Over 10 years ago, the Rosetta spacecraft left Earth to begin a long, lonely journey toward a ball of ice and rock. That four billion mile trek finally ended today, capped off with a nail-biting finale where Rosetta's washing-machine-sized lander, Philae, became the first thing we humans have ever landed on a comet.
Rosetta's target was Comet 67P/Churyumov-Gerasimenko, a lumpy ball whose orbit loops between Jupiter and Earth. There's no way to get there in one straight shot, so Rosetta took a long, circuitous path, with four planetary flybys that gave it gravity assists.
The last portion of the journey, where Rosetta orbited Comet 67P itself and let its lander Philae go, was the most complicated and perilous, threatening to undo everything that came a decade before. Once the lander Philae dropped from Rosetta, it was completely on its own. Its thruster system wasn't working, so Philae could only use its harpoon-like legs to screw itself into the ground. If Philae landed on anything but level ground, it would have fallen over with no way to get up.
So why did we spend all this trouble to study a barren piece of rock and ice? Only to answer a question as fundamental as the origins of our solar system. Before the sun and planets formed, our solar system was a cloud of gas and dust called "pre-solar nebulae." Comets are a sort of preserved pre-solar nebulae, a time machine into the early solar system. Philae is equipped with a bevy of instruments to help it study the makeup of Comet 67P's ice and rocks.
The mission from here is slated to last until August 2015, when the comet reaches its closest point to the Sun. Philae will be there as Comet 67P slowly melts into the streak we think of when we think of comets. It's the end of one long journey, and the beginning of another.