The "Giotto" and "Vega" missions gave planetary scientists their first view of Halley's surface and structure. Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as water, carbon monoxide, carbon dioxide and other ices) begin to sublimate from the surface of its nucleus. This causes the comet to develop a coma, or atmosphere, up to 100,000 km across. Evaporation of this dirty ice releases dust particles, which travel with the gas away from the nucleus. Gas molecules in the coma absorb solar light and then re-radiate it at different wavelengths, a phenomenon known as fluorescence, whereas dust particles scatter the solar light. Both processes are responsible for making the coma visible. As a fraction of the gas molecules in the coma are ionized by the solar ultraviolet radiation, pressure from the solar wind, a stream of charged particles emitted by the Sun, pulls the coma's ions out into a long tail, which may extend more than 100 million kilometres into space. Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus.