On Monday, China’s space program successfully launched another large piece of its space station. The 23-ton Mengtian module will provide pressurized volume for scientific experiments and is the final large living area to be added to the Tiangong space station.

China has constructed its modular space station—similar in form and function to the International Space Station, although smaller—on time and with few major issues. This can be considered a triumph for the Chinese space program, which now has capabilities second only to NASA and the commercial space industry in the United States.

However, the assembly of the space station has had one unfortunate side effect. To launch the main modules of Tiangong, including Monday’s flight, China has used a modified version of its powerful Long March 5B rocket. And as part of the overall mission profile, the vehicle’s massive core stage reenters Earth’s atmosphere in an uncontrollable manner.