Comet or asteroid: what is the difference?

Because looking up the difference between a comet or asteroid drives me nuts.

They are both:

  • Small chunks of “stuff” that hang around in the solar system. Most describe some sort of orbit, but some pass once and we never see them again.
  • The vast majority¬†can’t be seen with the naked eye
  • They can be made from variety of materials, including rock, ice, carbon-rich materials or nitrogen-rich materials
  • They are much smaller than our moon, have very little gravity and therefore an irregular shape.

Comets are:

  • More likely to be made from water ice or have a thick coating of water ice.
  • The evaporating ice is what gives them an atmosphere that is not bound by the gravity of the comet (because the comet is too small). The water vapour gives them their tail.
  • They originate in the outer solar system.

Asteroids are:

  • Mostly rocky
  • They hang around in the inner solar system (anywhere up to Jupiter) which is also where they are thought to have been formed

But:

  • Some asteroids have also been found to outgas water, while some comets have passed the sun so many times that whatever water ice they had on the surface has gone, leaving the rocky core.

So yeah, there isn’t a clear line to separate¬†between the two and no clear definition. For the sake of convenience, some people use the word asteroid for bodies that move in the inner solar system. The are more likely to be rocky.

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