Explain why artificial satellites in low Earth orbit must be given an occasional boost from their engines to maintain their orbital altitude.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Low Earth orbit satellites travel at altitudes between approximately 200 km and 2000 km above Earth's surface. At these altitudes, a very thin atmosphere still exists.
Model answer (4 marks)
The thin atmosphere at 200–2000 km exerts a drag force on the satellite.
The drag removes kinetic energy, so the satellite’s speed decreases.
With a lower speed the balance between gravity and centripetal force is lost, and the satellite falls to a lower orbit.
An engine boost increases the speed back to the required orbital velocity, restoring the original altitude.
The drag removes kinetic energy, so the satellite’s speed decreases.
With a lower speed the balance between gravity and centripetal force is lost, and the satellite falls to a lower orbit.
An engine boost increases the speed back to the required orbital velocity, restoring the original altitude.
Examiner tips
- Use the phrase ‘drag force’ and link it to loss of kinetic energy. Show the sequence: drag → speed loss → lower orbit → engine boost restores speed.
- Mention that the boost must be timed to counteract the gradual decay of the orbit.
Common mistakes
- Confusing drag with gravitational pull. Forgetting to state that the boost restores the correct orbital speed. Using vague terms like ‘air resistance’ without linking to energy loss.
Mark scheme (4 marks)
- The thin atmosphere exerts a drag force / air resistance on the satellite
- The drag force causes the satellite to lose (kinetic) energy / slow down
- As the satellite slows, gravitational force is no longer balanced / gravity pulls the satellite to a lower altitude
- Engine boost increases the satellite's speed / restores the correct orbital speed to maintain the original altitude
Key terms in this question
low Earth orbit · orbital altitude
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