A 20-ton piece of a Chinese rocket passed over New York City and Los Angeles before it crashed to the Earth this week, scientists tracking its descent say.

The debris, which came from a rocket that was launched in early May, is the fifth-largest piece of space junk to plunge uncontrolled through Earth’s atmosphere, according to experts who track space debris and satellites. It’s the largest object in nearly three decades to plunge to Earth unexpectedly, demonstrating the potential danger of such large objects as they make uncontrolled re-entries from low-Earth orbit.

The U.S. Space Force’s 18th Space Control Squadron, which tracks space junk and re-entries from Vandenberg Air Force Base, California, confirmed that the rocket stage passed through Earth’s atmosphere Monday at 11:33 a.m. ET as it was flying over the Atlantic Ocean. About 15 to 20 minutes before that, the debris sailed over New York City, according to Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

“On its last orbit of the Earth, this rocket stage happened to pass directly over Los Angeles and New York City,” McDowell said. “It’s just a strange coincidence that it happened to fly over two major urban areas on its last orbit, but if it had come down earlier, there would have been some drama.”

China launched the Long March 5B rocket on May 5 as part of an uncrewed test flight of a new crew capsule.

In low-Earth orbit, the rocket stage would have been traveling at roughly 18,000 mph, but friction would have significantly slowed the debris as it made the fiery journey through the atmosphere. McDowell estimated that any pieces that don’t burn up would hit the ground at 100 mph.

“It wouldn’t be enough to wipe out New York,” he said. “It might take out a floor of a building, but either way, that is still more than we need right now.”

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But even if space junk doesn’t rain down on a densely populated city, there are still clear dangers, McDowell said.

It’s thought that some of the surviving pieces of the rocket stage may have landed in Cote d’Ivoire in West Africa, where villagers and African media reported finding a roughly 40-foot pipe and other metal debris. While the source of the objects hasn’t yet been confirmed, McDowell said the locations fall along the predicted track of the rocket stage’s re-entry.

“The time and position are convincing,” he said. “It’s downrange of where it began to enter the atmosphere, and it’s right on the track.”

McDowell said there are some reports that debris may have hit the roof of a house in Cote d’Ivoire, but so far there are no reported casualties from the event.

While it’s common for spent rocket parts to fall back to Earth after boosting spacecraft or satellites into orbit, space agencies and satellite companies typically take extra precautions for decommissioned satellites or rocket stages of this size so they aren’t left to make uncontrolled re-entries.

“You might make it so that the engine can restart after you’ve delivered your satellite into orbit, so you fire up the engine and bring the rocket stage in over the South Pacific, where it’s not going to hit anyone,” McDowell said. “This rocket stage was just let in low-Earth orbit until friction brought it down. That’s definitely not the current best practice by international standards.”

This isn’t the first time parts of a Chinese rocket have fallen over populated areas. In March, a massive side booster was found downrange from a launch site in Xichang, in China’s Sichuan province. And in November, after a Long March 3B satellite launched two satellites into orbit, parts of the rocket’s boosters crashed into a nearby Chinese settlement.

Among the largest human-made objects to make uncontrolled re-entries are NASA’s Skylab space station, which burned up over the Indian Ocean and western Australia in 1979, and the Soviet Union’s Salyut 7 space station, which plummeted back to Earth in 1991 and broke up over Argentina.