A piece of China’s largest rocket falls back to earth this weekend. Nobody really knows when or where it will land.

On April 29th, a Long March 5B launched a large chunk of China’s next space station, Tiangong. Usually, the large booster stages of rockets return to Earth immediately after being dropped, but the 23-ton core stage of Long March 5B escorted the space station segment into orbit. Because of the friction caused by the rocket rubbing against the air up in the atmosphere, it has since lost altitude and will soon cause what is known as an “uncontrolled reentry” into the earth.

The likelihood of debris from the Long March 5B missile hitting anything or injuring someone is slim, but not zero.

Here’s what you need to know about the Long March 5B’s journey.

The Long March 5B booster could get back into the earth’s atmosphere anywhere between latitude 41.5 degrees north and latitude 41.5 degrees south. That means Chicago, which is just a fraction of a degree further north, is safe, but big cities like New York could be hit by rubble.

As of Saturday, the Aerospace Corporation, a nonprofit funded largely by the federal government that conducts research and analysis, has forecast a re-entry on Saturday at 11:30 p.m. Eastern Time. If it were right, it would land somewhere in the Atlantic with little risk to the people on land

Uncertainties over time – give or take 4 hours – and location stay big. On Friday evening, the aerospace forecast assumed re-entry into waters off New Zealand.

Since the booster is traveling at 18,000 mph, changing the minutes will move the debris hundreds or thousands of miles. Only a few hours before re-entry, the predictions become more precise.

The long March 5th is China’s largest missile and one of the largest currently deployed by any nation. The country’s space program needed a large, powerful vehicle to transport Tianhe, the main module of Tiangong, the new space station, which is expected to be operational by 2022 after more pieces were launched and connected in orbit.

The full rocket contained several parts. Shortly after takeoff, several smaller side amplifiers failed and crashed harmlessly in the Pacific. (The disposal of used, unwanted rocket pieces in the ocean is a common practice.) The core stage of the booster – a 10-story cylinder weighing 23 tons (curb weight) – put the Tianhe module into orbit.

For the past few decades, rocket stages entering orbit typically re-fire the engine after releasing their payloads, causing them to exit orbit and target an unoccupied area like the middle of an ocean.

China did not opt ​​for this launch, and so the big booster is now being returned to the surface in an uncontrolled manner.

China has a long history of letting parts of its space equipment come down where they like.

Missiles from one of its key launch sites, the Xichang Satellite Launch Center in Sichuan Province, routinely fell on rural areas and occasionally caused damage. China has since moved many of its launches, including the Long March 5B, to a new location in Wenchang, a town on Hainan, an island off the southeast coast.

Last year, when a Long March 5B rocket was first launched, a prototype of the Chinese space capsule and crew was lifted. The booster on this missile also made an uncontrolled re-entry, with some debris raining on an Ivory Coast village.

With more large parts of the Chinese space station scheduled to be put into orbit, more Long March 5B launches are expected by 2022. Unless the way China works changes, there is a chance someone will be injured from a fall Boosters will grow.

In March, a runaway SpaceX rocket stage reentered the Earth’s atmosphere near Seattle and surprised residents when it lit up the night sky. Parts of the missile landed on a farmer’s property in Washington state. In this case, the second stage engine was not fired as planned to safely shut it down.

And earlier in the history of space travel, the first space powers themselves had uncontrolled re-entries. The American Skylab, which operated in 1973 and 1974, disbanded when NASA scientists tried to control its descent in 1979. The 77-ton station largely disintegrated over the Indian Ocean, but debris scattered over Western Australia. President Jimmy Carter apologized.