Comets are small Solar System bodies which display episodes of emission and outgassing during at least part of their orbits, due to the existence of volatile molecules at or near their surfaces, which sublimate when heated by solar radiation. They are thought to be the icy remnants of the early phases of the outer planets formation. They are among the most dynamically complex objects in the Solar System, entering the inner planetary region either from a distant repository located at about 50 000 au from the Sun or diffusing chaotically from the trans-Neptunian belt.
Cometary collisions with the planets may be responsible for a fraction of the water present on Earth, and for the icy reservoirs that are thought to exist at the bottom of permanently shadowed craters on the Moon.
Near-Earth Comets (NECs) are define as comets having a period of revolution shorter than 200 years and a perihelion distance smaller than 1.3 au, thus allowing close approaches to the Earth. Their number is presently around 100, slowly increasing with time (1 or 2 entries per year). The chances for small comets to actually impact the Earth are in general significantly lower than those of near-Earth asteroids.
The NEO Coordination Centre has adopted as authoritative source of data the comet catalogue maintained by JPL and kindly made available to ESA.
Comet C/2011 W3 (Lovejoy) over Santiago de Chile
Credits: Y. Beletski/ESO
Comet C/1995 O1 (Hale-Bopp), 1997
Credits: D. Koschny