Ceres, the biggest asteroid in the asteroid belt, is only 946 km in diameter, and there are only six larger than 300 km.[1] Asteroids the size of 300 miles don

Ceres, the biggest asteroid in the asteroid belt, is only 946 km in diameter, and there are only six larger than 300 km.[1] Asteroids the size of 300 miles don't just appear out of nowhere. In order for them to hit Earth, they must be coming out of the deep space, and they have to hit Earth (radius 6371 km) out of a circle of radius 1 AU (150,000,000 km). To put it into perspective, if we can observe one "hostile" asteroid from outside the solar system crossing the circle made by the Earth's orbit every ten years (Note: we've never observed anything like that in our history, and we would surely have detected such a thing with 20th century technology), it will take 10 * (1 AU / 6371)^2 = 5.5 billion years before one hits Earth, on average. By that time the sun will destroy us anyway. I guess they are still more likely than a group of hostile jerk aliens sworn to exterminate humans once we set our foot on Mars, but not by a wide margin. [1] https://en.wikipedia.org/wiki/List_of_exceptional_asteroids https://en.wikipedia.org/wiki/List_of_exceptional_asteroids