What you'll learn
The Moon orbits the Earth, the Earth orbits the Sun, and artificial satellites orbit the Earth — all held in place by gravity. For AQA GCSE Physics you need to understand the objects in the solar system, how gravity keeps objects in orbit, why an orbiting object needs a particular speed, and the difference between types of satellite orbit. This guide covers the solar system, the role of gravity in orbits, circular motion and orbital speed, and natural and artificial satellites. By the end you should be able to explain how gravity produces orbits and why changing speed changes the orbit.
Key terms and definitions
Solar system — The Sun and all the objects that orbit it.
Orbit — The curved path of an object around a star, planet or moon.
Satellite — An object that orbits a larger object; may be natural (a moon) or artificial.
Gravity — The force of attraction between masses.
Centripetal force — The resultant force directed towards the centre that keeps an object moving in a circle.
Natural satellite — A moon that orbits a planet.
Artificial satellite — A human-made object placed in orbit.
Geostationary orbit — An orbit in which a satellite stays above the same point on the Earth's surface.
Core concepts
The solar system
The solar system consists of the Sun at the centre, with the eight planets orbiting it, along with dwarf planets, moons, asteroids and comets. The Sun is a star, and it is by far the largest object, holding everything in orbit through its gravity. Moons orbit planets, planets orbit the Sun, and the whole solar system lies within the Milky Way galaxy.
Gravity and orbits
Objects stay in orbit because of the force of gravity. The Sun's gravity pulls the planets towards it, and a planet's gravity pulls its moons towards it. Without gravity, an orbiting object would travel in a straight line and fly off into space. Gravity constantly pulls the object towards the centre, bending its path into a curved orbit. So gravity provides the force needed to keep an object moving in a circle around a larger body.
Circular motion and centripetal force
An object moving in a circle is constantly changing direction, even if its speed stays the same. A change in direction is a change in velocity, which requires a resultant force. This force is called the centripetal force, and it always acts towards the centre of the circle. For an orbit, gravity provides the centripetal force. This is a key idea: gravity is not slowing the object down or speeding it up; it is continually changing its direction, keeping it in orbit.
Orbital speed and the size of the orbit
For an object to stay in a stable circular orbit at a particular distance, it must travel at a particular speed. If the speed is wrong for that distance, the object will not stay in that orbit:
- Objects in orbits closer to the body they orbit must travel faster.
- Objects in orbits further away travel more slowly.
This is why the inner planets orbit the Sun faster than the outer planets. If a satellite's speed changes, its orbit changes: to move into a stable orbit at a different distance, its speed must change to the value that balances gravity at that distance.
Natural and artificial satellites
A satellite is anything that orbits a larger object. There are two types:
- Natural satellites — moons, which orbit planets naturally (for example, the Moon orbits the Earth).
- Artificial satellites — human-made objects placed in orbit for purposes such as communications, weather monitoring, navigation (GPS) and scientific observation.
Both are kept in orbit by gravity in the same way.
Types of artificial satellite orbit
Artificial satellites are placed in different orbits depending on their use. A geostationary satellite orbits high above the equator and takes exactly 24 hours to orbit, so it stays above the same point on the Earth's surface — useful for communications and television, because a dish on the ground can point at a fixed position. Other satellites are placed in low polar orbits, passing over the poles and closer to the Earth, so they travel faster and can scan the whole surface as the Earth rotates beneath them — useful for weather and imaging. The choice of orbit depends on the job the satellite has to do.
Why orbits are slightly elliptical
Although we often describe orbits as circular, most real orbits are actually slightly elliptical (oval-shaped). The planets orbit the Sun in ellipses, with the Sun near one focus, and the Moon's orbit around the Earth is also slightly elliptical. For GCSE, treating orbits as circular is usually acceptable, but it is worth knowing that the speed of an object in an elliptical orbit is not constant — it moves faster when closer to the body it orbits and slower when further away. This happens because gravity is stronger when the objects are closer, speeding the object up as it approaches and slowing it as it moves away. Knowing this helps explain why an object's speed can change during a single orbit.
Comets and their orbits
Comets are a good example of very elliptical orbits. A comet orbits the Sun on a long, stretched ellipse, so its distance from the Sun changes enormously during its orbit. When it is far from the Sun, gravity is weak and the comet moves slowly; as it approaches the Sun, gravity increases and the comet speeds up, moving fastest at its closest approach before slowing again as it travels back out. This is the same principle as for planets, but exaggerated because the orbit is so stretched. Comets show clearly how gravity and orbital speed are linked, and why an object in a non-circular orbit does not travel at a steady speed.
Worked examples
Example 1: Why an orbit is curved
Explain why a planet moves in a curved orbit rather than a straight line. Without a force, the planet would move in a straight line. The Sun's gravity constantly pulls the planet towards the Sun, changing its direction and bending its path into a curved orbit.
Example 2: Gravity as the centripetal force
What provides the centripetal force that keeps the Moon in orbit around the Earth? The Earth's gravity provides the centripetal force. It acts towards the centre (the Earth), continually changing the Moon's direction and keeping it moving in a circular orbit.
Example 3: Speed and orbital distance
A satellite moves to a higher orbit further from the Earth. How does its orbital speed change? At a greater distance, a stable orbit requires a slower speed, so the satellite in the higher orbit travels more slowly than it did in the lower orbit.
Example 4: Choosing an orbit
Why is a geostationary orbit useful for a communications satellite? A geostationary satellite takes 24 hours to orbit and stays above the same point on the Earth's surface. This means a ground dish can point at a fixed position without tracking the satellite, making it ideal for continuous communications and television signals.
Common mistakes and how to avoid them
A very common error is thinking a moving object needs a force in the direction of motion to keep it moving in orbit. In fact, the force (gravity) acts towards the centre, changing the object's direction, not driving it forwards.
Students often say gravity makes an orbiting object fall towards the planet. Gravity does pull it towards the centre, but because the object is also moving sideways at the right speed, it keeps missing and stays in orbit rather than falling in.
Another mistake is thinking all satellites travel at the same speed. Closer orbits require faster speeds; further orbits are slower. The speed must match the distance for a stable orbit.
When describing a geostationary orbit, remember it takes 24 hours and stays above the same point on the equator. Do not confuse it with a low polar orbit, which is faster and scans the whole surface.
Finally, remember that a change in direction is a change in velocity, so a resultant force (the centripetal force, provided by gravity) is needed even when the speed is constant.
Exam technique for "The Solar System, satellites and orbital motion"
Be ready to explain that gravity provides the centripetal force that keeps objects in orbit, acting towards the centre and changing the object's direction. This is a key idea worth stating clearly.
For orbital-speed questions, remember that closer orbits are faster and further orbits are slower, and that a stable orbit at a given distance needs a particular speed. If the speed changes, the orbit changes.
Distinguish natural and artificial satellites, and know the difference between geostationary orbits (24 hours, fixed above one point, for communications) and low polar orbits (faster, for weather and imaging). Use precise terms — orbit, gravity, centripetal force, geostationary — throughout.
Quick revision summary
- The solar system: the Sun, planets, dwarf planets, moons, asteroids and comets, held in orbit by the Sun's gravity.
- Objects stay in orbit because gravity provides a centripetal force acting towards the centre, continually changing their direction.
- A change in direction is a change in velocity, so a force is needed even at constant speed.
- For a stable orbit, the speed must match the distance: closer orbits are faster, further orbits are slower.
- Natural satellites are moons; artificial satellites are human-made (communications, weather, GPS, science).
- A geostationary orbit takes 24 hours and stays above the same point (communications); low polar orbits are faster and scan the surface (weather/imaging).