# Neptune orbits the Sun at a much greater distance than Mercury. Explain why Neptune's orbital speed is much lower than Mercury's orbital speed.

> Pearson Edexcel International GCSE Physics (4PH1) — 1.4 Astronomy · Explain · 4 marks

> Mercury orbits the Sun at an average distance of approximately 58 million km, whilst Neptune orbits the Sun at an average distance of approximately 4 500 million km.

## Mark scheme (4 marks)

1. Gravity (a non-contact force) from the Sun acts on both planets, keeping them in orbit
2. The gravitational force (pull) on Neptune from the Sun is weaker/smaller than on Mercury because Neptune is further away
3. The gravitational force provides the resultant force that causes the planet to accelerate (change direction) / keeps the planet in its circular orbit
4. A smaller resultant/gravitational force means a lower orbital speed is sufficient to maintain Neptune's orbit at its greater distance

## Key terms

- [orbital speed](https://www.gradenine.co.uk/glossary/orbital-speed)

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Source: [GradeNine](https://www.gradenine.co.uk/q/neptune-orbits-the-sun-at-a-e2201e67) · Published by Druglandscape Ltd.