Satellites

Introduction

A satellite is an object that revolves around a planet. It can be a natural or man-made device. For example, the moon is Earth's natural satellite. This chapter gives an overview of man-made satellites, their types, orbits and purpose.

What is an artificial satellite?

A man-made satellite is also called an artificial satellite. An artificial satellite is an Earth-orbiting device used for receiving and transmitting signals. Each satellite receives a signal and sends it back to Earth. The signal is received by Earth stations and then transmitted via cable, phone line, or microwave to its final receiver, for example a television set. The path a satellite follows is called an orbit. In the orbit, the farthest point from Earth is called the apogee. The closest point of the orbit is called the perigee.

Artificial satellites are usually custom-built to perform particular tasks. Exceptions include GPS satellites. GPS stands for Global Positioning System. Currently this system consists of 27 Earth-orbiting satellites. Initially these satellites were developed by the US as a military navigation system. Today, GPS is used for a large number of peaceful purposes.

The first artificial satellite

The first artificial satellite was launched in 1957 by the USSR. It was called 'Sputnik', which is Russian for 'satellite'. Due to Soviet government secrecy at the time, no photographs were taken of this famous launch. Compared with modern satellites, 'Sputnik' was very small and simply-designed. It looked like a metal ball with four antennae. It took about 98 minutes to orbit the Earth on its elliptical path. After 92 days, gravity took over and 'Sputnik' burned in the Earth's atmosphere. 'Sputnik' is a good example of just how simple a satellite can be. The 'Sputnik' launch was the start of the space age and the space race between the US and USSR. The 'Sputnik' launch also led directly to the creation of the National Aeronautics and Space Administration (NASA). See image 1.

How satellites are launched

Today, all satellites are launched into orbit by rocket or by space shuttle. The space shuttle is a space transportation system that can carry people and cargo. A satellite is cargo. For most satellite launches, the scheduled launch rocket, initially, is aimed vertically. This propels the rocket through the thickest part of the atmosphere.

Satellite orbit types

Satellites are usually classified according to their orbit. There are several different orbit classification systems. The most common system is based on the orbit's altitude (elevation above the Earth's surface).

• Low Earth Orbit (LEO: 200 to 2000 km above the Earth's surface)
  
  
• High Earth Orbit (HEO: above 2000 km)
  
  
• Medium Earth Orbit (MEO: general category of orbits centered on approximately 20 200 km)
  
  
• Geostationary Orbit or (GSO: 35 786 km above the Earth's surface)

Weather and communication satellites have geostationary orbit satellites. A geostationary orbit satellite is always positioned over the same spot on Earth. Many geostationary satellites are above the equator because the distance around the Earth is the largest there so its rotation is the fastest. Today a large number of different satellites from different countries and organisations are 'parked' along the equator. This means each satellite must be precisely positioned to prevent its signals from interfering with signals from nearby satellites. Satellites use a variety of light-sensitive sensors to determine their position. The satellite transmits its position to a ground station. See image 3.

The higher the orbit, the longer the satellite can stay in orbit. At lower altitudes a satellite interacts with the Earth's atmosphere. This interaction decreases the satellite's orbit until it falls back into the atmosphere and burns up. At higher altitudes there is a vacuum. A vacuum is a space with no, or very little, gas pressure. Apparently in space, a vacuum satellite can stay in orbit for centuries.

Satellites and the sun's radiation

Scientists collect important information from satellites about the sun's effect on the Earth's atmosphere and climate. The Solar Radiation and Climate Experiment (SORCE), a satellite mission of the National Aeronautics and Space Administration (NASA), has a five-year project to provide accurate measurements of the sun's radiation. Sun radiation means its light intensity. The satellite will collect data on the sun's light output, UV radiation and irradiation. Irradiation is a measure of the amount of light energy incident on a unit area of surface per unit of time. The sun's radiation data collected by the satellite will be much more accurate because the Earth's atmosphere will not interfere.

What are satellites for?

There are many types of satellites today. They perform different jobs and serve different purposes.

• Weather satellites help meteorologists predict the weather by taking photographs of high atmospheric levels.
  
  
• Communications satellites allow telephone and data conversations to be relayed through the satellite. Communications satellites are usually geostationary.
  
  
• Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
  
  
• Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite.
  
  
• Navigational satellites help ships and planes navigate. The most well-known are the GPS satellites.
  
  
• Rescue satellites respond to radio distress signals.
  
  
• Earth observation satellites observe the planet.
  
  
• Military satellites exist, but much of the actual application information remains secret.