Wandering through space, the Wirrn Queen comes upon space station Nerva in orbit around Earth. Intending to use the sleeping human race there as food for her children, she sabotages the station's systems, so that the colonists will not awaken. While doing this, the Queen is mortally wounded by the station security system. She manages to lay her eggs in the body of 1st Technician Dune before she dies. The Ark in Space.
Date is conjecture.
I'm estimating this happens about 1000 years before the colonists were to reawaken. The Earth is once again habitable, having recovered from the massive solar flares of 6000. The colonists on space station Nerva are meant to awaken at this time, but do not, due to the sabotage inflicted by the Wirrn Queen a thousand years before. A group of astronauts from the GalSec colony receive a distress call from Earth from one of their ships. This signal has been faked by Field-Major Stryre, a Sontaran.