Full coverage: China Tiangong-2 Space Lab Launch

So far, China's space program has sent 21 people into outer space. What kind of people qualify to become an astronaut? What kind of training do they have to go through?

Chinese astronauts go through rigorous tests, to see who can adapt to zero gravity best.

A special machine tests astronauts' vestibular system, a tiny organ that helps people maintain their balance.

"This test causes participants to experience a nervous reaction. Just like when we're car sick, we get a cold sweat, become nauseous, and may even puke," Wang Linjie, Aerospace Medicine Lab.

China's space program also tests astronauts' cognitive ability in zero-gravity conditions.

"You need good eye-hand coordination to navigate the spaceship. They are doing a good job! Their response time and accuracy is very good, exceeding our expectations," Wang Chunhui, Astronant Center Lab.

Besides undergoing zero gravity training, astronauts also need to prepare for super-gravity conditions. A centrifugal machine spins astronauts at a high velocity, so they can experience what it feels like to weigh seven to eight times more than they do on earth. It is the same feeling that astronauts experience during a spacecraft's launch.

The test lasts 40 to 50 seconds. Super-gravity conditions affect the astronauts muscles and makes breathing difficult.

Astronauts have to have excellent physical health. Astronauts' lungs are tested to see how they function when there is low air pressure and little oxygen. Fighter jet pilots undergo the same kind of test. In fact, all of China's current astronauts used to be jet pilots.

"Every process, every detail is put against strict standards. Ever since Shenzhou 10's mission was completed, every astronaut now goes through at least 4000 hours of training. We are fully prepared," said Li Xinke, party chief of China Astronant Center.