According to a note verbale from the Permanent Mission of China to the United Nations (Vienna), Tiangong-1's average orbital altitude is 349 kilometres, decaying at a daily rate of approximately 160 metres. Its re-entry into the Earth’s atmosphere is expected to occur between October 2017 and April 2018.
The unsigned diplomatic note (pdf) addressed to Secretary-General provided the update on the re-entry of the 8 metric tonne Tiangong-1 space lab, whose name means 'heavenly palace', on May 10.
In September Wu Ping, deputy director of the China Manned Space Agency (CMSA), stated that Tiangong-1 was orbiting at an altitude of 370 km and declining by 110 metres per day, with reentry expected in the latter half of 2017.
Space station stepping stone
The 10m long, 3.35m wide Tiangong-1 space lab was launched in September 2011 as a stepping stone on the way to a large modular Chinese space station, with the Tianhe core modulelooking set for launch in early 2019.
Crucially, Tiangong-1 demonstrated rendezvous and docking technologies, completing the procedures six times with the uncrewed Shenzhou-8 and crewed Shenzhou-9 and Shenzhou-10 spacecraft between 2011 and 2013.
Above: Shenzhou-10 docking with Tiangong-1 in June 2013.
Tiangong-1 hosted two three-person crews, including China's first woman in space, Liu Yang, in June 2012, and facilitated a live space lecture by Wang Yaping to millions of school children in June 2013, while providing tests of life support systems.
Tiangong-2, launched last year, last month received China's first cargo spacecraft, Tianzhou-1, in a successful test of on-orbit refuelling technologies crucial to the maintenance of a permanently crewed space station.
The communication went on to state that China would act to enhance its tracking and monitoring of Tiangong-1’s orbit and publish timely forecasts of its re-entry.
The China Manned Space Agency will also be providing information on Tiangong-1’s orbital status in real time through its website, in both Chinese and English.
Atmospheric drag - caused by collisions between the spacecraft and molecules - is responsible for the decay of the orbit Tiangong-1, as with other satellites in low Earth orbit.
The Earth's atmosphere is also likely to see to it that most of the spacecraft is not a threat to the surface.
Above: Wang Yaping delivers a lecture on physics in microgravity.
"According to calculations and analyses, most of the structural components of Tiangong-1 will burn up and be destroyed during the course of the re-entry. The probability of harm to aviation and on the ground as a result of Tiangong-1’s re-entry is very low," the note reads.
An Universe Today article - in constrast to many sensationalist reports - states that the likelihood of any one person being struck by a piece of space debris is 1-in-3,200, citing a NASA scientist discussing the upcoming reentry of the 6-ton UARS satellite in 2011.
Space junk regularly reenters the atmosphere, sometimes providing a surprise show for onlookers, as with the second stage of China's first launch of a Long March 7 rocket last year.
Conversely Russia's Mir space station was deorbited through a controlled reentry over the South Pacific in 2001.
China says it will announce the final re-entry time and the area likely to be affected well in advance and will issue the relevant information and early warnings in a timely manner, bringing information to the attention of the Secretary-General of the United Nations and the Office for Outer Space Affairs (UNOOSA) through diplomatic channels and to the public through news agencies.