A massive section of a rocket made its uncontrolled return to Earth on Saturday.

Aerospace experts had been tracking the debris for days, as it was initially unclear exactly when -- and where -- it will land.

The section is part of a rocket called Long March 5B, which China launched into orbit on July 24 to deliver a lab module to China's Tiangong Space Station.

Typically, rocket debris is meant to come back down in a controlled way, usually into the ocean to avoid populated areas. But during launch, the 23-metric-ton rocket booster reached orbit and is now being dragged toward Earth for an uncontrolled reentry, according to the Aerospace Corporation, a nonprofit that provides technical guidance on space missions to military, civil and commercial customers.

The exact point where the rocket booster would reenter the Earth's atmosphere couldn't be determined until within hours of reentry, experts said.

"The snag is that the density of the upper atmosphere varies with time -- there's actually weather up there -- and so that makes it impossible to predict exactly at what point the satellite will have plowed through enough atmosphere to melt and break up and finally reenter," astronomer Jonathan McDowell said in a Twitter conversation on the uncontrolled reentry of the rocket hosted by the Aerospace Corporation this week.

"If you're an hour off in predicting when that's going to happen, because it's going at 17,000 miles an hour, you're 17,000 miles off in where it's going to come down. And that's the big challenge with all this," he said.

The two previous launches of the Long March 5B rocket both had uncontrolled reentries, with rocket debris landing near the west coast of Africa in 2020 and in the Indian Ocean in 2021.