China is preparing to send three astronauts to live on its space station for six months—a new milestone for a program that has advanced rapidly in recent years.

It will be China's longest crewed space mission and set a record for the most time spent in space by Chinese astronauts. The Shenzhou-13 spaceship is expected to be launched into space on a Long March-2F rocket early Saturday morning from the Jiuquan Satellite Launch Center on the edge of the Gobi Desert in northwestern China.

The first crew who served a 90-day mission aboard the main Tianhe core module of the space station returned in mid-September.

The new crew has two veterans of space travel. Pilot Zhai Zhigang, 55, performed China's first spacewalk. Wang Yaping, 41, and the only woman on the mission, carried out experiments and led a science class in real-time while traveling on one of China's earlier experimental space stations. Ye Guangfu, 41, will be traveling into space for the first time.

The mission is expected to continue the work of the initial crew, who conducted two spacewalks, deployed a 10-meter (33-foot) mechanical arm, and held a video call with Chinese leader Xi Jinping.

China Manned Space Agency Deputy Director Lin Xiqiang said the rocket was fueled and ready to fly. "All systems conducting the Shenzhou-13 mission have undergone a comprehensive rehearsal. The flight crew is in good condition and our pre-launch preparations are in order," Lin said at a Thursday briefing.

The crew's scheduled activities include up to three spacewalks to install equipment in preparation for expanding the station, verifying living conditions in the module and conducting experiments in space medicine and other areas, Lin said.

China's military, which runs the space program, has released few details but says it will send multiple crews to the station over the next two years to make it fully functional. Shenzhou-13 will be the fifth mission, including trips without crews to deliver supplies.

When completed with the addition of two more modules—named Mengtian and Wentian—the station will weigh about 66 tons, a fraction of the size of the International Space Station, which launched its first module in 1998 and will weigh around 450 tons when completed. Lin said the two additional modules would be sent before the end of next year during the stay of the yet-to-be-named Shenzhou-14 crew.

China was excluded from the International Space Station largely due to U.S. objections over the Chinese program's secretive nature and close military ties. It made plans to build its own space stations in the early 1990s and had two experimental modules before starting on the permanent station.

U.S. law requires congressional approval for contact between the American and Chinese space programs, but China is cooperating with space experts from countries including France, Sweden, Russia and Italy.

Lin said China was expanding such cooperation, with Ye Guangfu having undergone training with the European Space Agency in 2016, and European astronauts participating in China's sea survival training in 2017.

"We welcome astronauts from other countries entering our space station and conducting international cooperation," Lin said. "We believe that after the station enters the operation and utilization phase, more foreign astronauts will visit our station."

China has launched seven crewed missions with a total of 14 astronauts aboard since 2003, when it became only the third country after the former Soviet Union and the United States to put a person in space on its own. Two Chinese astronauts have flown twice.

Along with its crewed missions, China has expanded its work on lunar and Mars exploration, including placing a rover on the little-explored far side of the Moon and returning lunar rocks to Earth for the first time since the 1970s.

China this year also landed its Tianwen-1 space probe on Mars, whose accompanying Zhurong rover has been exploring for evidence of life on the red planet.

Other programs call for collecting soil from an asteroid and bring back additional lunar samples. China has also expressed an aspiration to land people on the moon and possibly build a scientific base there, although no timeline has been proposed for such projects. A highly secretive space plane is also reportedly under development.

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