An artificial satellite is an object that people have made and launched into orbit using rockets. There are currently over a thousand active satellites orbiting the Earth. The size, altitude and design of a satellite depend on its purpose.
Sizes and altitudes of satellites
Satellites vary in size. Some cube satellites are as small as 10 cm. Some communication satellites are about 7 m long and have solar panels that extend another 50 m. The largest artificial satellite is the International Space Station (ISS). The main part of this is as big as a large five-bedroom house, but including solar panels, it is as large as a rugby field.
Altitudes of satellites above the Earth’s surface also vary. These are three common orbits:
- Low Earth orbit (LEO) – from 200 to 2,000 km, for example, the ISS orbits at 400 km with a speed of 28,000 km/hour, and time for one orbit is about 90 minutes.
- Medium Earth orbit (MEO) – most MEO satellites are at an altitude of 20,000 km, and time for one orbit is 12 hours.
- Geostationary orbit (GEO) – 36,000 km above the Earth. Time for one orbit is 24 hours. This is to match the rotation of the Earth so that the satellite appears to stay above the same point above the Earth’s surface. This is used for many communications and weather satellites.
The altitude chosen for a satellite depends on the job it is designed for.
Types of satellites
The GPS (global positioning system) is made up of 24 satellites that orbit at an altitude of 20,000 km above the surface of the Earth. The difference in time for signals received from four satellites is used to calculate the exact location of a GPS receiver on Earth.
These are used for television, phone or internet transmissions, for example, the Optus D1 satellite is in a geostationary orbit above the equator and has a coverage footprint to provide signals to all of Australia and New Zealand.
These are used to image clouds and measure temperature and rainfall. Both geostationary and low Earth orbits are used depending on the type of weather satellite. Weather satellites are used to help with more accurate weather forecasting.
Earth observation satellites
These are used to photograph and image the Earth. Low Earth orbits are mainly used so that a more detailed image can be produced.
These are used to monitor and image space. A satellite such as the Hubble Space Telescope orbits at an altitude of 600 km and provides very sharp images of stars and distant galaxies. Other space telescopes include Spitzer and Chandra.
International Space Station (ISS)
This is a habitable space laboratory. At an altitude of 400 km, the ISS travels at a speed of 28,000 km/h and orbits the Earth once every 92 minutes. Scientists inside the ISS are able to perform many valuable experiments in a microgravity environment.
Every satellite has some of the same basic parts:
- The bus – this is the frame and structure of the satellite to which all the other parts are attached.
- A power source – most satellites have solar panels to generate electricity. Batteries store some of this energy for times that the satellite is in the shadow of the Earth.
- Heat control system – satellites are exposed to extremely high temperatures due to exposure to the Sun. There needs to be a way to reflect and reradiate heat. Electrical components of the satellite can also produce a lot of heat.
- Computer system – satellites need computers to control how they operate and also to monitor things like altitude, orientation and temperature.
- Communication system – all satellites need to be able to send and receive data to ground stations on Earth or to other satellites. Curved satellite dishes are used as antennae
- Attitude control system – this is the system that keeps a satellite pointed in the right direction. Gyroscopes and rocket thrusters are commonly used to change orientation. Light sensors are commonly used to determine what direction a satellite is pointing.
- A propulsion system – a rocket engine on the satellite may be used to help place the satellite into the correct orbit. Once in orbit, satellites do not need any rockets to keep them moving. However, small rockets called thrusters are used if a satellite needs to change orbit slightly.
As well as these basic parts, satellites carry the equipment needed for their specific purpose.
Nature of science
The operation of the International Space Station depends upon collaboration between countries. It allows scientists to pool their resources in terms of scientific knowledge and finances. As a result, bigger and more substantial projects can be conducted to get more reliable results. This research is of benefit internationally.