a door and a spring mechanism. It is made up of anodized aluminum. They are mounted to a launch vehicle and carry CubeSats into orbit and deploy them once the proper signal is received from the launch vehicle. The P-POD Mk III has capacity for three 1U CubeSats however, since three 1U CubeSats are exactly the same size as one 3U CubeSat, and two 1U CubeSats are the same size as one 2U CubeSat, the P-POD can deploy 1U, 2U, or 3U CubeSats in any combination up to a maximum volume of 3U. CubeSats are being used for everything from environmental sensing and fundamental biology research to testing new space flight systems.
Poly Picosatellite Orbital Deployer (P-POD) and cross section
CubeSat forms a cost-effective independent means of getting a payload into orbit. Most CubeSats carry one or two scientific instruments as their primary mission payload. Several companies and research institutes offer regular launch opportunities in clusters of several cubes. ISC Kosmotras and Eurokot are two companies that offer such services.
The biggest advantage of Nano- and Pico-satellites is that they are a bargain. Most of the cost saving comes at the launch stage. Unlike conventional satellites, they don’t need a dedicated launch vehicle where they are the primary payload. Their affordability also comes from being built with off-the-shelf electronic circuit chips such as microprocessors and radio frequency transmitters and receivers. These are the same components that are inside smart phones, hand-held Global Positioning system units, and digital cameras.
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