The Solar System (OCR Gateway GCSE Physics)

• The Sun lies at the centre of the Solar System
  • The Sun is a star which makes up over 99% of the mass of the solar system

• There are 8 planets which orbit the Sun
  • There are 4 rocky planets: Mercury, Venus, Earth and Mars
  • There are 4 gas planets: Jupiter, Saturn, Uranus and Neptune
  • The gravitational field around planets is strong enough to have pulled in all nearby objects with the exception of natural satellites
  • We say the planets have 'cleared their orbits'
• There are also an unknown number of minor (dwarf) planets which orbit the Sun 
  • The gravitational field around a minor planet is not strong enough to have pulled in nearby objects
  • Therefore there may be other objects in the same orbit as the minor planet

• Some planets have moons which orbit them
  • Moons are an example of natural satellites

• Artificial satellites are man-made and can orbit any object in space
  • The International Space Station (ISS) orbits the Earth and is an example of an artificial satellite

• Asteroids and comets also orbit the sun
• An asteroid is a small rocky object which orbits the Sun
  • The asteroid belt lies between Mars and Jupiter

• Comets are made of dust and ice and orbit the Sun in a different orbit to those of planets
  • The ice melts when the comet approaches the Sun and forms the comet’s tail
  • The tail of a comet always points away from sun
  • The tail of a comet does not indicate the direction of travel of the comet

The objects in our solar system

Orbits

• There are many orbiting objects in our solar system
  • They each orbit a different type of planetary body

Orbiting Objects or Bodies in Our Solar System Table

• A smaller body or object will orbit a larger body
• In order to orbit a body such as a star or a planet, there has to be a force pulling things towards that body
  • Gravity provides this force

• The gravitational force exerted by the larger body on the orbiting object is always attractive
• Therefore, the gravitational force always acts towards the centre of the larger body
• The gravitational force is the centripetal force as it will cause the body to move and maintain in a circular path

Gravitational attraction causes the Moon to orbit around the Earth

The Orbits of Planets

• There are several similarities in the way different planets orbit the Sun:
  • Their orbits are all slightly elliptical (stretched circles) with the Sun at one focus (approximately the centre of the orbit)
  • They all orbit in the same plane
  • They all travel the same direction around the Sun

• There are also a few differences:
  • They orbit at different distances from the Sun
  • They orbit at different speeds
  • They all take different amounts of time to orbit the Sun

  

  Orbit of planets around the Sun

The Orbits of Moons

• Moons will orbit planets in a circular path
• Some planets will have more than one moon
• The closer the moon is to the planet:
  • The shorter the time it will take to orbit
  • The greater the speed in the orbit

The Orbits of Artificial Satellites

• A satellite needs to travel at a specific speed to maintain a circular orbit at a particular distance from the object
• If the speed of the satellite is too big:
  • The radius of the orbit will increase and the satellite will spiral into space
  • This is because the gravitational attraction cannot provide enough force to keep it in orbit

• If the speed of the satellite is too small:
  • The radius of the orbit will decrease and the satellite will move towards the object it should be orbiting
  • This is because the gravitational attraction is too strong to maintain a constant orbital radius

Diagram showing how the speed of an artificial satellite affects its orbit

• If an artificial satellite is to change the radius at which it is orbiting then the speed at which it is travelling must change
• To maintain a stable orbit:
  • If the speed increases the radius must increase
  • If the speed decreases the radius must decrease
• There are two main positions for orbiting artificial satellites:
  • These are Geostationary and Polar orbits
  • Both geostationary and polar orbiting satellites are used for communicating information
• A carrier wave is transmitted from Earth to a satellite which receives a greatly attenuated signal
• The signal is amplified and transmitted back to the Earth at a different carrier frequency
• The different frequencies prevent swamping of the signal transmitted from the Earth to satellite station

Geostationary Satellites

• Geostationary satellites orbit above the Earth’s equator
• The orbit of a geostationary satellite is 24 hours
• The geostationary satellite orbits at a height of 36 000 km above sea level
  • This is much higher than polar satellites
• Geostationary satellites are used for radio and telecommunication broadcasting around the world
  • This is due to the high orbit, as the satellite can communicate with a large area of the surface of the Earth at once

Polar Satellites

• Polar satellites orbit around the Earth’s north and south poles
• Polar satellites have a much lower orbit than geostationary satellites,
  • This is at around 200 km above sea level
• Uses of polar satellite include monitoring the weather, military applications, and taking images of the Earth’s surface
• There is a much shorter time delay for signals compared to geostationary orbit signals
• The signals and images are much clearer due to the lower orbit
• However, more than one satellite is required for continuous coverage of the Earth's surface
  • This is due to the low orbit

Geostationary and polar orbits around the Earth