An asteroid impact is a significant risk to humankind. By way of example, and as far as I know, there was a recent major impact on 10 May, 4830 years ago. Not an asteroid, but a comet that struck in the Indian Ocean causing tsunamis that flooded shorelines around the ocean perimeter. Where I live now, Perth, Western Australia, population 2.6 million, was then drowned to a flood depth of more than 150 metres.