Expert: James Gort - 10/16/2011

Question
What would happen to the earth if an asteroid hit the moon? Would we as people be affected. Would the moon suffer any effects - change in appearance, thrown out of orbit etc. Would the earth environment and atmosphere be effected? Would the size of the asteroid effect the outcome and if so what would the different outcomes be? In short what would happen to both the moon and the earth if an asteroid hit the moon?

Answer
Hello Taylor,

A lot depends on the size of the asteroid. If it had enough mass to eject material beyond the "escape velocity" of the moon, the earth undoubtedly would be bombarded by a meteor storm. Just due to earth's gravity, it would "sweep up" much of the ejected material. Some would go into earth orbit, but some would hit the earth.

As a example, an asteroid between 8 and 10 km wide hit the moon about 109 million years ago, and formed a huge crater (visible from earth with the naked eye) - Tycho. That didn't affect the moon's orbit substantially, but it probably caused a massive meteor shower on earth - with large meteoroids making it through the atmosphere and causing local damage.

Of course, if an asteroid was large enough to break up the moon or remove a large piece of it (say, 500 km across), that would be a different story. One effect would be the tides on earth - which would be diminished due to the lower mass of the moon fragments. However, such an event would also cause huge meteors to crash into earth, potentially causing widespread loss of life and even the extinction of some species who couldn't adapt (remember the dinosaurs).

So the main effect (other than tides) would come from the remnants hitting the earth. If the moon's orbit became more elliptical (it would be very unlikely that a collision would expel the moon from earth's orbit), that wouldn't have a large effect on earth. But if a large enough (maybe 1000 km across) asteroid hit the moon, the moon would be pulverized into small fragments, so the earth would have a ring system (like Saturn) instead of a single moon. Although I'm sure that would be beautiful to behold, not too many people on earth would be left to witness it - it would cause a devastating meteor shower on earth.

Hope that helps.

Prof. James Gort