The third and final module for China’s Tiangong national space station successfully docked with it on Monday, the China Manned Space Agency (CMSA) has confirmed. This third module is named Mengtian, and it joins the Tianhe core module and the Wentian experimental module, to complete Tiangong.

Assembly of the space station is not quite complete however, as Mengtian still has to be moved from its current docking position (in line with Tianhe) to its final location, attached to the port (left) docking ring, which will place it opposite Wentian and establish Tiangong’s definitive T-shape configuration.

Mengtian, CMSA has stated, has a length of 17.9 m, a diameter of 4.2 m and a volume of nearly 110 m³, of which about 32 m³ is usable by astronauts. It has a mass of some 22 t, and is designed to accommodate a spectrum of science experiments, including combustion, materials science and fluid physics.

The new module is also equipped with an airlock for payloads. Experiments placed in this airlock can be grasped by a robotic arm (which has a length of 5.2 m), which is mounted to the Wentian module, and then moved to ‘adapters’ on the outside of the module. Mengtian can also be used to launch CubeSats or other small spacecraft, with masses of up to 100 kg.

CMSA plans to launch a cargo spacecraft to Tiangong sometime in the next couple of weeks, and in late November or early December launch a new crew to take over the space station. This will mark Tiangong’s first crew rotation. There is a possibility that the space station could, in the future, be expanded to six modules, but this might require other countries to participate in the programme, converting it into an international space station.

However, the first stage of the Long March 5B rocket that lifted Mengtian into orbit also entered orbit. This has happened with the previous three Long March 5B launches, because the first stage is large. The first stage from this launch is expected to re-enter the Earth’s atmosphere during the next week or so. The problem is that, as with the previous Long March 5B first stage re-entries, it will be uncontrolled. This empty first stage has a mass of around 21 t and because of a host of factors, including solar activity, the timing and location of its re-entry cannot be accurately calculated. There is thus the possibility that it might cause damage when it impacts the Earth’s surface.