The Solar System, Satellites and Orbital Motion — AQA GCSE Physics (Separate)
The solar system is held together by gravity, which also keeps satellites and planets in orbit.
The solar system
Our solar system consists of:
- the Sun (a star) at the centre,
- the eight planets and dwarf planets that orbit the Sun,
- the moons (natural satellites) that orbit planets, and
- asteroids and comets.
The solar system is part of the Milky Way galaxy. Gravity from the Sun keeps the planets in orbit.
Orbital motion
For an object to stay in a circular orbit, there must be a force pulling it towards the centre — for orbits this is gravity (a centripetal force).
An orbiting object moves at a constant speed but its velocity is constantly changing, because its direction is always changing. A changing velocity means it is accelerating (towards the centre), even at constant speed.
Stable orbits and speed
For a stable orbit at a fixed radius, there is a specific speed. If the speed changes, the radius must change to maintain a stable orbit:
- a faster satellite orbits at a smaller radius,
- a slower satellite orbits at a larger radius.
Satellites
- Natural satellites are moons.
- Artificial satellites are used for communications, weather and GPS.
Exam tips
- Gravity provides the force that keeps planets and satellites in orbit.
- In a circular orbit, speed is constant but velocity changes (direction changes) → accelerating.
- For a stable orbit, faster = smaller radius, slower = larger radius.
- The Sun is at the centre; the solar system is in the Milky Way.