The Rosetta mission was a long-running and pricey project. The European Space Agency mission named Rosetta left the Earth in March, 2004 to land on Comet 67P/Churyumov-Gerasimenko 10 years later. Rosetta circled the sun four times, travelling a total of 6.4 billion kilometers. The spacecraft, now 300 million miles away from Earth, had to travel 34,000 miles an hour in order to come in to orbit with the comet. The lander mission cost around $275 million.
Secondly, the technical difficulty of Rosetta surpasses even the moon landing, especially in terms of the precision and planning needed to pull it off. Here are just a few characteristics that will give you an idea of why it was "mission impossible".
Comets have negligible gravity, which makes it hard to establish an orbit around them and causes a risk of the lander to simply bounce off the surface. Comets are also very mobile, they travel at a speed of 30 to 40 kilometers per second – fast enough to get from London to Paris in about 10 seconds.
On top of that there was no correction possible for Rosetta’s lander Philae. Given that comet 67P is only four kilometers in size imagine if the calculations had been off even by half a kilometer – 10 years of work wasted! So what were the scientists hoping to achieve by sending Rosetta on its decade-long and highly challenging journey? Nothing short of unlocking the clues about the origins of life on Earth. The thing is that comets were shaped in the initial phases of the solar system and are thought to contain the raw building blocks of the universe. Researchers also believe that comets could have been the initial source of water on our planet. By studying the composition of the 4.6-billion-old Churyumov-Gerasimenko comet, the Rosetta mission might not only confirm or deny these theories but also provide us with revolutionary information about the birth of the solar system.