A comet is composed of ice, gas and dust, and has a highly eccentric, elliptical orbit around the Sun. Comets become visible as they approach the Sun and can be much larger than they seem - the tails of some comets are over 1,000,000 km long. Examples include Halley's Comet and the Tago-Sato-Kosaka Comet.
Comets typically have orbital periods much longer than 1 Earth year, and are classified as either short-period or long-period comets, depending on whether the orbital period is less than or greater than 200 years.
In November 2014, the European Space Agency successfully landed a probe on a comet nucleus, the first time in history that this had been achieved. Philae sent back data for three days as it probed the surface of 67P/Churyumov–Gerasimenko.