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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. A comet’s orbit is highly elliptical, whereas a planet’s orbit is almost circular (or only slightly elliptical).
2. A comet’s orbit extends far beyond the Solar System, reaching much greater distances from the Sun than any planet’s orbit.
3. A comet moves fastest when it is closest to the Sun because the orbital radius is smallest.
4. The reduction in gravitational potential energy as it approaches the Sun is converted into kinetic energy, increasing its speed.
2. A comet’s orbit extends far beyond the Solar System, reaching much greater distances from the Sun than any planet’s orbit.
3. A comet moves fastest when it is closest to the Sun because the orbital radius is smallest.
4. The reduction in gravitational potential energy as it approaches the Sun is converted into kinetic energy, increasing its speed.
Examiner tips
- Use the exact terms ‘highly elliptical’ and ‘almost circular’ to match the mark scheme. Include the distance comparison. Explain the speed change with the potential‑to‑kinetic energy conversion. Keep each point brief and separate.
Common mistakes
- Confusing the shape of the orbit with the speed; writing that the orbit is ‘more circular’ for comets. Using vague phrases like ‘different shape’ without specifying elliptical vs circular. Failing to mention the conversion of potential energy to kinetic energy when explaining speed.
Mark scheme (4 marks)
- A comet has a more elliptical (oval-shaped) orbit whereas a planet's orbit is approximately circular / less elliptical.
- A comet's orbit often reaches far beyond the Solar System / extends much further from the Sun than a planet's orbit.
- A comet travels faster when it is closer to the Sun because its orbital radius decreases / gravitational potential energy decreases.
- The decrease in gravitational potential energy is converted to kinetic energy, so the comet's orbital speed increases.
Key terms in this question
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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