A 30-metre-long Chinese rocket weighing 21-tonnes is expected to crash to Earth this week - although no-one knows where it will land.
Long March 5B launched last week and delivered part of the Chinese space station Tianhe into orbit.
Experts now say the rocket could crash to Earth in the next few days - potentially in a populated area, according to Space News.
Spaceflight observer Jonathan McDowell said: "The Long March 5B core stage is seven times more massive than the Falcon 9 second stage that caused a lot of press attention a few weeks ago when it reentered above Seattle and dumped a couple of pressure tanks on Washington state."
The rocker is currently in low-Earth orbit circling the globe every 90 minutes - passing close to New York, Madrid, Beijing, Wellington and parts of Chile - as well as places in between.
The rocket may burn up as it enters the atmosphere, but there could also be large chunks of debris.
Holger Krag, head of the Space Safety Programme Office for the European Space Agency, said: "It is always difficult to assess the amount of surviving mass and number of fragments without knowing the design of the object, but a reasonable 'rule-of-thumb' is about 20-40% of the original dry mass."
