A low orbit gives detail and a short signal delay, while a high orbit gives wide coverage and a slower trip round. At lift-off, a rocket needs thrust greater than its weight.
This lesson is part of space technology within the SPM Science guide. It builds on explaining satellite functions.
How do low and high orbits differ?
The table uses original, rounded values for one exercise. Take the speed of radio waves as 3 × 10⁸ m/s.
| Satellite P | Satellite Q | |
|---|---|---|
| Height above Earth | 600 km | 36 000 km |
| Time for one orbit | 1.6 h | 24 h |
| Orbits per day | 24 ÷ 1.6 = 15 | 24 ÷ 24 = 1 |
| One-way signal delay | 600 000 ÷ 3 × 10⁸ = 0.002 s | 36 000 000 ÷ 3 × 10⁸ = 0.12 s |
Satellite P sees a small patch of ground in great detail and passes over a place several times a day. Satellite Q is a geostationary satellite that stays above one region and covers a much larger area, but the signal delay is 60 times longer.
Worked example: choose an orbit
A mapping agency wants detailed images of a city every few hours. Which satellite suits it?
- Need: detail and frequent views.
- Choice: Satellite P, the low orbit.
- Reason: it is close to the ground, so images show fine detail, and it makes 15 orbits each day, so it returns often. Satellite Q would cover a large area but with less detail and only one orbit a day.
The mistake that costs marks
The common slip is to write that a higher orbit means a faster satellite, because it is “further away and moving more”. The data shows the opposite, since Q takes 24 h and P takes 1.6 h.
| Step | Wrong | Right |
|---|---|---|
| Height and speed | Higher is faster | Higher takes longer per orbit |
| Detail | Higher sees more detail | Lower sees more detail |
| Coverage | Lower covers more | Higher covers more ground |
| Delay | Same at all heights | Longer for a higher orbit |
The fix is to read the table column by column before writing a comparison.
How does a rocket leave the ground?
The exhaust gases are pushed downward, and the rocket is pushed upward with an equal force. That upward force is the thrust.
Take a rocket of mass 500 000 kg with g = 10 m/s². Weight = 500 000 × 10 = 5 000 000 N. If the thrust is 7 500 000 N, the net force is 7 500 000 − 5 000 000 = 2 500 000 N upward. The acceleration is 2 500 000 ÷ 500 000 = 5 m/s², assuming the mass stays constant for that instant.
Check yourself
A rocket of mass 200 000 kg has a thrust of 2 600 000 N. Take g = 10 m/s². Can it lift off? Find its acceleration.
Answer
Weight = 200 000 × 10 = 2 000 000 N. The thrust of 2 600 000 N is greater than the weight, so it can lift off.
Net force = 2 600 000 − 2 000 000 = 600 000 N. Acceleration = 600 000 ÷ 200 000 = 3 m/s². Assumption: the mass is constant at that instant.
What to study next
Continue with reading space-technology data, then test the cluster with the space technology practice set.
If you want a teacher to go through orbit and launch comparisons with you, see online one-to-one Science tuition.