China's 10-storey, 23-ton Long March 5b rocket first stage is expected to make an "uncontrolled re-entry" back to Earth this weekend and the chances of it hitting a populated area are not zero, the NY Times has reported. According to the US government funded Aerospace Corporation, which has been tracking the rocket, it's expected to arrive on May 9th at 3:43 UTC (11:43 PM ET). However, that time is plus or minus 16 hours, so it's subject to change.
Both the US Space Command and Russia's Roscosmos agency are currently tracking the object. Scientists can't predict exactly where or when the first stage will reenter, because it depends on factors like solar activity that can make the atmosphere expand outward and create additional drag at the edge of space.
Our latest prediction for CZ-5B rocket body reentry is:— The Aerospace Corporation (@AerospaceCorp) May 6, 2021
🚀09 May 2021 03:43 UTC ± 16 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: https://t.co/p2AU9zE3y2 pic.twitter.com/MgzRAOTJnk
"The rocket stage’s orbital inclination of 41.5 degrees means that re-entry can be as far north as Chicago, New York City, Rome and Beijing and as far south as New Zealand and Chile," the Aerospace Corporation wrote in a Medium Post. The current thinking is that it will shower down over northeastern Africa, over Sudan.
One of the largest objects to famously re-enter the atmosphere and break up was the 77 ton Skylab back in 1979. In that case, it mostly fell into the Indian Ocean, but some pieces fell over sparse areas of Australia. The US President at the time, Jimmy Carter, apologized for the incident.
The Long March 5b is much smaller, but China has never provided rocket design details, so researchers can't predict exactly how it will break up in the Earth's atmosphere. "The general rule of thumb is that 20–40 percent of the mass of a large object will reach the ground," said Aerospace Corporation principal engineer Marlon Sorge.
It's unclear why China allows the large rockets to descend uncontrollably rather than using a deorbit maneuver to guide them to the ocean or an uninhabited region. Last year, another Long March 5B rocket made an uncontrolled re-entry, raining pieces onto a village in the Ivory Coast. “It could have been extremely dangerous,” he said. “We’re really fortunate in the sense that it doesn’t appear to have hurt anybody," said Jim Bridenstine, NASA's administrator at the time.