China's Tianzhou-1 cargo spacecraft completed its second docking with Tiangong-2 space lab at 2:55 p.m. Monday, after flying around the space lab.
Tianzhou-1 separated from Tiangong-2 on Monday morning and remained at distance of five kilometers behind the space lab for about 90 minutes.
Then, it was commanded to fly around Tiangong-2 from behind to a distance of five kilometers in front of the space lab. During the flight, both Tianzhou-1 and Tiangong-2 turned in a semicircle.
The experiments tested docking technology at different directions, which is of great importance to building a space station, according to the China Manned Space Engineering Office.
Tianzhou-1, China's first cargo spacecraft, was launched on April 20 from south China's Hainan Province, and it completed automated docking with the orbiting Tiangong-2 space lab on April 22.
The two spacecraft completed their first in-orbit refueling on April 27 and their second in-orbit refueling on June 15.
China is the third country, after Russia and the United States, to master refueling techniques in space, which is crucial in the building of a permanent space station.
As the International Space Station is set to retire in 2024, the Chinese space station will offer a promising alternative, and China will be the only country with a permanent space station.
Tianzhou-1 space freighter completes second docking with Tiangong-2 space lab
China's first cargo spacecraft, Tianzhou-1, has completed its second orbital docking with the Tiangong-2 space lab, testing a new manoeuvre required for the operation of the future Chinese Space Station.
The test began at 09:37 Beijing time (01:37 UTC) on Monday, with the craft uncoupling and Tianzhou-1 retreating to a distance of 5 kilometres.
The cargo spacecraft then manoeuvred itself around Tiangong-2, positioning itself 5 km ahead. Both craft then rotated 180 degrees in preparation for the second docking, occurring from the opposite direction to the first, with the procedure completed at 14.55 (06:55 UTC, 02:55 EDT).
The multi-directional space rendezvous technology tests help pave the way for the country to establish a large, modular space station in low Earth orbit, which will have docking ports to allow both Tianzhou and crewed Shenzhou craft to be docked contemporaneously.
Above: Animation of the second docking between TZ-1 and TG-2 (CMSA).
The test follows on from last week's successful second large scale orbital refuelling procedure.
Tianzhou-1 and the Tiangong-2 space station test bed module had been coupled for 58 days in orbit at around 390 kilometres above the Earth since April 22, following launch of Tianzhou-1 on April 20.
The Tianzhou-1 mission is the final step before construction of the CSS, which will begin with the launch of the already completed Tianhe core module in 2019.
Further tests, science experiments
With a length of 10.6 metres, a maximum diameter of 3.35m and a mass of 13 tonnes, Tianzhou-1 is China's largest spacecraft so far, designed for launch on a Long March 7 rocket to keep the CSS fuelled and its astronauts fully supplied.
The new spacecraft meant new challenges and higher-level requirements, compared with previous missions involving Shenzhou and Tiangong vessels.
After uncoupling following the second docking, the two spacecraft will then orbit separately for three months, during which time the cargo spaceship will carry out space science experiments, while Tiangong-2 will restart its own science tasks.
Above: A rendering of Tianzhou-1 refuelling Tiangong-2 (CMSA).
After the three-month free-flying period, the third and final docking will see Tianzhou-1 use fast-docking technology. It normally takes about two days to dock, while fast docking will take only six hours.
At the end of the five-month mission, Tianzhou-1 will be deliberately de-orbited and burn up in the Earth's atmosphere over the South Pacific. In future missions to the CSS, Tianzhou craft will also be carrying waste from the space station.
Above: A view of the inside Tianzhou-1 and its cargo while in orbit (Framegrab/CCTV).
China's cargo spacecraft starts independent operation
After flying with the Tiangong-2 space lab for about two months, China's Tianzhou-1 cargo spacecraft on Wednesday separated from the space lab and entered a stage of independent operation.
Controlled from Earth, Tianzhou-1 separated from Tiangong-2 at 9:47 a.m. Wednesday, remained 120 meters from the lab and then left to pursue its own course when conditions were deemed appropriate.
In this stage, Tianzhou-1 will orbit about 390 km above the Earth, conducting a variety of experiments.
It will also release a tiny satellite, CubeSat, and will carry out an automated fast-docking with Tiangong-2 and conduct the third refueling of the space lab.