I've been doing some work with my classes around the Yuri Milner Breakthrough Starshot project to send a tiny nano craft of mass around a few grams to alpha centauri. The starchip would travel at 20% of the speed of light. I've had them calculating various properties of the ship including kinetic energy. Since this would be around 10^14 J (about a Hiroshima bomb) it got the class thinking what would happened if little green men on alpha centauri were sending their own nano starchips toward the earth. If one collided with the earth what would happen? Some suggested it would burn up in the atmosphere and cause no damage (in which case could all this energy really just be dissipated in the atmosphere with no effect?). Some suggested it would reach the ground and cause a massive A-Bomb type explosion. Are there any natural objects this size travelling at the speeds suggested? What physics would be involved if something like this hit our planet. I'd like to give the class more to think about. Any suggestions.