Asteroids. Asteroids hit the Earth - this fact was made glaringly obvious by the impact event on February 15, 2013 over Chelyabinsk Oblast, Russia, that injured about 1600 people, and caused millions of dollars of property damage. This event was all over the news and social media - yet oddly, I've recently spoken with some people who had not heard it.
Asteroids hit the Earth more frequently then astronomers previously thought; using data from Earth-bound infra-sound detectors and satellites, it was discovered that between 2000-2013, Earth was hit 26 times by Asteroids, with explosions ranging from 1-600 kilotons; the 1945 Hiroshima bomb was 15 kilotons. Several of these impacts would have caused serious damage if they had exploded over a city, rather than harmlessly over the ocean or uninhabited regions.
Recent advances in imaging and data processing have allowed astronomers to discover thousands of Near-Earth Asteroids, and hundreds of thousands of Main-Belt Asteroids; as of this writing, there are 1590 known Potentially-Hazardous Asteroids (PHAs). It's estimated that there are a million or more Near-Earth Asteroids - we will never find them all at our current rate of discovery.
We have the technology to launch spacecraft that can find these Potentially Hazardous Asteroids, and we are devising methods to divert Earth-bound Asteroids. Dr. Ed Lu of the B612 Foundation says that it would only cost about as much as a freeway overpass to build a space telescope that could find these Asteroids, but as of this writing, NO world government has assigned the responsibility of Planetary Defense to any of their agencies.
Over a hundred scientists, astronomers, astrophysicists, astronauts, entertainers, media personalities, and others have signed the Asteroid Day Declaration, calling for a one hundred-fold acceleration in the discovery and tracking of Near-Earth Asteroids, and the creation of a global awareness event, to be held each year on June 30th - the anniversary of the Tunguska Impact Event of 1908.