Had the core stage reentered the atmosphere only a little bit earlier, perhaps 15 to 20 minutes, the rocket's debris could have rained down on the largest metro area in the United States.

A week ago, China launched the newest version of its largest rocket, the Long March 5B, from its southernmost spaceport. The launch proceeded normally.

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 perigee 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.

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 the order of 100 to 300kg 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 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.

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, perhaps 15 to 20 minutes, the rocket's debris could have rained down on the largest metro area in the United States.

The core stage was in an uncontrolled decaying orbit for 6 days and it missed New York by 15 minutes. That's 0.1% of the decay time.

That behavior is just unnecessary.