Arguably, an asteroid impact is the one single event that's most likely to wipe out human civilization. This may not be a day-to-day worry for you, but it is for astronomers, which is why the European Space Agency has started to plan a real live test run to see if it can successfully deflect the orbit of a near-Earth asteroid.
To be clear, the ESA has not spotted an asteroid that's likely to hit Earth, it's not launching a mission to save humanity, and as far as we know Bruce Willis is not involved in any way, shape, or form. Instead, the ESA will attack a binary asteroid that passes close enough to Earth to be reachable in a reasonable amount of time, take a whack at it, and see if its trajectory gets altered enough that if it had been headed for Earth, the whack would have caused it to miss.
As far as the whack goes, the ESA is going to employ the absolute simplest asteroid deflection strategy possible, which is to just plow a 660-pound spacecraft into the rock 14,000 miles per hour and hope that the force generated by the impact nudges the asteroid enough to shift its orbit in a measurable way. This is why the mission will be targeted at a binary asteroid, which is a smaller asteroid orbiting a larger one: by measuring how the orbit of the smaller asteroid is altered post-impact, we should get a pretty good idea of how successful the test was.
The asteroid in question is 65803 Didymos, which is 800 meters across, aka large enough to be quite dangerous. For reference, the Tunguska Event, an explosion over Sibera in 1908 that flattened something like 80 million trees, was probably just a cometary fragment about 100 meters across. So yeah, 800 meters would be bad times for Earth. Didymos has a little satellite about 150 meters in diameter that orbits it about a kilometer away, and the pair will be swinging by Earth between 2022 and 2023. The target will be the smaller satellite, not Didymos itself, and the ESA is hoping for a change in orbital velocity of about 1%.
The overall mission is called AIDA, for Asteroid Impact & Deflection Assessment, and will consist of two spacecraft: DART (Double Asteroid Redirection Test), which will be the impactor, and AIM (Asteroid Impact Monitor), which will watch the whole thing happen and then stick around to take more measurements. If this all sounds a little bit familiar, it's because we've already done something similar, in the form of Deep Impact. No, not the movie, but the mission which took place back in 2005, where we smacked a comet with a spacecraft to see what would happen. Here's a five second video of the impact:
AIDA is going to do the same sort of thing, but on a larger scale, with the explicit intention of testing out asteroid deflection technologies. This is basically our chance to figure out how to make a system like this work: next time, we may not have the luxury of a test.