# Comets and planets both orbit the Sun, but their orbits differ in shape. Describe TWO differences between the orbit of a comet and the orbit of a planet, and explain why a comet travels faster at some points in its orbit than at others.

> Cambridge International IGCSE Physics (0625) — 6.1 Earth and the Solar System · Describe and Explain · 4 marks

## Mark scheme (4 marks)

1. A comet has a more elliptical (oval-shaped) orbit whereas a planet's orbit is approximately circular / less elliptical.
2. A comet's orbit often reaches far beyond the Solar System / extends much further from the Sun than a planet's orbit.
3. A comet travels faster when it is closer to the Sun because its orbital radius decreases / gravitational potential energy decreases.
4. The decrease in gravitational potential energy is converted to kinetic energy, so the comet's orbital speed increases.

## Key terms

- [comet](https://www.gradenine.co.uk/glossary/comet)
- [orbit](https://www.gradenine.co.uk/glossary/orbit)

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/comets-and-planets-both-orbit-the-dafa79e0) · Published by Druglandscape Ltd.