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This lesson covers natural and artificial satellites, their orbits, and their uses, as required by the AQA GCSE Physics specification (4.8.1). This is a Physics-only topic. You need to understand the difference between natural and artificial satellites, the two main types of artificial satellite orbits (polar and geostationary), and how satellites are used for communication, observation, and navigation.
A satellite is any object that orbits a larger body due to gravitational attraction. Satellites can be natural (occurring naturally) or artificial (made and launched by humans).
| Type | Definition | Examples |
|---|---|---|
| Natural satellite | A celestial body that orbits a planet or other body naturally | The Moon (orbits Earth), Titan (orbits Saturn), Phobos and Deimos (orbit Mars), Europa (orbits Jupiter) |
| Artificial satellite | A human-made object placed into orbit around Earth or another body | International Space Station (ISS), GPS satellites, weather satellites, Hubble Space Telescope |
The Moon is Earth's only natural satellite. Key facts:
Exam Tip: When explaining why the Moon orbits Earth, always state that the gravitational attraction between the Earth and the Moon provides the centripetal force needed to keep the Moon in its roughly circular orbit. Do not simply say "gravity" without specifying what is attracted to what.
Humans have been launching artificial satellites since 1957, when the Soviet Union launched Sputnik 1 — the first artificial satellite. Today, there are thousands of active satellites orbiting Earth, serving many purposes.
To place a satellite in orbit, it must be accelerated to a high enough speed using a rocket. The satellite must reach a speed where its forward motion balances the gravitational pull of the Earth, resulting in a stable orbit. The required speed depends on the altitude of the orbit:
Once in orbit, no fuel is needed to maintain the orbit (in the absence of atmospheric drag). The satellite is in continuous free fall around Earth.
There are two main types of satellite orbit that you need to know for GCSE Physics:
A polar orbit satellite travels over both poles of the Earth in a low Earth orbit (typically 200–2,000 km above the surface).
| Feature | Detail |
|---|---|
| Altitude | Typically 200–2,000 km |
| Orbital period | Approximately 90 minutes to 2 hours |
| Orbital speed | Very fast (about 7.5–7.8 km/s) |
| Inclination | Passes over or near the North and South Poles |
| Coverage | As the Earth rotates beneath the satellite, the satellite eventually passes over the entire surface |
| Uses | Earth observation, weather monitoring, mapping, military surveillance, scientific research |
Because the Earth rotates beneath a polar orbit satellite, each orbit covers a different strip of the Earth's surface. Over the course of a day, the satellite can image the entire surface of the Earth.
A geostationary orbit satellite orbits the Earth above the equator at a specific altitude where its orbital period matches the Earth's rotation period (24 hours).
| Feature | Detail |
|---|---|
| Altitude | Approximately 35,786 km above the equator |
| Orbital period | Exactly 24 hours (same as Earth's rotation) |
| Orbital speed | About 3.1 km/s (slower than polar orbit) |
| Position | Remains above the same point on the Earth's surface at all times |
| Coverage | Fixed area — about one-third of Earth's surface |
| Uses | Communications (TV, telephone, internet), weather monitoring of a fixed region |
Because a geostationary satellite stays above the same point on Earth, a satellite dish on the ground can be pointed at it and remain fixed. This makes geostationary orbits ideal for communications and broadcasting.
Exam Tip: A geostationary satellite has an orbital period of 24 hours and orbits above the equator. These are the two key features — if you are asked to describe a geostationary orbit, always state both. A common mistake is to say "it doesn't move" — it does move, but it moves at the same rate as the Earth rotates, so it appears stationary relative to the ground.
| Feature | Polar Orbit | Geostationary Orbit |
|---|---|---|
| Altitude | Low (200–2,000 km) | High (35,786 km) |
| Period | About 90 min–2 hours | 24 hours |
| Speed | Fast (~7.5–7.8 km/s) | Slower (~3.1 km/s) |
| Position relative to Earth | Changes — passes over different areas | Fixed — stays above same point |
| Coverage | Can scan entire surface over time | Fixed region (about one-third of surface) |
| Main uses | Observation, mapping, weather | Communications, broadcasting |
| Number needed for global coverage | Several (but they scan the whole surface) | At least 3 (to cover the whole equator) |
graph TD
A["Satellite Orbits"] --> B["Polar Orbit (Low Earth Orbit)"]
A --> C["Geostationary Orbit"]
B --> D["Low altitude: 200-2,000 km"]
B --> E["Short period: ~90 min"]
B --> F["Passes over poles"]
B --> G["Uses: observation, mapping, weather"]
C --> H["High altitude: ~35,786 km"]
C --> I["Period: 24 hours"]
C --> J["Above equator, appears stationary"]
C --> K["Uses: communications, TV, weather"]
style A fill:#2c3e50,color:#fff
style B fill:#3498db,color:#fff
style C fill:#e74c3c,color:#fff
Artificial satellites serve many purposes:
Geostationary satellites relay television, telephone, and internet signals around the world. A signal is transmitted from a ground station up to the satellite (the uplink), the satellite amplifies and retransmits the signal back down to a receiving station on a different part of the Earth's surface (the downlink). Because the satellite remains in a fixed position relative to the ground, satellite dishes do not need to track the satellite.
The Global Positioning System (GPS) uses a constellation of about 24–32 satellites in medium Earth orbit (approximately 20,200 km altitude). A GPS receiver on the ground picks up signals from at least four satellites and calculates its position by measuring the time delay of the signals. GPS satellites are not geostationary — they orbit the Earth approximately every 12 hours.
Polar orbit satellites carry cameras and sensors to image the Earth's surface. They are used for:
Some satellites carry telescopes to observe the universe without the interference of Earth's atmosphere. Examples include the Hubble Space Telescope (visible and ultraviolet light) and the James Webb Space Telescope (infrared light).
Exam Tip: If asked about the advantages of placing a telescope in space rather than on Earth's surface, key points are: (1) no atmospheric distortion or absorption, (2) no light pollution, (3) can observe wavelengths (such as X-rays and certain infrared) that are blocked by the atmosphere. This links to the electromagnetic window topic covered later.
Several important principles govern satellite orbits:
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