The solar system is traveling through a region of our galaxy that contains a black hole whose gravity has changed the orbit of many Asteroids in the Asteroid belt in. It has recently been confirmed that a number of these large Asteroids are heading right in our direction. Our scientists have now calculated the size and mass of those objects and have concluded that an earth impact with any of those objects will lead to an extinction event.
To combat the situation the Space Shuttles have been brought out of retirement, upgraded with a number of nuclear weapons and reinstated into active service.
Your task as the commander of one of those shuttles is to ensure that none of the Asteroids make it to earth. You can expect 32 waves of asteroids.