Alien Legacy

In 2043, an alien probe originating from Alpha Centauri is detected in our solar system. Attempts at a peaceful first contact fail and a missile barrage is launched at Earth. Earth's ABM systems successfully destroyed all but one missile, and its sheer power of destruction prompted the world governments to invest significant effort into combating this threat from the stars. Within several years, great technological leaps were made and space travel was significantly improved upon. Battlefleets were assembled and dispatched across the vast distances of space to combat the alien menace. In 2119, fearing extinction, Earth built gigantic seedships designed as "arks" to propel people and the sum of human knowledge across the stars. One of these seedships, the UNS Calypso, is sent to Beta Caeli star system which is the farthest colony attempted yet. In 2135, a second seedship, the UNS Tantalus, is dispatched to the same destination in order to better the chances of human survival. The Tantalus is equipped with a more advanced fusion engine and thus will reach Beta Caeli some 21 years ahead of the Calypso. (From Mobygames description)