What is the name of a sci-fi story featuring interstellar war under limitations of orbital mechanics?

I read this story once, which described an interstellar war between earth and an alien species that sent one-penger capsules on centuries-long voyages to attack earth. The story described the orbital mechanics used to maneuver the enemy spacecraft and the ensuing reactions from humanity, planned according-to and limited-by real physics, to intercept the intruder. Any one know this story? ( I don't even remember if it was a book or a short story)