Full coverage: China Tiangong-2 Space Lab Launch

It usually takes years to get astronauts prepared for space missions. But thanks to advanced technology, the trainings are now well scheduled and more effective.

Space is a harsh environment. But over the years -countries with astronaut programs have learned how to handle complex tasks while in orbit high above the earth.

"In the old days, of course, we didn't know very much. Space was a real unknown," said former NASA astronaut Franklin Chang-Diaz.

Initial training for U.S. astronauts has changed over the years. Early on, scientists prepared them for the unknown -what could happen - what could go wrong in zero gravity.
The very first U.S. astronauts, selected in the late 1950's fit a rigid criteria
All were test pilots,- no taller than 5-11 -with a strong background in engineering.

"They performed a very important function which is to demonstrate that humans could actually survive in space," said Franklin Chang-Diaz.

A lot has changed during more than 50-years of space travel. NASA has broadened its ranks from military pilots to include citizen/scientists --- The average number of college years completed by an astronaut jumped from about four nearly eight-and-a-half.

Training has also graduated -- NASA now relies on what it calls the Neutral Buoyancy Tank to simulate space walks.

"There's a set of scenarios that we make sure we do so that we can fix whatever we think are the most likely things to go wrong on the space station," said NASA astronaut Cady Coleman.

Franklin Chang-Diaz retired from NASA in 2005-and is in the U.S. Astronaut Hall of Fame. He flew on seven space flights --including three grueling space walks.
Chang-Diaz says the time in NASA's giant pool to simulate weightlessness is critical.

"It is very important, -- very necessary in order to make sure you can tease out any potential mistakes you can make. You can optimize how you use your time -because your time is very precious out there," said Franklin Chang-Diaz.

These days -the space program also depends on virtual reality including finding your way BACK to the space station IF your tether breaks.

While missions are longer than early days of space flight -one thing has not changed. U.S. astronauts still spend hours in the classroom with books and computers. While it may not be glamorous, in the end, it helps with the most important goal returning safely to earth.