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D.1.15 (HL)—Atmospheric drag on orbits

Syllabus
First assessment 2025
Objective
—
Level
HL

Model Atmospheric Drag on an Orbit

HL only

Start with energy loss

Atmospheric drag opposes the satellite’s motion and removes mechanical energy from the orbit. The total orbital energy becomes more negative, so the satellite moves to a lower orbit. It does not simply slow while remaining at the same radius.

Explain the speed increase

For a circular orbit, v=GM/rv=\sqrt{GM/r}. As drag lowers rr, the new circular orbital speed is larger, so the satellite speeds up as it spirals inward even though drag is an opposing force at every instant.

Follow the feedback

At lower altitude the atmosphere is generally denser, and the higher orbital speed can increase the drag effect. Continued energy loss can therefore make the orbit decay further, eventually leading to re-entry, burning or impact depending on the body and conditions.

Common trap

Do not conclude that drag causes the final orbital speed to decrease merely because drag opposes motion. Distinguish the instantaneous force from the speed of the new lower circular orbit.

D.1.15 (HL) Exam Analysis

HL only

Assessment in practice

2–4 marks
How it is assessed

Questions ask for the effect of a small frictional force on orbital speed or explain the likely fate of a satellite in a decaying orbit.

Command terms

Suggest / State and explain

What earns marks

State that drag reduces total orbital energy, lowers the orbit, and leads to a higher circular speed at the smaller radius; then connect increasing density/speed to continued decay.

Watch for

Saying drag lowers speed without accounting for the lower circular radius, or omitting the reduction in total orbital energy.

Representative question

Question 1

[Maximum number: 4]

The satellite experiences a drag force due to the atmosphere of Earth. With reference to the results in (a) and (b)(i), state and explain the likely fate of this satellite.