A celestial body orbiting the sun (though some may be ejected from the solar system by planetary perturbations) that displays (at least during a portion of its orbit) some diffuseness and/or a "tail" of debris that points generally in the anti-solar direction. Both the diffuseness (generally called a coma) and the tail are composed of gas and/or dust of various atomic or molecular compositions, as is ascertained by spectroscopy. The coma and tail material come from a much smaller nucleus that is usually invisible due to the bright surrounding coma activity. Close-up pictures of a cometary nucleus did not occur until spacecraft fly-bys of Halley's comet in 1986. A more detailed explanation of what a comet is may be found in the Press Information Sheet on comet C/1995 O1 (Hale-Bopp).