Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (or atmosphere) and/or a tail - both primarily from the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.
Comets leave a trail of debris behind them. If the comet's path crosses Earth's path, then at that point may be meteor showers as the Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of the comet Swift-Tuttle. Halley's comet is the source of the Orionid shower in October.
Comets are distinguished from asteroids by the presence of a coma and/or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids.There are a reported 3,354 known comets as of November 2007, of which several hundred are short-period. This number is steadily increasing. The number of naked-eye comets averages to roughly one per year, though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it is often considered a Great comet.
Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible with the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma temporarily greatly increases in size. This happened in 2007 to Comet Holmes.