There is no internationally acception definition
of where space begins?


  • FAI – The International Aeronautical Federation, recognises the Karman line 60 miles above Earth’s surface as being the boundary between the atmosphere and space.
  • local space is known as the region around of our Solar System. (diagram)
  • space is not an empty vacuum – it is a near vacuum; each cubic kilometre contains a few drifting atoms of gas and dust.
  • in the region of our Sun – this interstellar medium consists of about 90% hydrogen, 9% helium and 1% dust. (diagram)
  • interstellar dust – tiny grains  composed mainly of silicate (silicon and oxygen) and graphite (carbon) as well as small amount of iron. (photo)
  • between galaxies – intergalactic medium consists mostly of ionized hydrogen – atoms of hydrogen gas that have had their electron stripped away.

How do we measure the distances to things in space?

  • space is so big – that miles becomes meaningless!
  • so that’s why people often use the word ‘astronomial’ to describe expectionally large number instead.
  • in local distances are often measured in AU (Astronomical Units); one AU is around 92 million miles as the average distance between the Earth and the Sun. (diagram)
  • interstellar space – usually measured in light years (ly); one light year is about 5.6 trillion miles (the distance light travels in one year.)
  • some astronomers prefer to measure large distance in parsecs (pc); one parsec equals 3.36 light years .


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