A question

Dear group:

I would like to ask a question, sorry If I ask too
much. Scientists have speculated that perhaps even
half the universe is made of antimatter. Imagine a
very unlikely event, but fascinating to write a story.
An antimatter asteroid a mile long, for some
mysterious reason, reach the moon and impact it. How
catastrophic would be the disaster. I know most of the
matter of the asteroid will disipate in the form of
gama rays, a lot of it, much of which it fall to the
Earth. Would it be the end!

Thank you very much for your knowledge and help.

Omar Vega