Final module docks at China’s Tiangong space station

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Huge Long March 5B rocket stage also in orbit with uncontrolled reentry expected in near future.

HELSINKI — A third module has arrived at China’s space station, completing the construction of the country’s crewed orbital outpost.

A Long March 5B rocket lifted off from the Wenchang Satellite Launch Center at 3:37 a.m. Eastern Oct. 31. Launch success was announced inside 25 minutes of launch with the Mengtian module in low Earth orbit.

Mengtian used its own propulsion to match orbit and rendezvous with the Tiangong space station—currently in a roughly 380 by 387-kilometer orbit—and connected with a forward port on Tiangong’s docking hub just under 13 hours after launch at 4:27 p.m. Eastern, China’s human spaceflight agency, CMSA, confirmed.

The new experiment module joins two earlier modules: the Tianhe core module and Wentian experiment module. Mengtian will soon be transpositioned to the port docking ring to complete the T-shaped arrangement of the space station. Wentian was transpositioned to the starboard docking ring in late September.

Mengtian (“dreaming of the heavens”) is a 17.9-meter-long, 4.2-meter-diameter and roughly 22-ton module designed to host a range of science experiments with areas of research including fluid physics, combustion and materials science and space technologies.

It has a total volume of nearly 110 cubic meters, including around 32 cubic meters available for use by astronauts, according to CMSA

A view inside of the Mengtian module ahead of launch. Credit: CMSA

Mengtian also has a payload airlock which will allow the small, 5.2-meter-long robotic arm launched with the Wentian module to grasp science experiments and install them on payload adapters on the outside of the module. The on-orbit release mechanism can deploy small spacecraft or CubeSats of up to 100 kilograms into orbit.

The crew of three astronauts comprising the Shenzhou-14 mission watched events from aboard Tiangong, waiting for the arrival of Mengtian.

Monday’s launch was the ninth of 11 missions mapped out for the construction and testing of Tiangong. The launch of a cargo spacecraft and separate crew launch will complete this phase before the end of the year.

A Long March 7 rocket arrived at Wenchang Oct. 11 and will be assembled to launch the Tianzhou-5 cargo mission. 

Launch of Tianzhou-5 could take place in the first half of November and will provide supplies for the upcoming Shenzhou-15 crewed mission. The Tianzhou-4 spacecraft will undock from Tiangong and be deorbited ahead of these missions. Shenzhou-15 will launch from the Jiuquan Satellite Launch Center in the Gobi Desert on  a Long March 2F rocket as early as late November, following the arrival of Tianzhou-5 at Tiangong.

The arrival of Shenzhou-15 at Tiangong will see China’s first crew handover, and mark the start of the operational phase of the space station.

China plans to keep Tiangong occupied for at least 10 years, conducting science experiments including international experiments through an initiative with UNOOSA. It could also host foreign astronauts in the future. 

China will add further capabilities to the Tiangong in the future. The Xuntian optical module—a co-orbiting, Hubble-class space survey telescope—is planned for launch in late 2023 or 2024.

Xuntian has a two meter aperture and 2.5 gigapixel camera. With its wide field of view it aims to survey around 40 percent of the sky over the course of ten years. It will be capable of docking with Tiangong for maintenance and repairs.

The space station itself could also be expanded from three to six modules, according to Chinese space officials. Such an expansion may depend upon other countries joining the project.

Long March 5B stage incoming

As with previous Long March 5B missions the large first stage of the rocket entered orbit and is likely to make an uncontrolled reentry within the next week or so.

U.S. Space Force 18th Space Defense Squadron tracked Mengtian in a 179 x 323 kilometer orbit inclined by 41.46 degrees hours after the launch, with the Long March 5B first stage in a 173 x 314 kilometer orbit.

The three earlier Long March 5B launches notably saw the large first stage of the rocket enter orbit and make uncontrolled reentries. The previous launch sent the Wentian module into orbit and saw the first stage reenter the atmosphere over Southeast Asia less than a week later.

Where and when the latest roughly 21-metric-ton empty stage will reenter cannot be accurately estimated. The stage will orbit the Earth once every 90 minutes, with the decay of its orbit dependent on atmospheric fluctuations. 

Variables including solar activity, which can puff up the atmosphere, leading to greater drag at higher altitudes.

The China Academy of Launch Vehicle Technology (CALT), the maker of the Long March 5B, did not comment on the previous incidents. It has however stated that it carries out passivation of spent stages, including venting remaining propellant depleting batteries, to prevent debris-causing explosions in orbit in line with international practices.

The wider issue of uncontrolled rocket body reentries is assessed in a Nature Astronomy paper published July 11. It estimates that, with current practices, there is a 10 percent chance of uncontrolled reentries causing one or more casualties over a decade.