It's not a case of "because they CAN"…it's actually a practice run for
So the joint US-European AIDA (Asteroid Deflection and Assessment) mission will crash a probe into a small asteroid called 65803 Didymos. It's only 160m wide, but the test will show if in principle a much larger asteroid threatening to wipe out human civilisation can be deflected the same.
Two spacecraft, one kamikaze and the other to monitor the effect, will be launched in October 2020. They'll travel about 6million km and reach the target mid-2022.
Dr Patrick Michel, lead investigator for the European Space Agency: "To protect Earth from potentially hazardous impacts, we need to understand asteroids much better - what they are made of, their structure, origins and how they respond to collisions."
Planet Earth has had a couple of near misses in the last few years: an asteroid the size of an aircraft carrier wizzed by - closer than the Moon! - in late 2011.
After all, if/when the situation is for REAL, Bruce Willis will be too far into his dotage to save us!