Yesterday, the Philae lander successfully landed on a comet. (You can follow its landing as live-cartooned by +Randall Munroe
) It's a little bit hard to understand just how insane a task that was, so let me try to give you a bit of context.
First, there's the problem of simply getting there. Comet 67P is about 4km across on its long side, and the Rosetta mission had to travel about 6.4 billion
kilometers in order to reach it. For a sense of scale, this is sort of like being able to throw a rock to the other end of a football field and correctly hit the leftmost edge of a particular red blood cell. Not the center, mind you, but a particular dot on its surface.
But a flight like this isn't a simple throw, and the animation below shows you why. To efficiently get out to distant parts of the solar system requires a manoeuver called a "gravitational assist," or a "slingshot." It turns out that if you fly close by an object that's pulling you in with gravity, and you fire your engines hard and forward just as you're at the closest point, you get a much bigger speed boost than you would if you fired your engines at any other time. So interplanetary flight paths will often do things like boost into an orbit around the Sun, swing back around some other convenient planet -- sometimes the Earth, sometimes Venus, sometimes Mars -- and use it to speed up. More complicated flight paths might do more than one swing. Rosetta did four
gravitational assists, flew close enough to two asteroids to take pictures, and then pulled into orbit around a comet. This had to basically be planned out from the beginning, and re-planned hastily when the original launch window slipped: it turns out that a many planet-route that you plan on one day isn't much use on a different day.
Once it encountered the comet, Rosetta put itself into orbit around it, and dropped the Philae lander. Landing was an even more interesting challenge, because comets lack the one thing that we most often use to land, namely gravity. On Earth or Mars or the Moon, if a lander drops, it will generally go "thud," and your biggest problem is not going "thud" very quickly and being smashed to bits. On a comet, if a lander drops, it will generally bounce off and then fly back into space. Worse, you can't assume that you can just hit hard and embed yourself in the surface of the comet: we had no idea
what surface we would be landing on, whether it be nearly-impenetrable rock, gravel, ice, dust, level, sloped, cratered, or something completely different.
So Philae's landing plan had a few steps. It would fly up to the comet, then fire harpoons into the surface, while firing a thruster engine to stabilize itself. It would then reel itself in, and fire that thruster to push itself hard towards the surface, and then screw itself down to the rock. The idea was that hopefully one
of these would work.
In practice, the one turned out to be the screws, because both the thruster and the harpoons failed to work, for reasons still unknown. But somehow it appears to have landed anyway.
And what do we get for all of this? Our first chance to study a comet from really up close. Comets are made from the matter at the farthest reaches of our Solar System, and carry in them a snapshot of what our solar system was made of in its earliest days.
You can see more about the mission planning here: http://www.esa.int/Our_Activities/Space_Science/Rosetta/The_long_trek
and at http://en.wikipedia.org/wiki/Rosetta_(spacecraft)
More about Comet 67P at http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko