Seeing is believing. But seeing a huge rock half the size of Earth smash into Jupiter in July 1996 was incredible. Can you imagine what would happen if Earth collided with a huge space rock? It would be disastrous. In fact, it is thought that a meteor killed the dinosaurs millions of years ago. 
A rock like the one that hit Jupiter could split Earth in two. Doing nothing about it is like waiting for the end of the world. So scientists plan to start project to identify all space rocks that could hit Earth.
    
be in danger
Seeing is believing. = To see is to believe. 眼见为实
incredible = unbelievable 难以置信的
Earth 地球(相对于其它星球)     the earth 指地球本身     on earth 在地球上(固定用法), 究竟adv.in the world
The bus collided with the car and killed two people.
When the typhoon hit the island, it caused many disasters.
meteorite 陨石    meteor 流星
do nothing about sth       We should do something about our traffic problem.
identify 辨别，鉴别