Satellites are usually classified according to the type of orbit they are in. There are four types of orbit associated with satellites, and the type of orbit dictates a satellite's use.
Low Earth Orbits
Satellites in low Earth orbits are normally military reconnaissance satellites that can pick out tanks from 160 km above the Earth. They orbit the earth very quickly, one complete orbit normally taking 90 minutes. However, these orbits have very short lifetimes in the order of weeks compared with decades for geostationary satellites. Simple launch vehicles can be used to place these satellites of large masses into orbit.
Meteorological satellites are often placed in a sun-synchronous or heliosynchronous orbit. These satellites are in polar orbits. The orbits are designed so that the satellite's orientation is fixed relative to the Sun throughout the year, allowing very accurate weather predictions to be made. Most meteorological satellites orbit the Earth 15 to 16 times per day.
Earth-synchronous or geosynchronous satellites are placed into orbit so that their period of rotation exactly matches the Earth's rotation. They take 24 hours to make one rotation. However, the plane of orbit for these satellites is generally not the equatorial plane. Apart from geostationary satellites (see below), the satellites are used for communications at high latitudes, particularly in Russia and Canada. The orbits are called Molniya orbits. The satellites are placed in highly elliptical orbits which enable them to appear to hover above one point on the Earth for most of the day. In twenty four hours they move over the Earth in a figure of eight pattern centred on a fixed longitude, moving slowly where they can be useful and quickly where they are of little use.
The majority of communications satellites are in fact geostationary satellites. Geostationary satellites like geosynchronous satellites take 24 hours to complete a rotation. However, geostationary satellites are positioned directly over the equator and their path follows the equatorial plane of the Earth. As a result geostationary satellites don't move North or South during the day and are permanently fixed above one point on the equator of the Earth.
Most video or T.V. communications systems use geostationary satellites. Geosynchronous and geostationary satellites are typically orbiting at 35,788 km (22,238 miles) above the surface of the planet (42,000 km from its centre).
Modern satellites have a mass of several thousand kilograms, compared with just 180 kilograms for Sputnik. Modern satellites are placed in space using launch vehicles like the Arianne Rocket or the Space Shuttle. Once in space, most satellites obtain their power from the Sun using solar panels. Satellites travelling deep into space often carry additional nuclear power supplies.
Pupil Worksheet Questions on Types of Satellite
Read the article on "Types of Satellite" and answer the following questions.
1. What is the Earth's natural satellite?
2. What was the first satellite in space?
3. Most of the modern satellites orbiting the Earth carry very little fuel, but are in space for many years. Where do they get there energy from?
4. What does the term orbit mean?
5. What does the term stable orbit mean?
6. A satellite orbits the Earth 15 times a day how long does it take to make one orbit.?
7. Many modern satellites are carried into space using the Space Shuttle. Why are they not launched directly into space?
8. Voyager 2 reached Uranus in 1986. It used both nuclear power and solar power. Why did it require two sources of fuel?
9. The closer a satellite is to the Earth the less time it takes to orbit the Earth. What would happen to a satellite if it was too close to the Earth? It is not in a stable orbit!
10. How high is a geosynchronous satellite above the centre of the Earth?
11. Use the information in the article to work out the speed of a geostationary or geosynchronous satellite.
Remember For a circle, Distance = 2IIr where r is the distance from the centre of the Earth, and:-
Speed = Distance÷Time
12. Satellites send information as a microwave beam. What are microwaves also used for?
13. Clementine is a satellite orbiting the Moon. What is its function? Why cannot Earth based satellites do the same job?
14. Could a satellite send information to the Earth during a solar eclipse?
15. Recently a fault developed with the Hubble Space Telescope. Several astronauts were sent to fix it. How could the astronauts move such a heavy object? What is your weight in space?
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