Space Exploration

Astronomical Terms and Our Solar System

  • The universe consists of many galaxies separated by empty space.
  • A galaxy is a large cluster of stars (eg the Milky Way)
  • A star is a large ball of matter that is undergoing nuclear fusion and emitting light and other forms of electromagnetic radiation. The sun is a star.
  • The Sun and many other stars have a solar system. A solar system consists of a central star orbited by planets.
  • A planet is a large ball of matter that orbits a star (eg Earth or Jupiter). Planets do not emit light themselves.
  • An exoplanet is a planet existing around another star, outside of our solar system.
  • Many planets have moons which are natural satellites. A moon is a lump of matter that orbits a planet eg The Moon orbits the Earth or Deimos and Phobos orbit Mars.
  • A dwarf planet is an object that orbits a star but is not large enough or roughly spherical enough to be classed as a small planet.
  • A asteroid is an orbiting object which is even smaller than a dwarf planet.


In space terms, a satellite is generally defined as an object which orbits a planet. The Moon is Earth's one natural satellite, but we also have thousands of artificial satellites that have been put into space orbiting the planet.

An artificial satellite orbits the Earth by travelling at a high velocity at a set distance above the planet. The satellite is attracted by the Earth's gravity and the manner in which it 'falls' enables it to orbit the planet.

One of the most common types of satellite is one that travels around the Earth at the same rate as the Earth rotates on its axis.

This means that the satellite appears to 'hover' above the same point (on the equator) on the Earth's surface. A receiver can be pointed at this satellite, allowing for a link for information to pass to be established. These satellites are known as geostationary and have to be placed at a height of 36,000 km and at a velocity that means the satellite takes 24 hours to complete one revolution of the Earth.

When a satellite is in a lower orbit, such as a weather satellite, it has to travel at a greater velocity in order to remain in orbit. On a clear night you can see these satellites pass by with the naked eye.

Conversely, if a satellite is at a higher orbit it needs to travel at a lower velocity in order to remain there.

Many aspects of our day-to-day life is dependent upon satellites:

  • Global Positioning Satellites (GPS) allow us to use a phone or other device, such as a sat nav, to determine our location to within a few metres.
  • Television networks rely heavily on satellites to transfer signals from one area to another eg live reporting from major events.
  • Our weather forecasts are based upon data taken from satellite systems which have monitored the area…


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