The Rosetta mission to the asteroid 67P/Churyumov–Gerasimenko is an engineering marvel and demonstrated some new capabilities for space travel. There are multiple challenges in the mission that had to be overcome. Some say “what’s the ig deal, we’ve landed on stuff before, why bother going to a comet?” Well let me tell you what the deal is.
On March 2nd, 2004, the ESA launched an ambitious mission that wouldn’t bear fruit for 10 years (now that’s some patience). The mission was Rosetta, and its goal was to track down an asteroid moving at about 11.5 km/s, which is about 25,000 mph. Yeah, that’s fast. So the first challenge was to catch something moving that fast. To appreciate the difficulty in this, consider a marble or some other small object moving at thousands of mph two miles up in the sky above you on the ground. You can’t see it at that distance, and even if you could it would be moving so fast that you’d miss it if you blinked. Now you need to have to figure out how to get to that marble with a grain of sand, and not ram into it when you get there. So this was the first time we have ever landed on a comet.
Some may say we didn’t really land on the comet. While it is true Philia did have a rough landing and bounced into a crevase, it wasn’t easy. We’ve landed on Mars, the moon, and on Earth. But the difference between those places and comets is gravity, or lack there of. Gravity is proportional to the mass of an object. Compare to a planet or moon, a comet is miniscule, and thus has very little gravity. So Philia didn’t so much land on the comet, but flew towards it very slowly and hoped it didn’t hit too hard. A good analogy is in pool (billiards) when you want to have the cue ball very lightly tap a ball on the edge of a pocket. Philia had to lightly tap the surface of the comet. Unfortunately the distance between the comet and Earth meant Philia had to do it all by itself, with no help from Earth controllers. So you have a tiny robot, in the depths of space, trying to lightly tap a comet moving at some 11 km/s. Not easy. The fact Philia got ot the surface without crashing is an accomplishment It also send back images and other data while it was active.
Philia’s mothership, Rosetta, also had a difficult time. Because of the lack of gravity, Rosetta can’t simply orbit the comet like it would Earth. Instead it had to use its thrusters to keep it in a triangular orbit around the comet. If its thrusters were to stop working, Rosetta would quickly sail away from the comet. It’s path around the comet looks a bit like a pinball’s path between a bunch of bumpers. That motion is very complicated however for the comet is constantly moving, so Rosetta has to constantly adjust its thrust in order to keep up and orbit the comet. It is fiendlishly difficult orbital mechanics problem, and we have solved it, adding another notch to the accomplishements of the mission, and thus humanity.
Now lastly, the why did we bother going there? The foremost scientific question answered by the mission to date is whether Earth’s water came from comets, and the answer seems to be no. The question of where did water come from is an integral part of our understanding of how life started on Earth. We know that all organisms on Earth require water to live, and that when the planet was first formed it was just a hot ball of rock. So how did it get all the water we have today? The answer is obviously it came from outer space. Water doesn’t magically appear, so something had to deliver water to the early Earth. The most prevalent theory up to now has been that comets, which are mostly ice, crashed into the Earth and turned into water.
Earth water has a certain signature based on the percent of water that is called heavy water, or deuterium (you may know have heard of it as the stuff used for fusion). Our water has a relatively low percentage of deuterium, which makes fusion a bit difficult. The amount of deuterium in water is based on the environment, especially the radiation environment. The sun has a certain characteristic radiation that causes some water molecules to turn into deuterium. So, if we find another water source, say a comet, that has the same deuterium percentage, then that is evidence that Earth water came from comets. However the Rosetta mission has found that the water on the comet has 3 times the percentage of deuterium that Earth water has. This means the comet’s water was exposed to a different environment. This does not necessarily mean some of Earth’s water didn’t come from comets, this comet could also be a wierd one and other comets do match our water signature. But it is a new scientific finding that add a new piece of knowledge to the collective understanding of us and the universe.
So in conclusion, the Rosetta mission has demonstrated our technical abilities to catch up to, orbit, and land on a comets. It has also provided new knowledge to further our understanding of everything. And that is what space is about: achieving new technical capabilities and learning new things.