NEO's and the moon
NEO's are near Earth objects and can. E seen through telescopes.
NEO's are difficult to see as they don't give off much light, they may have unusual orbits and are small compared to the universe.
If there was enough warning about an NEO coming our way, we could try and deflect it before it hit us.
In a very young solar system, a planet the size of Mars is thought to have collided with Earth. The two planets merged together and the heavier elements sank to the bottom of the Earth. The rest of the debris was thrown into orbit to merge together and therefore create the moon.