the impactor, sometimes called theia, is thought to have been a little smaller than the planet mars. theia collided with earth about 4.533 ga. models reveal that when an impactor this size struck the proto-earth at a low angle and relatively low speed (), much material from the mantles (and proto-crusts) of the proto-earth and the impactor was ejected into space, where much of it stayed in orbit around the earth. this material would eventually form the moon.