Long March 5B: Out-of-control Chinese rocket tipped to be right over NZ when it comes down

A huge out-of-control piece of Chinese space junk is currently expected to be right above New Zealand when it comes down. 

The Long March 5B rocket, launched on April 29, is edging closer to a fiery and unpredictable end, and where in the world it will come down remains a mystery. 

But right now, early Saturday afternoon, one tracker has it flying over Northland at its estimated reentry time of 4:19pm on Sunday. 

It's currently within touching distance of the atmosphere - at just 152km into the sky at its closest approach, having come down 48km in the past 24 hours.

The rocket - also known as CZ-5B - was used to put a module for a new Chinese space station into orbit. But unlike most rockets, which are discarded before they get too high, it entered orbit too. It's not clear whether it was meant to or not, Chinese officials remaining tight-lipped.

It's about 21 tonnes and 30m long, and the US is keeping a close eye on it.

"We're hopeful that it will land in a place where it won't harm anyone," US Defense Secretary Lloyd Austin told BBC News. "Hopefully in the ocean, or someplace like that."

A similar rocket a year ago burned up over the Atlantic, but parts of it rained down in western Africa. 

"The chance of someone being hurt is maybe a percent or so," Jonathan McDowell, Harvard-Smithsonian Center for Astrophysics astronomer told The Atlantic.

"The chance of you being hurt is 8 billion times smaller than that, so don't worry about it."

It's hard to know when it will make its final descent. The density of the upper atmosphere is key, and it changes from day to day, location to location. 

Aerospace Corporation earlier this week picked it would be Monday (NZ time), give or take 40 hours either side. It's since narrowed its prediction to 4:20pm on Sunday, give or take eight hours. A graphic on its site shows the Long March 5B will be right above Northland at that time.

Another tracker, orbit.in-now.com, has it coming down about 4:40pm Sunday - but travelling at almost 28,000km/h, it will be likely be somewhere over the Pacific if not already over the United States by then. 

The truth is, the predictions currently have a 16-hour window - eight hours either side. So no one really knows. 

The US military has said it won't be shooting it down to ensure it doesn't land anywhere people live. 

China has claimed it will mostly burn up on reentry, a spokesperson saying it "has a very low probability of causing harm to aviation activities and the ground".

State-controlled newspaper the Global Times said fear of the rocket was "Western hype".