Artificial satellites

An artificial satellite is an object that people have made and launched into orbit using rockets. There are currently over 3,000 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

Navigation satellites The GPS (global positioning system) is made up of a minimum of 24 satellites (and allows for up to 32), 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.

Communication satellites 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.

Weather satellites 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.

Weather satellite orbits

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Weather satellites operate in two orbit types. The GEO mode allows the same geographic area to be viewed continuously from a very high altitude. With the LEO polar orbit, complete Earth coverage is possible.

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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.

Astronomical satellites These are used to monitor and image space. A satellite such as the James Webb Space Telescope provides very sharp images of stars and distant galaxies. It operates in a halo orbit – between about 250,000 and 832,000 km, which keeps it out of both Earth and Moon's shadow. Other space telescopes include Hubble 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.

International Space Station (ISS)

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The International Space Station (ISS) is a habitable artificial satellite that has been placed in a low Earth orbit. It completes 15.7 orbits per day and is maintained at an orbital altitude of between 330 km and 410 km.

Image courtesy of NASA.

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Satellite design

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.

Making space accessible

To get a satellite into space is a very expensive business that few can afford. Rocket Lab, a company set up by New Zealander Peter Beck, was set up with a vision to “make space more accessible”.

Peter has a vision that affordable access to space will enable powerful global change – for example allowing developing nations better access to internet. In 2012, Peter initiated the Electron programme to focus on providing cost effective rockets and launch services that will allow organisations better access to space and satellite technology.