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.
But this article does not deal with these kinds of artificial satellites.
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
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.
Types
of orbits
Many types of orbits exist, but most artificial
satellites orbiting Earth travel in one of four types:
1) High altitude geosynchronous orbit.
2) Medium altitude orbit.
3) Sun synchronous, polar altitude orbit.
4) Low altitude orbit.
Most orbits of
these four types are circular.
High altitude orbit
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.
Medium altitude orbit
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.
Sun-synchronous orbit
1)
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.
Low altitude orbit
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 satellite
Artificial
satellites are classified according to their mission. There are six main types
of artificial satellites:
v 1 Scientific
research satellite.
v 2 Weather
satellite.
v 3 Communication
satellite.
v 4 Navigation
satellite.
v 5 Earth
observing satellite.
v 6 military
satellite.
Scientific research
satellite
Scientific research satellite 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 satellite
1) Weather satellites help scientists study weather
patterns and forecast the weather. Weather satellites observe the atmospheric
conditions over large areas.
2) 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 function as a search and rescue system. They are equipped to detect
distress signals from all commercial, and many private, planes and ships.
3) 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
satellite
1) 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
satellite
1)
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.
2) 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
satellite
1)
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 satellite
1) 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.
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.
History
•
In 1955, the United States Soviet Union announced plans to launch
artificial satellites. On Oct.
4, 1957 , the Soviet Union launched
Sputnik 1, the first artificial satellite. It circled Earth once every 96
minutes and transmitted radio signals that could be received on Earth. On Nov. 3, 1957 , the Soviets
launched a second satellite, Sputnik 2. It carried a dog named Laika, the first
animal to soar in space. The United
States Jan. 31, 1958 ,
and its second, Vanguard 1, on March
17, 1958 .
•
In August
1960, the United States
•
Since the
1970's, scientists have created new and more effective satellite instruments
and have made use of computers and miniature electronic technology in satellite
design and construction. In addition, more nations and some private businesses
have begun to purchase and operate satellites. By the early 2000's, more than
40 countries owned satellites, and nearly 3,000 satellites were operating in
orbit.
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