The way you would put it to a nine-year-old is to say, "If a meteorite was coming to hit the Earth, how would you figure out if it was going to or not?"
9살된 아이한테 말하려면 "운석이 다가와서 지구에 부딪힌다면 그게 부딪힐지 아닐지  어떻게 알 수 있지?"