What Is a Satellite?

A satellite is an object that moves in an orbit around another, more massive object.

There are two main types of satellites:

• Natural satellites – naturally occurring objects that orbit planets or other bodies.
• Artificial satellites – human-made objects placed into orbit for a particular purpose.

The Moon is Earth's natural satellite. Earth also has thousands of artificial satellites and other spacecraft orbiting it.

 

 

 

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Natural Satellites

Natural satellites are usually called moons.

Examples include:

• Earth's Moon
• Phobos and Deimos orbiting Mars
• Europa and Ganymede orbiting Jupiter
• Titan orbiting Saturn

Natural satellites vary greatly in size, composition, and distance from their planets.

Artificial Satellites

Artificial satellites are machines launched into space and placed into specific orbits.

They can carry:

• Cameras
• Radio transmitters and receivers
• Scientific instruments
• Weather sensors
• Navigation equipment

Their orbit is chosen according to the job the satellite needs to perform.

Circular Orbits

A satellite in a circular orbit remains approximately the same distance from the object it is orbiting.

Gravity continuously pulls the satellite toward Earth. However, the satellite also has a large tangential velocity.

These two effects combine to produce an orbit.

 

 

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The gravitational force acts toward the centre of Earth and provides the centripetal force needed to keep the satellite moving in a circular path.

Without gravity, the satellite would move away along a path tangent to its orbit.

Orbital Speed

The speed required for a circular orbit depends on the satellite's distance from Earth.

For a circular orbit:

\( v = \sqrt[]{ \frac{GM}{r} } \)​

where:

• v = orbital speed
• G = gravitational constant
• M = mass of the object being orbited
• r = distance from the centre of that object

This relationship tells us:

Smaller orbital radius → greater orbital speed

Larger orbital radius → lower orbital speed

Geostationary Orbits

A geostationary satellite appears to remain above the same point on Earth's surface.

To do this, the satellite must:

• Orbit above the equator.
• Travel in the same direction as Earth's rotation.
• Have an orbital period equal to Earth's rotation period — approximately 24 hours.
• Be approximately 35,786 km above Earth's surface.

 

 

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The satellite is still moving rapidly through space. It only appears stationary because it moves around Earth at the same rate that Earth rotates.

Why Are Geostationary Satellites Useful?

Because they remain above approximately the same location, ground-based antennas can point continuously toward the same satellite.

This makes geostationary satellites particularly useful for:

• Television broadcasting
• Telecommunications
• Internet services
• Weather monitoring

Low Earth Orbit

Many artificial satellites operate in Low Earth Orbit (LEO), generally a few hundred to around 2,000 km above Earth's surface.

Satellites in LEO travel around Earth much faster than geostationary satellites.

For example, the International Space Station orbits roughly 400 km above Earth's surface and completes an orbit in about 90 minutes.

 

 

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LEO is commonly used for:

• Earth observation
• Scientific research
• Imaging
• Some communication systems
• Human spaceflight

Because LEO satellites are relatively close to Earth, they can provide detailed observations and lower communication delay.

Polar Orbits

A polar orbit carries a satellite over or near Earth's North and South Poles.

As the satellite orbits, Earth rotates underneath it. Over time, the satellite can observe much of Earth's surface.

 

 

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Polar and near-polar orbits are particularly useful for:

• Mapping
• Weather observations
• Environmental monitoring
• Measuring ice coverage
• Monitoring forests and oceans
• Earth imaging

Common Uses of Artificial Satellites

Artificial satellites have become an important part of modern technology.

Communication

Communication satellites transmit information between different parts of Earth.

They can carry:

• Television signals
• Telephone communications
• Internet data
• Emergency communications

Navigation

Navigation satellite systems allow receivers to determine their position on Earth.

Satellite navigation is used in:

• Smartphones
• Cars
• Aircraft
• Ships
• Surveying

Weather Forecasting

Weather satellites observe:

• Clouds
• Storm systems
• Atmospheric conditions
• Ocean temperatures

These observations help meteorologists track storms and produce weather forecasts.

Earth Observation

Satellites can repeatedly photograph and measure Earth's surface.

Scientists use them to study:

• Deforestation
• Agriculture
• Wildfires
• Pollution
• Glaciers
• Sea ice
• Natural disasters

What Affects Orbital Motion?

Several factors affect the motion of a satellite.

Orbital Radius

The distance between the satellite and the centre of the object it orbits affects both its speed and orbital period.

For circular Earth orbits:

Higher orbit → lower orbital speed → longer orbital period

Lower orbit → higher orbital speed → shorter orbital period

Mass of the Central Object

A more massive central object produces a stronger gravitational field.

For example, a satellite orbiting a very massive planet experiences different orbital conditions from one at the same orbital radius around a less massive planet.

Velocity

A satellite must have the correct velocity to maintain its desired orbit.

If its velocity changes, the shape or size of its orbit can change.

Spacecraft therefore use engines or thrusters to adjust their orbits when necessary.

Gravity

Gravity provides the inward force necessary for orbital motion.

A satellite can be thought of as continuously falling toward Earth while moving forward fast enough to keep missing it.

This continuous free fall produces an orbit.

Comparing Satellite Orbits

Example: Choosing an Orbit

A scientist wants to monitor changes in forests across the entire Earth.

A polar or near-polar orbit would be useful because the satellite can observe different areas as Earth rotates beneath it.

A television company wants to transmit signals continuously to the same region.

A geostationary orbit would be useful because the satellite remains above the same part of Earth.

Did You Know?

Satellites in orbit are still affected strongly by Earth's gravity. Astronauts do not appear weightless because there is no gravity in space.

Instead, astronauts and their spacecraft are falling together around Earth. This condition is called free fall and produces the experience of apparent weightlessness.

Key Terms

• Satellite – an object that orbits another object.
• Natural satellite – a naturally occurring satellite, such as a moon.
• Artificial satellite – a human-made object placed into orbit.
• Orbit – the path of an object around another object.
• Circular orbit – an orbit with approximately constant distance from the central object.
• Geostationary orbit – an orbit in which a satellite appears stationary above one point on Earth's equator.
• Polar orbit – an orbit that passes over or near Earth's poles.
• Low Earth Orbit (LEO) – an orbit relatively close to Earth's surface.
• Orbital speed – the speed of an object travelling through its orbit.
• Orbital period – the time required to complete one orbit.
• Centripetal force – the inward force required for circular motion.

Key Takeaways

• Satellites can be natural or artificial.
• The Moon is a natural satellite, while communication and weather satellites are artificial.
• Gravity provides the centripetal force that keeps satellites in orbit.
• Satellites in lower circular orbits move faster and have shorter orbital periods.
• Geostationary satellites orbit above the equator with a period of approximately 24 hours.
• Different satellite orbits are chosen for different purposes.
• LEO, polar, and geostationary orbits each have advantages for particular applications.