In the last book of Stephen Hawking, he thought an asteroid impact is the greatest threat to life on Earth. you have probably heard about The Chelyabinsk meteor incident.
A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and large enough to cause significant regional damage in the event of impact. They are defined as having a minimum orbital intersection distance with Earth of less than 0.05 astronomical units (19.5 lunar distances) and an absolute magnitude of 22 or brighter. 98% of the known potentially hazardous objects are not an impact threat over the next 100 years.
Science Loop video.