Space boffins are watching the skies for a 23 metric ton Chinese rocket booster that is expected to crash back to Earth.

The debris measures 53.6 meters in length and is a remnant of a mission earlier this week to deliver the Wentian laboratory module to China's Tiangong space station. Wentian itself is an exciting addition to the Chinese orbital complex and is the first module to extend the existing Tianhe core module, which was launched in 2021.

However, being a hefty beast, Wentian required a hefty rocket. In this case, the heavy-lift Long March 5B, used previously to launch Tianhe. Other variants of the Long March 5 were used to launch the Chang'e 5 lunar mission and the Tianwen-1 mission to Mars.

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The problem is that the massive first stage of the Long March 5B also performs the duties of the upper stages of other rockets and so has not been dumped downrange. Instead it entered orbit and, inevitably, will come back down to Earth. The problem is working out when and where .

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While other rockets (including some of China's own) are capable of restarting their engines on orbit and disposing of themselves in a controlled manner, this does not apply this time around. Instead, the descent will be uncontrolled as the orbit decays and, [4]according to federally funded researchers at The Aerospace Corporation "there is a non-zero probability of the surviving debris landing in a populated area."

Our latest prediction for [5]#CZ5B rocket body reentry is: 

🚀 31 Jul 2022 07:52 UTC ± 22 hours

Reentry will be along one of the ground tracks shown here. It is still too early to determine a meaningful debris footprint. Follow this page for updates:  [6]https://t.co/SxrMtcJnj0 [7]pic.twitter.com/xsDqC8rOEt — The Aerospace Corporation (@AerospaceCorp) [8]July 26, 2022

How much debris? The Aerospace engineering and space experts reckon that something this big won't simply burn up in the atmosphere. Instead, the rule of thumb of 20 to 40 percent of the mass reaching the ground is quoted.

[9]Space junk damages International Space Station's robot arm

[10]Chinese rocket plunges into Indian Ocean, still lands sharp rebuke from NASA

[11]Spent Chinese rocket stage set to make an uncontrolled return to Earth

[12]China launches first module of new, crewed, Tiangong-3 space station

To make matters worse, approximately 88 percent of the world's population live under the currently estimated footprint of the debris, although that figure and footprint will shrink in the coming days as new data becomes available and predictions are refined. With luck, there is every chance that whatever returns to Earth will simply fall into the ocean.

This is not China's first rodeo when it comes to the uncontrolled re-entry of rocket stages; debris from two previous Long March 5B launches also tumbled back to Earth. Other countries, including Russia and the US have similarly had issues with uncontrolled entries of large objects; the former with the 1991 return of Salyut 7 and the latter with the infamous scattering over Western Australia of Skylab in 1979.

Earlier this morning, China's Tianzhou-3 cargo spacecraft made a [13]controlled re-entry into the atmosphere, with a small amount of debris deposited into the South Pacific. It is a shame that the same capability has not been applied to the leftover bits of the Long March 5B. ®

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[4] https://aerospace.org/article/aerospace-experts-tracking-massive-uncontrolled-reentry

[5] https://twitter.com/hashtag/CZ5B?src=hash&ref_src=twsrc%5Etfw

[6] https://t.co/SxrMtcJnj0

[7] https://t.co/xsDqC8rOEt

[8] https://twitter.com/AerospaceCorp/status/1552072423343673344?ref_src=twsrc%5Etfw

[9] https://www.theregister.com/2021/06/01/iss_canadarm2_space_junk_collision/

[10] https://www.theregister.com/2021/05/10/long_march_misses_people/

[11] https://www.theregister.com/2021/05/04/china_rocket_long_march/

[12] https://www.theregister.com/2021/04/29/tianhe_tiangong_3_launch/

[13] https://mp.weixin.qq.com/s/TopCU98QpYZqQziNg0BlRA

[14] https://whitepapers.theregister.com/