Comets have historically induced a sense of awe and dread in humans. Comets are now understood to be bodies rather loosely held together. When they approach the Sun the increasing input of thermal energy causes the ices and dust that make up much of the comet to be ejected and form a huge 'fuzzy ball' or coma surrounding the much smaller parent body or nucleus. The effect of the solar wind, a stream of particles emitted by the Sun, is to produce the tails for which comets are famed.