The Earth Shield Project — Marsell Morris

A giant asteroid the size of Pluto is circling the Sun in the Kuiper Belt until it is struck by a most unlikely, smaller, rogue asteroid from the other side of the galaxy, and is sent on a trajectory that will, in a year and a half, collide with Earth. With no existing technology to prevent the collision, Humankind, searches for a way to avert the coming catastrophe in the short time available and finds a drug addicted physicist who emerges as the hero.