What colour are comets when they are not near the Sun?

Do comets look as vibrant when they’re not getting a helping hand from the Sun?

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Comets are black before they near the Sun but when they approach it, they burst into bright colours just like comet ISON

Comets are black before they near the Sun but when they approach it, they burst into bright colours just like comet ISON

Asked by Sam Harvey

The heart, or nucleus, of a comet is a collection of frozen water and gases as well as other carbon-based materials. As a result, comets far away from our Sun are effectively black since they have one of the lowest albedos – a measure of how much light they reflect – of any object we have observed.

As a comet gets closer to the Sun, some of these frozen gases sublimate creating the coma – the envelope of atmosphere that surrounds a comet. These gases can reflect sunlight and turn our dark object into a bright, yellow-white body. One of the two tails a comet produces, the ion tail – a collection of charged particles pushed away by the solar wind – will begin to glow with a blue tint.

Answered by Josh Barker from the National Space Centre

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