The comet’s tail
Comets are “dirty snowballs” that spend most of their life at the edges of the Solar System. Due to gravitational interaction with the gas planets, their orbit can be subjected to changes that bring it close to our star. If the distance is such that solar radiation becomes rather intense, part of the ice melts and the nucleus expels gas and dust in all directions forming a gaseous envelope called “coma”, whose size can be even 10 times the size of the Earth.
As a result of the interaction with solar radiation, or more precisely, as an effect of the pressure of electromagnetic radiation caused by the solar photons colliding with the dust particles, the matter of the coma extends into the characteristic tail. This trail of dust and gas, which can extend for hundreds of millions of kilometres, always faces away from the Sun, whether the comet is getting closer to or further away from our star. The combination of the movement of the comet and the radiation pressure results in the tail’s slightly curved shape.
Moreover, solar radiation partly ionises the gas of the coma which becomes plasma made up of particles, atomic nuclei and free electrons that are electrically charged. The magnetic field generated by the solar wind directs and channels the plasma particles into a tail that has a typical blue colour that remains distinct from the dust. It is for this reason that comets are characterised by two distinct tails.