China's fourth manned spacecraft lifted off at 6:37 pm on Saturday, sending its first female astronaut into space and history.

Shenzhou IX, carrying female astronaut Liu Yang and male astronauts Jing Haipeng and Liu Wang, blasted off from the Jiuquan Satellite Launch Center in Gansu province into a blue sky.

The crew will stay in space for more than 10 days, during which time they will perform scientific experiments and the country's first manual space docking - a highly technical procedure that brings two vessels together in high-speed orbit.

Successful completion of the space rendezvous and docking - which will see Shenzhou IX attach itself to the Tiangong-1 module currently orbiting Earth - will take China one step nearer to setting up its own space station in 2020.

At the Beijing Aerospace Control Center on Saturday, hundreds of scientists monitored and reported the spacecraft's status every few seconds. Big screens showed the three astronauts smiling and waving at the monitoring camera when the carrier rocket blasted off.

At 6:47 pm, Shenzhou IX entered its orbit. Chang Wanquan, chief commander of the country's manned space program, announced that the lift-off had been a complete success.

Wu Bangguo, chairman of the National People's Congress Standing Committee, who was watching the launch at the Jiuquan Satellite Launch Center, congratulated the space scientists for the successful lift-off.

And State Councilor Liu Yandong read a congratulatory letter from President Hu Jintao, who is in Denmark for a state visit, saying that: "I feel very glad to hear of the success of launching the Shenzhou IX manned spacecraft and I would like to extend warm congratulations and sincere regards to all those participating in the research and tests (of the country's space program)."

The rendezvous and docking between the Shenzhou IX spaceship and the orbiting space lab module Tiangong-1 will mark a major breakthrough in the country's manned space program, Hu said in the letter.

In about two days, the spacecraft will conduct a robotic docking with Tiangong-1 and astronauts will move to the space lab module and conduct scientific experiments.

For them, the challenges ahead are tough, including conducting the country's first manual docking and living in weightlessness for the longest time in China's space history.

According to the plan, the manual docking is scheduled for the middle of the mission. Astronauts will maneuver the spacecraft close to Tiangong-1 - both moving at a speed of 7.9 km per second - in a limited time and with minimal fuel consumed.

Though the astronauts have practiced manual docking for more than 1,500 times each, their performance is subject to huge pressure and the zero-gravity environment, said Chen Shanguang, head designer of the astronaut system.

"It is a huge test for astronauts' ability to judge spatial position, eye-hand coordination and psychological abilities," he said.

History shows space dockings have high risks.

The United States and Russia (the former Soviet Union) have conducted more than 300 docking missions since 1966. Among them, the US has failed twice, while Russia (the former Soviet Union) has failed 15 times. Most of these failures happened in the initial stages of their space exploration.

China plans to send three spacecraft to rendezvous and dock with the orbiting Tiangong-1 space lab module. Last November, China achieved a robotic docking between unmanned Shenzhou VIII and Tiangong-1.Zhou Jianping, chief designer of the manned space program, said that China needs to conduct multiple missions to test its ability in space rendezvous and docking, a key technology for assembling a space station.

"One successful mission does not mean all similar missions will be successful, because the space docking results could vary due to different types of space vehicles and different space environment. So carrying out multiple spaceflights will enhance the maturity and reliability of China's space docking technology," he said.

Another big challenge astronauts will face is the zero-gravity environment, in which they will stay for more than 10 days, longer than the past three manned missions combined.

Mission commander Jing, who stayed in space for three days in 2008, said that living in weightlessness feels like standing on one's head.

"You feel the blood has all gushed to the head. The head and the eyes feel swollen, and the nose seems to get stuck. On the Earth, many people can stand upside down for just a few minutes," he said. The crew will experience that sensation for more than 10 days.

Research showed that human beings in weightlessness usually suffer fluid redistribution, bone and muscle loss and space-motion sickness that can cause dizziness and nausea.

Astronauts returning from long-duration space missions are also likely to suffer orthostatic intolerance, which means they feel dizzy when they try to stand up from a lying posture after returning to Earth.

Female astronauts are more likely to develop the intolerance and will usually take a longer time to recover than their male counterparts, Chen said.

The three astronauts may not be able to walk out of the spacecraft's re-entry capsule because of this condition after landing in the central part of the Inner Mongolia autonomous region in late June, he said.

"We will simplify the welcoming ceremony for the returning astronauts. When they return, they will be escorted to Beijing for recovery in a lying or sitting posture," he said.