P3aL5  Satellites

Key Words

Artificial satellite - a satellite that has been made by scientists and launched into orbit on a rocket.

Centripetal force - a force acting towards the centre of the circle.

Geostationary orbit - orbits the Earth once every 24 hours.

Gravitational force - force due to gravitational attraction.

Natural satellite - a moon or lump of rock that orbits a planet.

Orbit - path of a satellite around a planet.

Polar orbit - an orbit that passes over the poles.

Speed - the rate at which distance is covered.

Test Yourself

Homework
Physics GCSE
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Grade E

Satellites are objects that orbit around a larger body.  Artificial satellites are sent up on rockets into orbit to carry out the following jobs:

  • Communications;

  • Global positioning system (GPS) for navigation;

  • Monitoring of weather;

  • Military (or spy) satellites;

  • Research.

All of these have solar panels on to provide the electrical energy and all have a means of sending back signals to ground stations (without which they are not much use).

 

Natural satellites can be things like the Moon or just large rocks that are caught in orbit.  Mars' moons are just that.

 

Artificial satellites are put into two main kinds of orbit:

  • Polar orbit, in which the satellite goes round the Earth several times a day, sweeping the entire surface as the Earth rotates;

  • Geostationary orbit in which the satellite goes round once every 24 hours (23 h 56 min to be precise), so that it remains above the same point all the time.  They are used for telecommunication and broadcasting.  Your satellite dish points towards the ASTRA satellite.

 

Grade C

Orbits occur because of gravity, not because there is no gravity.  Where satellites orbit the Earth, the acceleration due to gravity is between about 2 to 9 m/s2.  So there is still an appreciable pull.

 

An orbiting satellite is actually falling to the Earth all the time, except it misses it.  Let's look at this in more detail.

 

Imagine a large gun on a high mountain.  It fires a shell which travels a certain distance.

 

 

If the gun is more powerful, the shell travels further.

 

In fact the surface of the Earth is, of course, curved.  So the range is a little bit further still.

 

 

Now suppose we have a higher mountain and a more powerful gun.  This time the shell travels even further.  It follows the curvature of the Earth until it finally hits the ground.  Gravity is acting towards the centre of the Earth.  The gravity pulls the shell down until it eventually hits the ground, quite a long way from the gun.

 

 

If the gun is powerful enough, the shell will leave the barrel at such a speed that it never actually hits the ground.  It goes around the Earth in orbit.  (The label should say "massive gun...")

 

 

So an orbiting satellite is constantly falling to the Earth.  However it travels at such a speed that it always misses the Earth.  Since space is a vacuum, there is no friction, so the satellite does not slow down.

 

If the satellite is slowed down, by firing retro-rockets, it will fall to the Earth's surface.  This procedure is vital for getting cosmonauts back safely.

 

If a satellite is accelerated to a higher speed, it will move into a higher orbit.  At a high enough speed it will leave the orbit altogether. 

 

Space controllers use the gravity of planets to accelerate space probes to very high speeds.

 

Grade A

There is a project that uses a pair of satellites 220 km apart to detect very small differences in the Earth's gravitational force.  As gravity changes, the satellites change orbits slightly, which is detected by distance measuring equipment that is accurate to a micrometre (1 × 10-6 m).  Their speed is also measured with GPS.