An out-of-control Chinese space lab is expected to fall to Earth within days, according to the latest estimate from the European Space Agency (ESA), which is monitoring its descent.
While posing minimal risk to humans, the uncontrolled re-entry of the space lab is a blot on China's ambitious space program. The 8.5 ton Tiangong-1 "ceased functioning" on March 16, 2016, China told the United Nations in May 2017, without specifying why.
Space experts stress the potential danger to humans is tiny -- the odds of debris from the vessel hitting a human are estimated to be less than one in 1 trillion. That compares with a one-in-1.4 million chance of a person in the US being struck by lightning.
However, Alan Duffy, a research fellow in the Centre for Astrophysics and Supercomputing at Swinburne University of Technology in Australia, said that China's secrecy around the space mission made the risks difficult to assess.
"The international community doesn't know what the craft is made of, and that makes estimating the danger more challenging, as hardened fuel containers could reach the ground while lightweight panels won't won't," he said.
Space station prototype
The 40-foot long Tiangong-1, or "Heavenly Palace," was launched in September 2011. Along with its successor -- the Tiangong-2, which launched in 2016 -- it was a prototype for China's ultimate space goal: a permanent, 20-ton space station that is expected to launch around 2022.
In its UN submission anticipating the craft's fall to Earth, China said the probability "of endangering and causing damage to aviation and ground activities is very low."
In January, Zhu Zongpeng, the chief designer of the space lab, told the state-run China Youth Daily newspaper that China had been monitoring the Tiangong-1. He predicted that most of it would burn up when it entered the atmosphere while the rest would fall into the sea.
Since March 14, China has been giving daily updates on the altitude of the vessel. On Sunday, the Tiangong-1 was at an average altitude of 216.2 kilometers (134 miles), down from 286.5 kilometers on December 24, 2017.
Markus Dolensky, the technical director at the International Centre for Radio Astronomy Research in Australia, said if skies were clear witnesses may potentially see a "series of fireballs streaking across the sky."
"It is now nearing its fiery demise as it gradually gets slowed down by the fringes of the Earth's upper atmosphere," he said.
It's not uncommon for space debris, such as spent satellites and rocket stages, to fall to Earth although vessels that are capable of supporting human life are much rarer.
The last human space outpost to fall to Earth was the 135-ton Russian space station Mir in 2001. That was a controlled landing, with most parts burning up upon return and the rest landing in the ocean.
The first US space station, the 74-ton Skylab, fell to Earth in an uncontrolled reentry in 1979. Some debris fell in sparsely populated Western Australia, incurring no damage except for a $400 fine for littering.
Experts said it's difficult to establish exactly where the space lab will fall, but it's expected to descend within a latitude of 43 degrees north and south of the equator.
"Some parts of the upper atmosphere are thicker than others meaning the craft slows unpredictably and since it travels around the Earth in just 90 minutes even an uncertainty of a two minutes means the craft could fall anywhere along a 1,000 kilometer track," Duffy said.