our solar system travels at high velocity through our galaxy. sometimes as we go on this crazy carousel ride we enter into dangerous territories. The gravitational pulls of Other galactic bodies can sometimes launch huge meteors or comets that can come flying directly in our direction. we are mostly lucky as the planet jupiter and our sun with their huge mass has a strong gravitational pull and they take the brunt of the hits. but then sometimes if it goes just the right way we ourselves can be the target. The earth has sustained many catastropic hits in the past and will continue to do so, but it only happens every 33000 years or so (i'm guessing) give or take.