So the Earth might have had a second moon

An interesting study is making the rounds today: the possibility that Earth had two moons for a while, after which the smaller one crashed into the other, leaving us with one.

The reason for the theory: one side of the moon is quite different from the other (the article explains in exactly which ways), and the impact of a body about 1200 in diameter at a slow speed could explain that. The theory doesn't claim that the moon formed in any other way but the most accepted theory, that of a giant impact with a body about the size of Mars, but suggests that the impact created two moons that coexisted for a time until they eventually collided.

The article calls the other moon 'a tiny second moon', but a diameter of 1200 km is anything but: that would make it larger than Ceres, larger than Enceladus. 1200 km is actually the diameter of Charon, Pluto's largest moon.

The theoretical impact is a slow 2.3 or so km per second. According to the asteroid impact calculator, the average speed for asteroid impacts is 17 km/s and for comets it's 51 km/s.

So let's see what happens when such a body were to hit Earth at a low speed? The minimum speed of impact for an object hitting the Earth is actually 11 km/s but luckily the asteroid impact calculator doesn't mind inputting a lower number. The results are:

(1200 km diameter, made of porous rock, lands 5000 km away in water 5000 m deep, hits at a 30 degree angle)

To see the results, click here. No fireball is created at this velocity.