NEAs have orbits that can cross the orbit of the Earth. Collisions of asteroids with the Earth have taken place frequently over geological history. Comets can also collide with the Earth but the risk of a comet impact is thought to be much lower than that of an NEA impact, although given the potentially high relative velocities the effects in the case of a comet impact could be much more devastating.
The phenomenon of collisions in the history of our Solar System is a very fundamental process, having played the major role in forming the planets we observe today. There is no doubt that major collisions of asteroids and comets with the Earth will continue to occur in the future.
Can we protect our civilization from the next major impact? Various initiatives are being taken by space agencies, including ESA and NASA, and research groups around the world to identify potential future impactors, investigate their physical characteristics, and develop strategies to mitigate the risk. The DLR Department of Asteroids and Comets contributes to this effort with a number of programs: