A comet is composed of ice, gas and dust, with an 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.
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.