Comet Halley. Coloured ultraviolet image showing the content of neutral hydrogen in the coma of Comet Halley. The image was taken on March 13th 1986 from a sounding rocket at 300 km above the Earth. The coma is an almost spherical halo of gas and dust, seen here as coloured concentric shells, surrounding the body (lower centre, blue) of the comet. The coma develops when a comet is in the vicinity of the Sun and it might extend for tens of millions of kilometres. The colour code underlines the variations in the gas density which decreases the further the distance from the cometary body.