Comet exploration spacecraft, computer artwork. The unmanned probe (upper frame) is approaching a comet, a body of ice and dust from the outer parts of the solar system. Comets are studied because they contain material from the early stages of the formation of the solar system. Several probes have been launched to study comets. The Stardust probe has collected samples of dust from Comet Wild-2, and will return to Earth in 2006. The Rosetta probe is another mission to study a comet, and it should reach its target in 2015.