Satellites and orbits (AQA GCSE Physics): Revision Notes
Satellites and orbits
What are satellites?
Satellites are objects that move around other, larger objects in space. They follow curved paths called orbits.
Gravity is the force that keeps satellites moving in their orbits. Without gravity, satellites would fly off in straight lines into space.
The relationship between gravity and orbital motion is fundamental to understanding how all objects move in space. Gravity acts as the invisible "rope" that keeps satellites moving in curved paths rather than flying away in straight lines.
Natural satellites
Natural satellites formed by themselves through natural processes over millions of years. They were not made by humans.
Examples of natural satellites:
- The eight planets in our Solar System - they orbit the Sun
- Moons - they orbit planets (like our Moon orbits Earth)
These natural satellites have been moving in their orbits for billions of years.
Artificial satellites
Artificial satellites are manufactured objects that humans have built and launched into space using rockets.
Examples of artificial satellites:
- GPS satellites in geostationary orbits around Earth - used for sat nav and positioning
- Weather satellites in low polar orbits around Earth - used for forecasting, military purposes, spying, or observing Earth
- Space probes sent to orbit and study the Sun, other planets, or asteroids in our Solar System
Real-World Example: GPS Constellation
The GPS system consists of at least 24 satellites orbiting Earth at an altitude of about 20,200 km. These satellites are positioned so that at least four are visible from any point on Earth's surface at any time, allowing your phone or GPS device to determine your exact location.
These satellites serve many useful purposes for life on Earth.
How satellites stay in orbit
All satellites - both natural and artificial - follow circular or near-circular orbits.
The gravitational force acting on a satellite stays the same at every point in its circular orbit. This happens because the distance (radius) from the centre stays fixed.
Understanding the difference between speed and velocity is crucial for orbital mechanics:
- Speed is how fast something is moving
- Velocity includes both speed AND direction
In circular orbits:
- Speed stays constant - the satellite doesn't speed up or slow down
- Velocity constantly changes - because the direction of movement is always changing
- The satellite is constantly "falling" towards the centre but never reaches it
Motion in the solar system
Different objects move in orbits based on their mass:
- Planets have much smaller masses than the Sun, so they orbit the Sun in near-circular paths
- Moons have smaller masses than planets, so they orbit planets
- Artificial satellites have much smaller masses than planets or moons, so they can be made to orbit either planets or moons
The closer an object is to the Sun, the faster it moves in its orbit. This means:
- Earth moves faster than Jupiter
- Jupiter moves faster than Saturn
Worked Example: Orbital Speeds in Our Solar System
Comparing orbital speeds around the Sun:
- Mercury (closest to Sun): ~47.9 km/s
- Earth: ~29.8 km/s
- Jupiter: ~13.1 km/s
- Neptune (furthest): ~5.4 km/s
Notice how the speed decreases as distance from the Sun increases!
Remember!
Key Points to Remember:
- Gravity provides the force that keeps all satellites in circular orbits
- Natural satellites (like planets and moons) formed naturally over millions of years
- Artificial satellites are human-made objects launched by rockets for specific purposes like GPS and weather monitoring
- In circular orbits, speed stays constant but velocity constantly changes direction
- Objects closer to the Sun move faster in their orbits than objects further away