Grab your helmets: A Chinese rocket booster in space the length of two tractor-trailers will plummet back to Earth sometime soon. No one knows exactly when — maybe today.

UPDATE: Nov. 4, 2022, 9:40 a.m. EST The U.S. Space Command announced that two pieces of the Long March rocket booster likely reentered Earth over the Pacific Ocean just after 4 a.m. MDT on Nov. 4. The first fell over the south-central Pacific, the second over the Northeast Pacific. The exact locations of impact have not yet been reported by the People's Republic of China.

Analysts had little data to make an accurate prediction on where in the world the 50,000-pound piece of junk would crash. But, as of the middle of the week, they knew some places where it wouldn't: Most of Europe, as well as everywhere above Chicago's latitude, could be ruled out, according to Aerospace Corporation, a federally funded nonprofit organization that has been tracking this rogue core stage and other previous ones.

This will be the fourth time China has allowed its biggest rocket, Long March 5B, to re-enter Earth's atmosphere without having any control over where it falls. The booster was used during China's final launch on Oct. 31 to construct the Tiangong space station, which means "heavenly palace." The rocket carried the last piece to build the orbiting laboratory.

"We, the world, don't deliberately launch things this big, intending them to fall wherever. We haven't done that for 50 years," Ted Meulhaupt, an Aerospace space debris expert, told reporters on Wednesday. "All the large re-entries that have been uncontrolled in the last few years, except for these, were accidents. Something went wrong. It wasn't supposed to happen."

As of mid-Thursday, the modelers estimated the booster would reemerge between 4 a.m. and 1 p.m. ET Friday.

Why is China allowing this to happen… again?

It's not clear. The lack of information from China to the rest of the world leaves room for speculation.

The point of this mission and the previous three was to send extremely heavy modules into space to build a space station. Any extra weight built into the rocket, such as extra fuel, heat shields, or technology to perform a controlled landing, would take away from what could be hauled into orbit. Some believe that is the reason China has opted to let the rocket fall where it may.

What are the odds of getting hurt?

Most of the junk that plunges back to Earth burns up or splashes down. The odds of space stuff hitting the ocean are good, given two-thirds of the planet is covered in water.

But the uncertainty of where this large hunk of metal will land presents a risk to humans that is well above commonly accepted levels among spacefaring nations. About 88 percent of the world's population lives within the swath of the rocket's potential strike zone, according to Aerospace. And when the booster comes down, statistical modelers estimate between 10 and 40 percent of its weight in material will survive to reach Earth's surface.

Scared?

The Aerospace team says the risk to a person of getting hit is six per 10 trillion.

"You're 80,000 times more likely to get hit by lightning, so no, don't worry about it," Meulhaupt said.

Still, the rocket puts 7 billion people worldwide at some level of risk.

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Why is it hard to forecast where it's going?

There are far too many variables. The density of the atmosphere, the level of solar activity, and the direction of the rocket are all needed to compute the drag force the object will be subject to, which can dramatically impact its trajectory.

Scientists outside of the Chinese space program also need to know how the rocket is built and what it's made of to accurately predict how the materials will break apart and scatter. Aluminum, for example, burns up easily, but titanium has an extremely high melting point.

What do other spacefaring nations think about it?

Many space programs have condemned China for the re-entries, calling them out as reckless, including NASA, but there aren't any specific international laws or treaties that control how a country or company disposes of its launch vehicles.

The Space Liability Convention of 1972, however, concluded that if something bad happens and the debris causes damage, the launching state is responsible. Just a few years later, the one and only claim made under the convention was filed.

"You may recall, back in the late '70s, the Kosmos 954 satellite came down and spread radioactive material across part of Canada," said Marlon Sorge, another space debris expert from Aerospace. "The Soviet Union was responsible for dealing with fixing that."

What could China do about it?

Technology exists to prevent uncontrolled re-entries.

Aerospace experts say space programs can choose materials that don't survive the scorching-hot conditions of re-entry as well, or they could build a launch vehicle that allows the mission control team to guide it down. Doing the latter would require power to restart the engines and technology to steer it to an unpopulated area, like the middle of an ocean.

For NASA's mega moon rocket, the Space Launch System, the U.S. space agency has a disposal plan for every potential spaceflight to put it on a course to splash down at sea.

Outside experts observing the Chinese launches don't believe the state's space program has made any such modifications that would allow for that.