AtlantisConstruction begins on what is intended to be the fourth and final member of the current Space Shuttle fleet, Atlantis. With four orbiters in service, NASA will be one step closer to the routine, almost-weekly schedule of orbital flights envisioned in the Nixon-era mandate for the shuttle. Refinements and improvements in the process of constructing the shuttle fleet make Atlantis the lightest shuttle to date, over three tons lighter than Columbia.