Well, this is my first post here. Been lurking for a while and have read a lot of the RKKV (Relativistic Kinetic Kill Vehicle) questions in my search for a solid answer to my question. Alas, no such luck. So time to stop lurking...
At some point in the past a relativistic weapon was fired during a some war somewhere by someone that had no idea Earth or its solar system even existed. The shot itself was a miss and the triggerman likely got a proper chewing out. The misfire was noted, reports sent up, but for one reason or another, the top brass never followed up on that one rogue RKKV that got launched off towards a distant arm of the galaxy.
Jump forward in time to near present day Earth. The RKKV has been flying along, minding its own business, and by fluke or bad luck, its path takes it into our solar system. Then it hits the Moon. Good thing too, maybe, because it was heading straight for the Earth. The Moon takes the hit in line with the Earth --- becoming the impactor that stops the RKKV from hitting us dead on.
This is where my question comes in:
Just how bad would this be?
I've played around with Universe Sandbox² a few times trying to see just what would happen [to no real avail] and I've read up on RKKVs on Atomic Rocket. I've even done some back of the envelope maths to try and see just what would happen. However, I'm not entirely sure what would happen.
This particularly RKKV is made of a material with a density of 1000g/cm³ that has been shaped into a kilometer long rod with a diameter of 20 meters [314,159,265,358.98cm³]. It was travelling at a speed of .2c at the time of impact.
My math-fu simply isn't good enough to properly figure out what this would do, but the event itself is a key event in a story I've been working on for a bit now. The event itself is aptly named the "Oops" event and the fact that there is anyone on Earth left to name the event demands that it be a survivable scenario for the critters on Earth.