UPSC Prelims 2011 Q3: Science & Technology – Space Technology & Astronomy
Satellites used for telecommunication relay are kept in a geostationary orbit. A satellite is
said to be in such an orbit when:
1. The orbit is geosynchronous.
2. The orbit is circular.
3. The orbit lies in the plane of the Earth’s equator.
4. The orbit is at an altitude of 22,236 km.
Select the correct answer using the codes given below:
- (a) 1, 2 and 3 only
- (b) 1, 3 and 4 only
- (c) 2 and 4 only
- (d) 1, 2, 3 and 4
Answer: (a) 1, 2 and 3 only
Explanation
What makes an orbit 'geostationary'
A satellite is geostationary when it appears fixed over one point on Earth's equator.
| Condition | Required? | Reason |
|---|---|---|
| 1. Geosynchronous | Yes | Its orbital period must equal Earth's rotation period (about 23 h 56 m) |
| 2. Circular | Yes | An elliptical orbit would change the satellite's speed and apparent position |
| 3. In the plane of Earth's equator | Yes | An inclined orbit would make the satellite drift north-south, not stay fixed |
| 4. At an altitude of 22,236 km | Not a separate condition | This altitude is the consequence of conditions 1-3 (the height needed for a geosynchronous circular equatorial orbit), not an independent requirement |
Geostationary vs geosynchronous:
| Term | Meaning |
|---|---|
| Geosynchronous | Period matches Earth's rotation; orbit can be inclined or elliptical |
| Geostationary | A special case of geosynchronous: circular + equatorial, so the satellite stays fixed overhead |
Answer: (a) 1, 2 and 3 only.
Why each option is right or wrong
- (a) CORRECT. Geosynchronous, circular and equatorial together define a geostationary orbit.
- (b) Incorrect. Circular is also required; a non-circular geosynchronous equatorial orbit would not keep the satellite fixed.
- (c) Incorrect. Geosynchronous and equatorial are both required, not just circular and a fixed altitude.
- (d) Incorrect. The 22,236 km altitude is a result of the other three conditions, not a fourth independent one.
Key point
Geostationary orbit = geosynchronous + circular + equatorial. The ~35,786 km (22,236 mile) altitude follows from these three, it is not a separate defining condition.
Remember
Geostationary = geosynchronous + circular + equatorial.
Common mistakes
- Treating the altitude figure as a defining condition rather than a consequence.
- Confusing geosynchronous (period matches Earth) with geostationary (also circular and equatorial).
- Forgetting that an inclined geosynchronous satellite still moves north-south in the sky.
Exam tip
A specific number (like an altitude) in an options list is often the 'odd one out' — a result, not a cause.
Sources
- NCERT Class XI Physics — Gravitation (satellites: geostationary orbit)
- NASA — What is a geostationary orbit?