Wellington in path of China rocket's uncontrolled re-entry

The Long March-5B Y2 rocket carrying the core module of China's space station, Tianhe, blasts off from the Wenchang Spacecraft Launch Site on April 29. Photo / Getty Images

A huge rocket launched by China last week is slowly and unpredictably heading back to Earth, with New Zealand potentially in its path.

The 22.5 tonne Long March 5B, a variant of China's largest rocket, was launched successfully from Wenchang last Thursday.

The rocket's core stage delivered its payload directly into Earth's low orbit but a glitch has seen its core stage also in orbit.

The rocket is likely to make an uncontrolled re-entry over the next days or week as the atmosphere drags it crashing to Earth.

An astronomer who tracks objects orbiting Earth, Jonathan McDowell, told SpaceNews that its path takes it "a little farther north than New York, Madrid, Beijing and as far south as southern Chile and Wellington, New Zealand".

Still no TLEs for the Vega launch. One more TLE for object B from the Tianhe launch, whose slow decay rate confirms it is the CZ-5B core stage pic.twitter.com/0dVkUkcpjA

— Jonathan McDowell (@planet4589) April 30, 2021

Satellite trackers have detected the 30m-long rocket travelling at more than seven kilometres per second.

There had been speculation that the Long March 5B core would perform an active manoeuvre to deorbit itself, but that appears not to have happened.

If the rocket doesn't substantially break up it will be one of the largest instances of uncontrolled re-entry of a spacecraft and could potentially land on an inhabited area.

Where and when the new Long March 5B stage will land is impossible to predict.

The Long March-5B-Yao 2 carrier rocket blasts off from the Wenchang Space Launch Centre in Hainan Province on April 29. Photo / Getty Images

The high speed of the rocket body means it orbits the Earth roughly every 90 minutes. A change of just a few minutes in re-entry time results in re-entry point thousands of kilometres away, SpaceNews reported.

The largest and most famous re-entry incident was Nasa's 76-tonne Skylab, whose uncontrolled re-entry scattered debris across the Indian Ocean and Western Australia in 1979.

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China's latest mission is linked to construction of their space station.

Long March 5B was delivering a module, named "Tianhe", or "Harmony of the Heavens", which will become living quarters for three crew members once the massive structure is complete.

The space station, named "Tiangong" (Heavenly Palace), is expected to have a mass of up to 100 tonnes – roughly one-fifth the mass of the ISS, which is 420 tonnes.