Vacations in the us for If we use the distance from Earth to the Sun as a measure Astronomical Unit or AU, most comet nuclei react to the Sun’s heat when they are within AU roughly million km of the Sun. However, some comets, like Hale-Bopp, which have a high proportion of exotic ices like carbon monoxide, begin producing fountains of gases some AU from the Sun! When active comets are the same distance from the Sun as Earth, their nuclei have jets and are surrounded by a coma consisting of gas, dust, and rocks and they may have a gas tail and a dust tail. Comets are most productive when closest to the Sun, a point called their perihelion from the Greek peri, meaning near, and helios, meaning Sun. As a comet moves away from the Sun, it will react less and less to it and eventually will cease reacting at all. Most comet nuclei are between and km in diameter, although some are considerably larger than this. Hale-Bopp is reckoned to be about ? km in diameter. The Great September Comet of is believed to have had a nucleus of km in diameter. Vacations in the us 2016.