A week ago, China launched the newest version of its largest rocket, the Long March 5B, from its southernmost spaceport. The launch proceeded normally and represented another success for China as it seeks to build a robust human spaceflight program. Over the next few years, this rocket will launch components of a modular space station.

Notably, because of this rocket’s design, its large core stage reached orbit after the launch. Typically, during a launch, a rocket’s large first stage will provide the majority of thrust during the first minutes of launch and then drop away before reaching an orbital velocity, falling back into the ocean. Then, a smaller second stage takes over and pushes the rocket’s payload into orbit.

However, the Long March 5B rocket has no second stage. For last week’s launch, then, four liquid-fueled strap-on boosters generated most of the thrust off the launch pad. After this, the core stage with two YF-77 main engines pushed an experimental spacecraft into orbit before the payload separated.

This left the large core stage, with a mass slightly in excess of 20 tons, in an orbit with an average altitude of about 260km above the Earth. Because the apogee of this orbit was only about 160km above the planet, the core stage was slowly drawn back toward the planet as it interacted with the planet’s upper atmosphere.

This is a rather large object to make an uncontrolled return to Earth. According to Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics and keen observer of satellites, this is the largest vehicle to make an uncontrolled reentry into Earth’s atmosphere since 1991, when the Soviet Salyut 7 space station broke up over Argentina.

Engines likely survived

The core stage is estimated to have a mass of about 21 tons, including extra fuel on board, but it’s not clear how much of the rocket survived its interaction with the atmosphere. Although he did not have access to a detailed model of debris, McDowell estimated that at the very least, dense components of the rocket’s engines would have survived.

“I would not be surprised if several bits with masses of order 100 to 300 kg hit the surface,” he told Ars. “I would be a bit surprised if anything as big as 1 metric ton did.”

The US Space Force’s 18th Space Control Squadron confirmed that the core stage re-entered Earth’s atmosphere at 11:33am ET (15:33 UTC) on Monday at a location over the Northern Atlantic Ocean. At this point, the core stage would have been at an altitude of 80km and rapidly descending toward Earth. McDowell said there were some reports emerging about possible debris found downrange in Cote d’Ivoire.

It is perhaps worth noting that before it entered Earth’s atmosphere, the core stage track passed directly over New York City. Had it reentered the atmosphere only a little bit earlier, the rocket’s debris could have rained down on the largest metro area in the United States.

China has previously shown a disregard for debris from its rocket launches, however. It frequently launches rockets from pads surrounded by land. This has led to debris from first and second stages falling on villages in the country.

It is not clear whether future launches of the Long March 5B rocket will continue to send its core stage into an unstable orbit or if this was a one-off instance during the rocket’s test flight. Certainly this will be discouraged, at the very least, by other nations.

https://arstechnica.com/?p=1675084