The Rosetta spacecraft is preparing to release the lander Philae onto the comet 67P/Churyumov-Gerasimenko. So far we’ve gotten some great images of the comet, and if successful Philae will be the first soft landing on a comet. Given the number of successful missions we have nowadays, it can be hard to wrap your head around just how challenging this kind of thing is.
This video might give you a better idea. It traces the orbital motion of Rosetta leading up to the landing of Philae. After achieving orbit around 67P, Rosetta is then maneuvered to within 10 km of the comet. This is necessary to scan the surface with enough precision to determine a landing site. It then goes into a higher orbit again, before dive-bombing the comet so Philae can reach the landing site accurately. The low mass and irregular shape of 67P means you can’t simply rely upon gravitational freefall. Once the landing is made, Rosetta will then go back to a higher orbit before reversing orbital direction so it is in a good position to relay data from Philae to Earth.
This orbit is a dance of gravity and thrusters. It has been calculated based upon the best data we have, and it has to be right. One small error and the spacecraft could slam into the comet, or skip past without ever achieving its final mission. Of course all of this is after the 10-year mission just to get to the comet and match trajectory. As anyone who’s played Kerbal Space can tell you, this is an extraordinarily complex set of orbital dynamics.
If you ever happen to meet someone who does this kind of orbital logistics, shake their hand and say thanks. Without their work our spacecraft wouldn’t arrive at their destination, and we wouldn’t have the opportunity to do some awesome science.