The European Space Agency's Rosetta spacecraft will finally complete its 10 year journey to intercept the "rubber duck" comet 67P/Churyumov–Gerasimenko, with contact expected to be made today. (Update: 6/8/14 10:50): The Rosetta probe has now successfully commenced its powered orbit of the comet.) Now comes the hard part.
The Rosetta spacecraft is currently travelling at about 36,000 mph; that's roughly mach 47. Comet 67P (known as the 'rubber duck' comet due to its distinctive shape), meanwhile, is moving through space at 34,175 mph. In order for the Rosetta to intercept and enter orbit around the comet, it's going to have to match the comet's speed within a range of just 1 mile per hour.
So, to slow itself down sufficiently, the Rosetta performed a series of thruster burns in June and July to cut 1,500 miles per hour off its trajectory, while another pair of "insertion burns" performed over the last two days should put the spacecraft in optimal position to snag the passing space rock.
Once that happens, the Rosetta will spend 18 months examining the comet before deploying a landing craft known as the Philae. This would mark the first time in recorded history that human technology has not only orbited a comet, but landed and sampled it as well. [Space]