that’s basically chemistry. At relativistic speeds the electrons of the projectile don’t have play a significant role. It’s going to be atomic nuclei hitting atomic nuclei and the time it takes to go through the earth is like two microseconds for the projectile going at ( 1 - 10^-9 ) c. Even that, I suppose, is too long for the particle beam to scatter momentum from fusing with other particles, creating gamma rays, creating exotic particles etc. But we could just always go even closer to c? (on paper)