I'm pretty sure the title was "T". It's about an alien race at war with humans. They decide to end the human race by destroying Earth before humans evolve. They send some humans through space and back in time to do the job.
I think the humans have single letter names (A, B, C, etc) They have raised/cloned the humans just for the job and to save space in their ships they have only a head, a body and one arm. Most don't make it.
The aliens don't know that there were 2 more planets in the past and they count the planets in from Pluto. One human hits a planet and completely destroys it and T hits a glancing blow and just breaks up his target forming the asteroid belt.