Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of kilometers.
Featured Mission: Rosetta
Europe's Rosetta mission is on a 10-year mission to explore comet 67P/Churyumov-Gerasimenko. The spacecraft will orbit the comet and study it as it approaches the Sun and deploy a lander to make the first landing on a comet.
Read More About Comets