I recently asked a question in Space Travel SE where we were exploring ideas on how to get a space craft that weighs 1 kg to travel at 0.1c (one tenth the speed of light). At this rate, it would be possible to reach Proxima Centauri, the nearest star after our sun, in 43 years. We concluded that repeated gravity assists with the solar system would become impossible once the spacecraft significantly exceeds solar system escape velocity, which is much much less than 0.1c.
While composing the question and much reflection afterwards, I couldn't help asking myself if there are objects in our Milky Way galaxy that are 1 kg or greater in mass that are traveling at speeds of 0.1c or greater. Without taking relativistic effects into consideration since they are still insignificant at 0.1c, the kinetic energy using E=0.5mv2 that one kilogram of mass traveling at 0.1c would be 449.4 trillion joules. This is the amount of energy equivalent to an earthquake of 7 on the Richter scale or a large hydrogen bomb.
Although such an object would not strictly be categorized as a relativistic bomb, it would still cause a very significant explosive release of energy upon collision with a massive object. The only events that come to mind that would be capable of accelerating a 1 kg or greater mass object to 0.1c or greater speed would be supernova. There may be others like gamma ray bursts, quasars, etc.
I am having trouble understanding if generation of such relativistic bombs can be generated by supernova explosions. What physical laws would prevent this or make it unlikely? If it is indeed possible, why don't we observe them? How can we observe them if possible?
How large would a mass have to be, traveling at 0.1c, that if it struck the earth, would cleave the earth in 2?