PARIS: China’s Long March 5B rocket is in an uncontrolled free-fall towards Earth and no one knows where or exactly when it will burn through Earth’s atmosphere, but the risk of debris hitting an inhabited area remains very small, experts said on Friday.

Re-entry has been forecast to happen between 2100 GMT on Saturday and 0700 GMT on Sunday (between 2am and 12 noon on Sunday in Pakistan). That range will continue to narrow in the coming hours.

China launched the first module of its Heavenly Palace space station on April 29, a milestone in Beijing’s ambitious plan to establish a permanent human presence in space.

The module was propelled by a powerful rocket, whose first stage is currently descending Earthward.

If Chinese ground engineers have no control over the booster stage’s trajectory, it is not due to a technical failure or some unexplained glitch.

The Long March 5B rocket was designed that way.

From a low Earth orbit, bodies are drawn gradually by gravity towards the surface of the planet.

Such objects are usually consumed by heat as friction from an increasingly dense atmosphere increases on approach.

But the Long March rocket is so massive -- up to 18 tonnes -- that it is unlikely to burn up entirely.

“Given the size of the object, there will necessarily be big pieces left over,” said Florent Delefie, an astronomer at the Paris-PSL Observatory.

“If the rocket is composed of materials that don’t separate on re-entry, it’s even riskier,” he added. “That would appear to be the case for the Long March 5B.”

This was no doubt clear to Chinese flight engineers.

“The Chinese should have anticipated a controlled re-entry with a retrorocket, which is what the Russians did when they removed the Mir station from Earth orbit,” explained Nicolas Bobrinsky, head of engineering and innovation at the European Space Agency.

One thing is sure: the spent booster is orbiting on an inclination to the equator of about 41 degrees. This means that any debris will fall within 41 degrees North and 41 degrees South, which includes the tropics and a generous band on either side.

Greece, Spain and north Africa are within that belt, as are most of the United States and China. France and Germany lie outside it.

Given the rocket’s altitude-- 150 to 250 kilometres -- its re-entry is very difficult to anticipate.

Because lower levels in the atmosphere are more prone to variations in density, “we can’t really say exactly when that will happen,” Bobrinsky said.

“But even one hour before impact, the level of uncertainty will be high,” he added.

Published in Dawn, May 8th, 2021