The Service Module carries the environmental controls and the life support system for the entire station. Until the Service Module is in place, no other parts can be attached. NASA chief Daniel Goldin said the space station is being built to see how people can live and work safely in space for long periods of time. He said it will also be a world-class laboratory that could lead to discoveries and technical advances that affect all our lives. What makes the space station such a unique laboratory is the near absence of gravity.
Gravity is the pulling force that holds objects to the surface of a planet.
Beyond Earth's atmosphere, the planet's gravity force grows weaker. The space shuttle, for instance, experiences only a tiny pull, called microgravity. It's strong enough to keep the shuttle in Earth's orbit, yet weak enough that objects inside the shuttle have almost no weight. So they float in space. Nothing works quite the same in microgravity as it does on Earth.