A satellite is anything that orbits around another object. Moons are natural satellites that orbit around planets, whereas artificial satellites are objects that people have made and launched into orbit using rockets.

There are over a thousand active satellites orbiting the Earth with a range of functions. We are becoming increasingly dependent upon them, especially in the areas of weather monitoring, navigation assistance, communications, Earth and space observation and national security.

The science ideas that allow satellites to remain in orbit are principally ones of gravitational forces and circular motion.

Gravity and satellite motion

Gravity is a force that attracts all objects towards each other. It is gravity that keeps the Earth’s natural satellite, the Moon, and its largest artificial satellite, the International Space Station (ISS), in orbit around the Earth. However, both these satellites have sideways motion in the form of orbital speed that keeps them from falling back to Earth. Find out more in the article Gravity and satellite motion.

Natural satellites

Moons are natural satellites that orbit around planets. Other space objects such as planets, asteroids and comets can be regarded as satellites.

The Galilean moons of Jupiter can be viewed with the aid of binoculars or a simple telescope, and the timing of the Moon’s passage around the Earth has led to the definition of two types of month – sidereal and synodic.

Artificial satellites

There are currently over a thousand active artificial satellites orbiting the Earth. The size, altitude and design of a satellite depend on its purpose. Although communication, weather, navigation and observation satellites come in a wide range of sizes, they do have common basic parts such as a power source, propulsion system, computer system and communication system. Learn about how satellites are launched into space and how some of them have become space junk.

Communications satellites

Communications satellites are used to send and receive radio signals for television, phone or internet communications. Large concave reflectors called satellite dishes are normally used to send and receive these signals. Without these satellites, the connectivity of the ‘global village’ would collapse.

Revolutionary ideas

Since the earliest times, humans have made observations of the night sky. These observations, particularly of the Earth, Moon, Sun and planets (visible to the naked eye), led to the development of models to explain the movement of these natural satellites as seen in the night sky. Over time, there was a move from the geocentric model to the heliocentric model. Find out more in the article Our solar system – revolutionary ideas and the timeline History of satellites.

Meet the scientists

Dr Allan McInnes is a senior lecturer in the Electrical and Computer Engineering Department of the University of Canterbury. His extensive experience in the satellite industry along with his excellent communication skills feature in several of the video clips.

Dr Wolfgang Rack is a senior lecturer in the Gateway Antarctica programme at the University of Canterbury. One of his research projects involves measuring sea-ice thickness in Antarctica using the CryoSat-2 satellite

Dr Adrian McDonald is a senior lecturer in the Physics and Astronomy Department of the University of Canterbury. His ozone depletion work and Antarctic climate measurement research take him to Antarctica for 2–3 weeks each year.

Kelvin Barnsdale is senior research engineer in the College of Engineering at the University of Canterbury. His expertise in radio frequencies and electronics led to work on Space Shuttle missions and designing GPS systems.

Warwick Holmes is an avionics systems engineer who helped to build, test and launch the Rosetta spacecraft.

Take up the challenge

Student activities include a mix of hands-on and literacy-based investigations. Consider beginning a teaching sequence with Gravity and satellites – true or false? The activity is supported by the teacher resource Alternative conceptions about gravity and is a useful pretest to establish student understanding.

Students turn their eyes to the night sky to observe natural satellites and to spot artificial satellites – like the ISS – as they pass overhead. Back indoors, students can hunt for satellites online with a webquest.

Two activities use models to teach science concepts. Scale model for satellite orbits explores satellite altitudes and satellite motion. Investigating satellite dishes uses an umbrella as a curved reflector to increase a cellphone's Wi-Fi signal.

Key terms

For explanations of key concepts, see Investigating satellites – key terms.

    Published 27 March 2013, Updated 21 February 2017