The key relationship:
- Orbit closer to planet → need to move FASTER to stay in orbit
- Orbit further from planet → move SLOWER
- The Moon orbits slowly (~1 km/s); the ISS orbits fast (~7.7 km/s) — ISS is much closer!
Why? Closer = stronger gravity = need more speed to avoid falling in. Further = weaker gravity = less speed needed.
Orbital period and radius:
- Larger orbital radius = longer orbital period (takes longer to complete orbit)
- Mercury (closest to Sun) = 88 days; Neptune (furthest) = 165 years
This study notes covers Orbital Speed and Radius within Orbits for GCSE Physics. Revise Orbits in Space Physics for GCSE Physics with 14 exam-style questions and 10 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 8 in this topic. Use this study notes to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Orbits
What is a protostar?
Explain why a main sequence star remains stable (in equilibrium) for billions of years.
Quick recall flashcards
What is a polar orbit used for?
Mapping - the satellite orbits at ~800 km altitude covering the whole Earth as the planet rotates beneath it
What is Low Earth Orbit (LEO)?
Orbit at 200-2000 km altitude with ~90 minute orbital period. Used for ISS, imaging, and Earth observation