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Less than two days after launching from southern China, the Tianzhou 1 supply ship glided to an automated linkup with the Chinese Tiangong 2 space laboratory around 240 miles (385 kilometers) above Earth on Saturday, ready for several months of technical demonstrations to prepare for assembly of a space station in the next few years.

The Tianzhou 1 cargo craft, about the size of a double-decker bus, docked with the Tiangong 2 module at 0416 GMT (12:16 a.m. EDT) Saturday after an autonomous computer-controlled rendezvous. A docking ring between the two craft retracted a few minutes later to create a firm connection as they sailed over Chinese ground stations.

The high-altitude linkup occurred after a journey from Tianzhou 1’s launch pad at the Wenchang space center, a modern facility on Hainan Island in the South China Sea. China completed construction of the launch complex in 2014 and conducted the first launches there last year.

Tianzhou 1 blasted off on top of a Long March 7 rocket Thursday and reached a preliminary orbit around 10 minutes after liftoff. The craft’s on-board thrusters fine-tuned its orbit for final approach to Tiangong 2 over the last two days.

The robotic supply ship will pump propellants into the Tiangong 2 mini-space station, which launched last year and hosted two astronauts for a month-long stay in October and November. The refueling experiment will validate key technologies China needs to build and maintain a much larger orbital complex starting as soon as next year.

The multi-piece space station should be completed by 2022, when it will be ready to support three-person crews for expeditions lasting as long as six months.

China has not released a timeline for the refueling tests, but officials said Tianzhou 1 will dock with Tiangong 2 three times and remain with the space lab for two months, then spend around three months in solo flight conducting more experiments before re-entering Earth’s atmosphere later this year.

Chinese engineers consider the refueling demonstration one of the final major tests before kicking off construction of a permanently-staffed space station due to be complete by 2022.

“This means a lot because this one of the most important for China establishing its own space station,” said Wang Jianshe, deputy director of the science and technology commission at the China Academy of Aerospace Propulsion Technology. “Only by doing this can the long-term operation of a future space station be guaranteed.”

Transfer ports located around the docking mechanisms on both spacecraft will be used to route liquid propellants from Tianzhou 1’s storage tanks into the Tiangong 2 space lab.

“The spacecraft can only carry a limited amount of propellant into space,” Wang told China’s state-run CCTV television network. “After all the propellant has been used, the spacecraft can no longer function. In order to extend its life span, refueling in orbit has to take place. There are many challenges during this process — for example, the docking technology, the recycling of high-pressure gas, and the refueling itself.”

Tianzhou means “heavenly vessel” in Chinese.

Designed to accommodate up to 14,300 pounds (6,500 kilograms) of payloads, the Tianzhou spacecraft is similar in purpose to cargo freighters that fly to the International Space Station, such as the Russian Progress supply ship and the commercial Cygnus and Dragon carriers built by Orbital ATK and SpaceX.

In design and capability, the Tianzhou is most like Russia’s Progress and Europe’s now-retired Automated Transfer Vehicle, which carried dry goods, water and propellant to the orbiting outpost. The U.S. commercial supply ships and Japan’s HTV logistics vessel cannot refuel the space station.

“We’ve established contingency plans while designing the refueling system,” Wang said. “In the event of a propellant leak during the process, the system can locate its position immediately, and isolate the leak, and recombine to continue the refueling.”

Crews on China’s space station will need fresh equipment, experiments and other supplies during long-duration missions. The longest Chinese spaceflight to date was the Shenzhou 11 crew’s visit to the Tiangong 2 space lab last year, a mission that lasted approximately 32 days.

“For example, the daily supplies of the astronauts, including food and clothing, extravehicular spacesuits, as well as drinking water with special tanks,” said Bai Mingsheng, chief designer of the Tianzhou 1 spacecraft at CASC. “We will see if the Tianzhou 1 spacecraft meets the demand of transporting and resupplying various goods through this launch.”

According to the government-managed Xinhua news agency, ground crews packed a “test load” of items inside Tianzhou 1’s pressurized cabin, simulating the equipment a three-person crew would need for one month in space. The payloads include crew provisions, water tanks and oxygen and nitrogen vessels designed to replenish the space lab’s breathable atmosphere.

“The Tianzhou 1’s carrying capability is designed according to the scale of the space station, aiming to achieve the highest carrying capacity, with the lowest structural weight,” said Bai Mingsheng, chief designer of the Tianzhou 1 spacecraft at the China Aerospace Science and Technology Corp., in an interview with CCTV before the launch. “There is an index for the spacecraft’s carrying capacity, or payload ratio, as it is called. The payload ratio of Tianzhou 1 reaches 0.48, which ranks fairly high in the world.”

With a launch mass of nearly around 28,460 pounds (12,910 kilograms), Tianzhou 1 is the heaviest spacecraft ever launched by China, slightly bigger than the Tiangong 2 space lab itself.

The Tianzhou 1 spacecraft measures around 34.8 feet (10.6 meters) long and 11 feet (3.4 meters) in diameter. Once in orbit, the spaceship extended its power-generating solar panels to a span of approximately 49 feet (15 meters) tip-to-tip.

Chinese officials said last month that the core component of the country’s space station will launch on a heavy-lift Long March 5 rocket some time in 2018. The Tianhe 1 module will be joined by China’s second supply ship, named Tianzhou 2, for a cargo delivery before the first astronaut crew visits the orbiting facility aboard the Shenzhou 12 spacecraft.

Two more modules will be added to the space station by 2022, completing assembly of a complex that is expected to have a mass of more than 60 metric tons, about one-seventh that of the International Space Station, and comparable to the mass of NASA’s Skylab station in the 1970s.

While no crews will be present during Tianzhou 1’s mission, the freighter carries several experiments, including one on “non-Newtonian” gravity, according to Chinese media reports.

Other research investigations will study the proliferation and differentiation of human embryonic stem cells into germ cells to gather data on the possibility of human reproduction in space, Xinhua reported.

The stem cells and embryos of mice are also on-board Tianzhou for an experiment into how animals and humans could regrow lost tissues and organs, Xinhua said. Researchers also sent up an experiment to test out a new medicine for osteoporosis.

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