The European Space Agency's Rosetta spacecraft has been flying toward its target, Comet 67P/Churyumov-Gerasimenko, for 10 years. This month, at long last, it arrives. Astronomers think that comets were the building blocks of our solar system, and by studying one up close, they can better understand these planetary raw materials. Rosetta will also document how a comet's tail forms by watching its target shed dust and gas as it nears the sun. But that's only if its lander can do something never before attempted: land on a comet hurtling through space.