A natural or human-made body orbiting a star, planet, or other solar body.
Look up in the night sky. What is that light moving across the sky? It's moving fast in a straight line, and it's not blinking or twinkling. It's not a plane, and it's not a meteor. It's a satellite!
A satellite is any object that orbits (goes around) a planet or a star. Our moon is a natural satellite because it orbits the Earth. The Earth itself is a natural satellite of the sun because it revolves around the sun. All of the planets in our solar system are satellites of the sun. Mars, Jupiter, Saturn, Uranus, and Neptune all have multiple moons that orbit them. Their moons are satellites of those planets. Our solar system has over 200 known natural satellites.
There are also artificial, man-made satellites that are in orbit around the earth. These are the fast-moving objects you can often see in the night sky. They are deliberately launched into orbit around a body in space, usually with a specific job that involves receiving and sending back information to Earth. Why are they out there? How do they work? Let's find out more about satellites.
The First Satellites
The first artificial satellite was called Sputnik 1. It was launched in 1957 by the Soviet Union (USSR). Sputnik was a small aluminum ball, about the size of a beach ball, with four long antennas and powered by batteries. Inside Sputnik were radio transmitters that sent out a "beep, beep" sound that was heard all over the world. Sputnik only transmitted a signal for about three weeks, but this small and simple satellite marked the beginning of the Space Age. Soon after, Sputnik 2 carried the first living passenger into orbit, a dog named Laika. In 1958, the United States launched its first satellite, Explorer 1. A year later, NASA's Explorer 6 sent the first satellite pictures of Earth, and by 1962, the first orbiting satellite provided long-term service to the Earth.
Since that time, the number of satellites and their complexity has continued to increase. Since 1957, more than 8,000 satellites from more than 50 countries have been launched. Today, about 3,600 man-made satellites remain in orbit, with about 1,000 of them currently functioning. If you go outside on a clear night and see a bright light speeding across the sky, it may well be a satellite reflecting the light of the Sun.
How Satellites Work
Satellites come in many shapes and sizes. They can be as small as your hand or as large as a truck. But most have similar parts. The bus, or container, is the main body of the satellite. Attached to the bus are antennas that receive and send signals back to Earth. Because satellites have to power themselves, they all have a power source, usually solar panels or batteries. Earth-orbiting satellites have instruments, such as cameras and sensors, that are pointed toward Earth to gather information about our planet. Other satellites have instruments facing toward space to collect data from the solar system and universe. Most artificial satellites orbit the Earth, but some orbit other planets, such as Mars, Venus, and Saturn, and still others orbit the Sun.
Some satellites help people send information around the world. Information from a ground station on Earth can be sent to an orbiting satellite that bounces it back to receivers at another location on Earth. Other types of satellites take pictures or gather data about our planet and send that data back to Earth. Satellites in space help us overcome the limitations of Earth's geography. They can collect and send more information, more quickly, than instruments on the ground.
Satellites are launched into space on rockets. How do they stay in orbit? A satellite orbits Earth when its speed is balanced by the pull of Earth's gravity. Without this balance, the satellite would fly in a straight line off into space or fall back to Earth. Orbital velocity is the speed needed to achieve a balance between gravity's pull on the satellite and the satellite's tendency to keep going. At the correct orbital velocity, gravity pulls down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface.
The orbital velocity of the satellite changes depending on its altitude above Earth. Those satellites that are closer to the Earth must move faster to stay in orbit. The speed a satellite must travel to stay in orbit is about 17,500 mph (28,200 km/h) at an altitude of 150 miles (242 kilometers.) However, in order to maintain an orbit that is 22,223 miles (35,786 kilometers) above Earth, a satellite orbits at a speed of about 7,000 mph (11,300 km/h).
The higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, traces of Earth's atmosphere create drag, which sometimes causes the orbit to change. At higher altitudes, where there are no traces of atmosphere, a satellite can stay in orbit for centuries.
Can satellites collide in space? It's possible. NASA and other international organizations keep track of satellites in space. When a satellite is launched, it is placed into an orbit designed to avoid other satellites. But orbits can change over time, and with more satellites, the chance of a crash increases. In 2009, an American satellite and a Russian satellite accidentally collided in space. Watch NASA scientists avert a close call between two satellites.
Different Kinds of Satellites
When it comes to satellites, it's all about the orbit. Space scientists decide on the orbit for a satellite depending on its job. Some satellites orbit at a low altitude, just a few hundred miles above the Earth. Others orbit the Earth thousands of miles out in space. Some orbit around the Earth's equator and others go over Earth's north and south poles.
Low Earth orbit (LEO) satellites orbit in a region between 111 miles to 1243 miles (180-2000 km) above Earth. These satellites may orbit the Earth many times a day and are often used for observing the Earth. Many of these are polar-orbiting satellites that orbit Earth in a north-south direction from pole to pole. As the earth spins below them, these satellites can scan the entire globe.
Medium-Earth orbits (MEO) satellites orbit at an altitude between 1,243 miles and 22,223 miles (2,000-36,000 km) above the Earth. Navigation satellites work well at this altitude.
Geostationary satellites, often used for communication, orbit Earth at an altitude greater than 22,223 miles (36,000 km.) These satellites orbit Earth from west to east over the equator. Their orbital period is the same as Earth's rotation period: 24 hours. Because they move in the same direction and speed as the Earth is spinning, they are always above the same location, so from the ground they don't appear to be moving.
In the picture below, the yellow areas show what part of Earth each kind of satellite sees during its orbit.
The Uses of Satellites
Satellites are used for many purposes. Even though they are hundreds or thousands of miles out in space, satellites are part of our everyday life on Earth. They make us safer, broadcast entertainment, and make life more convenient. Without satellites, some of us couldn't watch television, or figure out how to travel from one place to another. Some of us might be endangered by bad weather that we didn't know was coming, or we might be unable to make a long-distance phone call. Satellites often affect our lives without us even realizing it. Let's take a look at some of the jobs satellites do.
Communication satellites allow television, radio, internet, and telephone signals to be sent live anywhere in the world. Before satellites, long-distance transmissions were difficult and had many barriers. But with satellites, signals can be beamed from one location upward to a satellite and almost instantly be redirected down to many locations anywhere on the Earth. Today, much of what you see on TV is transmitted by satellite signal.
Communication satellites allow video conferencing for businesses and classrooms. Where people live far from cities, communication satellites provide access to education and medical help that would otherwise not reach them. Satellites even allow your parents to use a bank card to make purchases at stores or gas stations. Learn more about communication satellites.
In the past, people used the positions of the sun and the stars, maps, and compasses to find their way. Today, satellite navigation systems such as GPS (Global Positioning System) allow people to figure out exactly where they are and how to get to where they want to go, making it almost impossible to get lost. A system of 30 satellites circling the Earth make up the Global Positioning System. The satellites constantly send out signals, your GPS receiver picks up those signals, and the distance from those satellites is calculated to determine your exact location.
Often GPS receivers are built into cars and cell phones. Other uses of GPS systems include use by military submarines for navigating under the sea, use of GPS for the treasure-hunt game of geocaching, and use of a handheld navigator that helps guide blind people to their destination. Learn more about GPS navigation satellites.
First responders rely on satellites as they help people in trouble. Satellite systems provide vital information and connect emergency response teams. Search and rescue satellites are designed to provide a way for ships at sea and planes in the air to communicate from remote areas. These satellites locate distress beacons sent by ships, aircraft, or individuals in remote or dangerous regions. Radio distress beacons directly linked to a search and rescue satellite can lead rescuers quickly to an emergency location. Learn more about search and rescue satellites.
Earth observation satellites do lots of jobs! Because of their global view, they are able to observe large areas of Earth at one time. Satellites facing the earth enable us to better understand environmental changes on Earth. Satellite images can survey the earth and provide pictures and data about almost any region in the world - even your own house! Without satellites, such measurements would require huge amounts of land-based inspection.
Satellites provide information about oceans, land, clouds, and ice. They can detect underground water and mineral sources. They provide information that helps farmers know which crops to plant and helps city planners know how to improve traffic flow. They monitor wildfires, volcanoes, smoke pollution, hurricanes, and oil spills so that emergency workers can respond to disasters. Weather satellites observe the Earth from space to help meteorologists forecast weather patterns and track storms. Satellites are some of the best sources of data for climate change research. They monitor ocean temperatures, sea levels, the sizes of glaciers, changes in tree cover, and gases in the atmosphere, such as ozone and carbon dioxide. Scientists can use this information to create models of climate change. Learn more about NASA's earth science missions.
Satellites help scientists study space. They can provide much more data than instruments on Earth can. Satellites can see into space better than telescopes on Earth's surface because Earth's atmosphere doesn't interfere with the view from satellites. Many of these satellites are operated by NASA and other countries' scientific organizations.
Satellites that face toward space have many different jobs. Some watch for dangerous rays coming from the sun. Some explore asteroids and comets. Some fly close to or orbit other planets, looking for evidence of water on Mars or capturing pictures of Saturn's rings.
The Hubble Space Telescope orbits around Earth once every 95 minutes, at about 5 miles per second. It takes pictures of planets, stars, galaxies, and black holes. Scientists have learned a lot from Hubble's pictures. It is considered by some to be the most valuable satellite ever built.
The largest satellite is the International Space Station. It is the size of a soccer field. It is so big that scientists had to send up pieces of it to be assembled in space. The ISS is the only satellite that has people living on board, providing a home for a rotating crew of six astronauts from different countries. They do experiments and study what happens to people when they live in space. Scientists will use lessons from the space station to get astronauts ready for future space travel.
If thousands of satellites have been launched but only a portion of them are currently functioning, what happened to the rest? Many satellites have a working life of 10-15 years. When a satellite wears out and stops working, it sometimes falls back toward Earth and burns up in the Earth's atmosphere. Other satellites continue to orbit the Earth, joining the category of orbiting objects known as space debris, or space junk.
Space debris includes obsolete satellites whose work is finished and thousands of fragments of space materials. The United States Space Surveillance Network estimates that there are more than 21,000 objects greater than 4 inches (10 cm) orbiting the Earth, with only 5% of those being working satellites. These objects come from exploding rockets, accidentally dropped tools or pieces of unneeded equipment. These bits of space junk are satellites too, as they are in orbit around the earth, but they can be hazardous to working satellites and spacecraft traveling in and through Earth's orbit. Sometimes, the International Space Station has to be moved to avoid impact with chunks of space debris. These days, when a new satellite is launched, its engineers are required to have a plan for the end of its working life: either it will be returned to the earth or sent far out into space into "graveyard orbit," away from the busy community of working satellites.
Fun Facts About Satellites
- Vanguard-1 is the oldest man-made satellite still in orbit. It has orbited Earth for 62 years, and is expected to continue to do so for nearly another two centuries.
- Today, engineers have developed small, lightweight satellites that measure about 4 inches square, known as CubeSats. They are much less expensive than large satellites and can share launchers to get them up into orbit. CubeSats can measure space weather, take pictures of the earth, and monitor radiation - but they are smaller than a football.
- What do space satellites have to do with endangered species? Satellites provide clear migration images that scientists can use to help protect threatened animals from drought, habitat loss, and poachers.
- Large satellites like space stations and other spacecraft returning to Earth might not entirely burn up before reaching the ground. Spacecraft operators can make sure that any debris falls into an area far from where anyone lives. This designated area in the Pacific Ocean is called the Spacecraft Cemetery.
- Some satellites are launched and operated by governments and scientific organizations. Others belong to private companies and students. Could you launch a satellite someday? Yes!