HELSINKI — A large rocket stage used to launch the final module for China’s space station made an uncontrolled reentry into the atmosphere Friday after precautionary airspace closures took effect in southern Europe.

The roughly 21-metric-ton dry mass Long March 5B rocket stage reentered over the south-central Pacific Ocean at 6:01 a.m. Eastern (10:01 UTC) Nov. 4, according to the U.S. Space Command, just over four days after its launch.

The reentry event had been tracked by U.S. and European authorities which provided predictions for the reentry. China’s human spaceflight agency, CMSA, regularly released orbital data updates for the rocket stage but provided no predictions. 

China launched the third and final module for its Tiangong space station with the fourth Long March 5B rocket Oct. 31. The Mengtian module successfully docked with the station 13 hours later.

As with China’s previous space station module launches, the Long March 5B first stage also acts as the upper stage for the mission and entered orbit, which is exceptional in international spaceflight. 

The stage reentered over the Pacific Ocean but not before causing issues for air traffic in Europe, due in part to the uncertainty in predicting reentry events.

A section of airspace over northern Spain was closed early Friday based on a bulletin issued by the European Aviation Safety Agency (EASA) Nov. 3 based on predictions from the EU Space Surveillance and Tracking (EUSST). France also closed airspace south of Corsica from 9:30 to 10:30 a.m. local time.

 The bulletin noted a number of cities potentially to be affected based on ground tracks including Lisbon in Portugal, Barcelona and Madrid in Spain, Marseille in France, and Rome in Italy.

The advisory recommended National Aviation Authorities regularly monitor and take into consideration predictions of the Long March 5B reentry and further recommended that, “the respective authorities of the affected Member States to consider implementing and notifying airspace restrictions on a path of minimum 70 km and up to 120 km on each side of the estimated re‐entry trajectory, a few minutes before and after the time window.”

The large prediction windows for the reentry are due to the challenges of modeling, including variables such as atmospheric fluctuations which affect how quickly the orbit of an object decays. Prediction windows are on the order of hours even within a day on the reentry. 

Objects in low Earth orbit travel at nearly eight kilometers per second, meaning a deviation of even a few minutes means a reentry more than a thousand kilometers away.

The Long March 5B reentry is a large and prominent symptom of a wider problem. A recent Nature Astronomy paper published earlier assesses that current practices mean there is a 10 percent chance of uncontrolled reentries causing one or more casualties over a decade.

Around 50 objects with a mass of more than one ton reenter the atmosphere randomly each year, according to the Aerospace Corporation.

Long March 5B reentries account for four of the six largest objects making uncontrolled reentries. The reentries of the U.S.’s Skylab in 1979 and the Soviet Union’s Salyut 7 in 1991, at roughly 77 tons and 40 tons respectively, are the only higher mass events.

China’s next apparent scheduled use of the Long March 5B is to launch the Xuntian observatory around late 2023 or 2024. Xuntian is a Hubble-class space telescope which will co-orbit with the Tiangong space station.

Andrew Jones covers China's space industry for SpaceNews. Andrew has previously lived in China and reported from major space conferences there. Based in Helsinki, Finland, he has written for National Geographic, New Scientist, Smithsonian Magazine, Sky...