(Source: CGTN)

The Tianzhou-1 cargo spacecraft has two main tasks: docking and refuelling.

In the two days after the launch of Tianzhou-1, all its efforts go into catching up and trying to dock with the Tiangong-2 space lab, which was sent into orbit last year. When docking is complete, the new combined craft will continue to orbit for two months. During this time the cargo ship will attempt to refuel the space lab three times. This process is really like air to air refuelling, just at much higher altitude and higher speed. It also needs more sophisticated manoeuvres to control it, in case the dangerous propellant leaks into outer space. In-orbit liquid propellant transfer is critical to ensure a sustainable fuel supply for the spacecraft, and is a cornerstone in building China's space station.

After the first mission is completed, the two crafts separate and prepare for a second docking. This time, they need to perform some acrobatic movements. Tianzhou-1 speeds up and dives under the Tiangong space lab to get behind and catch it on its other side. The two crafts spin around at the same time and link up when they are both in position. Trust me, this is not just for fun but is a very necessary exercise. China's ultimate goal, the space station, consists of different modules and components. All of these need to be docked to each other. So it's necessary to practice docking from front and back.

The two dockings will adopt what scientists call "the conventional measure". It's an established method but will take two days to finish. But here comes the third dock... Tianzhou-1 needs to verify the so-called express link-up, the latest technology to speed up the entire docking process from days to just 6 hours. If it succeeds, China would be the second country to master this technology, after Russia.

Tianzhou-1's entire mission will take months. But once it's finished, it means China is just one step away from its ultimate dream, to build a space station by 2022.