How big is a comet nucleus ?

How big is a comet nucleus ?
Comet nuclei come in a range of sizes. Most are between a few and ten kilometres in size while the largest ones have an average size of
about 20 km. The comet nucleus is nearly 50 % water ice and therefore the typical density may be around 1 gm/c.c. Thus an average comet has a mass that is billion times smaller than that of the earth. The nucleus of comet Halley was found to be peanut shaped by the spacecraft Giotto, 15 km long and 8 km cross section. Almost all the comet nuclei whose shapes have been determined, have a similar shape.

Where do comets come from ?

Where do comets come from ?
Trillions of comet nuclei are stored at the edge of our solar system. This cloud of
comets, called the Oort cloud in honour of the Dutch astronomer who proposed this idea first. It is thought to be at a distance of more than one trillion kilometers (one followed by twelve zeros) from the earth and the sun. Comet nuclei may also be found in a second location that is much closer viz., the Kuiper belt, a region between the
orbits of Neptune and Pluto. About a billion comets may be stored here. Notice that both locations are very far away from the sun. At the low temperatures found at these locations comet nuclei remain frozen mud balls! They are so tiny and so dark that they cannot be detected even with the most powerful telescopes at these distances.