They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed. ... More about comets

Up until about the middle of the twentieth century, astronomers still possessed an imperfect notion of how and where asteroids and comets originally formed.

Subject: How do comets form? ... How do comets, asteroids, and meteors form? Please explain the difference between them . Also how do the ones that have an orbit, if that is the correct term, achieve it?

Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. ... For this reason, comets are said to be short-lived, on a cosmological time

Up until about the middle of the twentieth century, astronomers still possessed an imperfect notion of how and where asteroids and comets originally formed.

A TRIP THROUGH TIME ON DUSTY COMETS ... Comets are frozen relics from the solar nebula, a cloud of ... As the comets formed, dust from the nebula was incorporated into the ices that comprise a comet.

Missions to Asteroids ... Current and Past Missions ... Asteroid Fact Sheet

Comet Hale-Bopp May Have Formed Near Neptune ; By Greg Clark; Staff Writer; posted: 08:52 am ET; 05 June 2000; ... Scientists aren't certain where comets come from, but until now,

it is composed of sodium gas (11). Following so many new findings, comet theorists are completely rethinking how comets are formed and what they contain.

Comets are born of fire as well as ice, the first results from the US space agency ... Comets formed in the cold, outer-reaches of the early Solar System, and were never exposed to such extreme heating.