In planning for Servicing Mission 4 to Hubble, crew members divide their time between NASA's Johnson Space Centre, Houston, working underwater on a Hubble mock-up to simulate the effects of weightlessness, and NASA's Goddard Space Flight Centre, Greenbelt, Maryland, where they practice scheduled mission tasks on a Hubble mock-up inside a large clean room facility. Many Goddard engineers are trained divers. These engineers work along side the astronauts while in the Neutral Buoyancy Lab to aid in their training. This underwater experience helps HST engineers understand what the astronauts need as they work together to refine tools and procedures to service Hubble