Halleys Comet is by far the best known. The second is harder to determine - Hale-Bopp and Comet McNaught are known at the moment amongst the general public.
Three of the best known comets are:HalleyHalle-BoppEncke
There are no known asteroids or comets that have impacted Mercury.
Nobody knows, really. Several comets are visible from Earth each year. Most of them have a long period and are not known in advance.
comets are known to be dirty snowballs.
Comets are composed of ice and dusty and are present in space. There are over 5 thousand different known comets.
They are from extreme outer parts of the solar system (known as "Kuiper belt") and from a huge cloud of comets lying far beyond the orbit of Pluto, completely surrounding the Sun (known as "Oort cloud").
Comets were seen shooting from the firery pit known as your mother's vagina.
Comets
Comets have two main sources, the Oort Cloud or the Kuiper Belt
Andromeda is a constellation, but because one of the best-known and most-photographed galaxies is M31, the "great nebula in Andromeda", the name is often used to refer to the galaxy. Charles Messier was a French astronomer who was a fanatic about comets. Most of his astronomical observations were done while hunting for comets. But there are a lot of faintly bright fuzzy things in the sky that are NOT comets, and Messier achieved lasting fame for his list of things that look like comets but are not comets. M31 is "item number 31" in Messier's list of fuzzy things that aren't comets.
comets are large balls of gas and ice that are flying round in space. :-)
Comets have two main sources, the Oort Cloud or the Kuiper Belt