Just before they do, a military contingent from Earth arrives to help defend the city against the impending Wraith attack long enough for Earth's latest battleship to arrive. The season ends with a cliffhanger, while the city is still under siege by the Wraith. Season two began airing in the United States on July 15, 2005, and it picked up where Season 1 ended. The Atlantis expedition successfully avoids being culled by the Wraith by making them believe Atlantis had been destroyed, and they recover semi-regular contact with Earth, thanks to the Daedalus and the new Zero Point Module (ZPM) recovered by SG-1. Sheppard is promoted to Lt. Colonel and former Runner Ronon Dex replaces Lt. Ford, who went missing in action (MIA) at the end of the battle with the Wraith. The central plot of the second season is the development of Dr. Beckett's retrovirus, which can, theoretically, turn a Wraith into a human. While an incomplete version makes a young Wraith girl lose all her humanity and almost turns Sheppard into an Iratus bug, a more developed version is tested on a living Wraith, "Michael", with mixed results. Michael's Wraith faction proposes an alliance with Atlantis, but they betray the team. The season closes again with a cliffhanger—the Wraith are heading for the rich feeding grounds of Earth. Season three premiered in the United States on July 14, 2006, picking up where season 2 ended. Having stopped the Wraith from reaching Earth and having failed to develop a working Wraith retrovirus, the expedition faces its third year in the Pegasus galaxy with the Wraith still a threat and a new, powerful enemy bent on destroying the expedition and Atlantis: the Asurans, self-replicating nanobots, also known as Replicators. The situation becomes complicated when an experiment gone awry drains their only ZPM, leaving them without a power source for the city's shields. Soon thereafter, they find a lost Ancient vessel and unwillingly turn over the city of Atlantis to its crew. The SGC sends General O'Neill and Richard Woolsey to try to negotiate an agreement between Earth and the Ancients to allow the expedition to return to Atlantis. The Ancients are then subsequently killed by an invading Asuran force while O'Neill and Woolsey send off a distress call to Earth and go into hiding. The main members of the Atlantis expedition on Earth disobey their orders and go back to the city, rescue O'Neill and Woolsey, and repel the Asuran invasion.