Long March 5B, a Chinese Rocket, Expected to Tumble Back to Earth


This article was revised shortly after publication to reflect an updated forecast from The Aerospace Corporation.

No, you are almost certainly not going to be hit by a 10-story, 23-ton piece of a rocket hurtling back to Earth.

That said, the chances are not zero. Part of China’s largest rocket, the Long March 5B, is tumbling out of control in orbit after launching a section of the country’s new space station last week. The rocket is expected to fall to Earth in what is called “an uncontrolled re-entry” sometime on Saturday or Sunday.

Whether it splashes harmlessly in the ocean or impacts land where people live, why China’s space program let this happen — again — remains unclear. And given China’s planned schedule of launches, more such uncontrolled rocket re-entries in the years to come are possible.

The country’s space program has executed a series of major achievements in spaceflight in the past six months, including returning rocks from the moon and putting a spacecraft in orbit around Mars. Yet it continues to create danger, however small, for people all over the planet by failing to control the paths of rockets it launches.

On Thursday, the Aerospace Corporation, a nonprofit largely financed by the federal government that performs research and analysis, predicts re-entry will occur on Saturday at 11:43 p.m. Eastern time. If that is accurate, debris could shower down over northeastern Africa, over Sudan.

Uncertainties over the time — give or take 16 hours — and location remain large. A day before, Aerospace’s prediction put re-entry more than one hour earlier, over the eastern Indian Ocean.

When the booster burns up depends, for instance, on the sun. A rise in the intensity of the solar wind — charged particles spewed out by the sun — would puff out Earth’s atmosphere, increasing atmospheric drag on the rocket booster and speeding its fall. The tumbling of the rocket stage also complicates calculations.

The United States Space Command and Russia’s space agency are both tracking the rocket core. The Russian statement noted that the re-entry would not “affect the territory of the Russian Federation.” The Space Command promised regular updates ahead of a potential re-entry.

Because the booster is traveling at 18,000 miles per hour, a change of minutes shifts the debris by hundreds or thousands of miles. It is only a few hours before re-entry that the predictions become more precise.

“It’s an engineering decision based on probabilities,” Dr. McDowell said. He said the Chinese engineers could have designed the trajectory to remain suborbital, falling back to Earth right after launch, or they could have planned an additional engine firing to drop it out of orbit in a way that posed no possible danger.

“It’s not a trivial thing to design something for a deliberate re-entry, but it’s nevertheless something that the world as a whole has moved to because we needed to,” said Ted J. Muelhaupt, principal director of Aerospace’s Center for Orbital and Re-entry Debris Studies.

China plans many more launches in the coming months as it completes construction of the country’s third space station, called Tiangong, or “heavenly palace.” That will require additional flights of the mammoth rocket and the possibility of more uncontrolled re-entries that people on the ground will watch nervously, even if the risk from any single rocket stage is tiny.

“It’s in the shared interests of all nations to act responsibly in space to ensure the safety, stability, security and long-term sustainability of outer space activities,” Jen Psaki, the White House press secretary, said on Wednesday, adding that the United States hoped to promote “responsible space behaviors.”

Falling debris has long bedeviled spaceflight.

Mr. Muelhaupt said it could be 10 tons spread over hundreds of miles. “Think about three pickup trucks’ worth of debris,” he said.

The largest cascade of space debris onto the surface occurred when the space shuttle Columbia disintegrated over Texas in 2003 as it re-entered the atmosphere en route to a landing in Florida. The seven astronauts aboard died, but no one on the ground was hurt as 85,000 pounds of debris fell on sparsely populated areas. But had the disaster occurred a few minutes earlier, heavy pieces of the spacecraft like the engines could have hit the ground near Dallas at hundreds of miles per hour.

China’s new space station is intended as an alternative to the International Space Station. The current orbiting outpost, jointly built by NASA, Russia and other partners, has kept humans continually in space for more than two decades now. But Chinese astronauts have been excluded by a U.S. law prohibiting cooperation with China in space.

After the launch of what will be the station’s main living quarters on April 29, China’s leader, Xi Jinping, called it “an important pilot project in the building of a powerful nation in both technology and space,” according to the state television network, CCTV.

Chinese space officials have not publicly addressed the uncontrolled re-entry since then, despite attention and worry around the world.

The Global Times, a newspaper controlled by the Chinese Communist Party, on Wednesday quoted scientists and experts saying that there was little danger and that the space administration had “carefully considered” the prospect of falling debris.

The newspaper, which often reflects the views of more hawkish officials, said the concern and criticism reflected Western efforts to discredit China’s space program.



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