According to the book of Genesis, God punished the fallen angels for spreading their seed among humans and creating the giants.
His punishment would last for 70 generations. Assuming a generation is 70 years, 4900 years from that date was roughly 1900.
Assuming that has any validity, aliens are fallen angels coming to torment us. With the power of Christ, we are more powerful than them, though we have to recognize our spiritual power.