It appears that the Tiangong 1 mission had ended unexpectedly due to a dysfunctional battery charger, a source close to the Chinese space industry disclosed.
While all eyes are now fixed on the recently launched Tiangong 2 space laboratory module and the upcoming expedition missions, its predecessor Tiangong 1 is on an uncontrolled course to come crashing down to Earth in late 2017.
The Tiangong 1 space module was intended to serve as a ‘target vehicle’ for perfecting the orbital rendezvous docking technique. Since its launch in September 2011, a total of three expedition missions (one unmanned and two manned) have performed rendezvous dockings, in both automatic and manual mode, with the space module. The designed operational lifespan of the space module is two years.
According to the original mission description document published by the China Manned Space Agency (CMSA), at the end of its mission the Tiangong 1 module would perform a controlled destructive re-entry into the South Pacific Ocean. The mission description document did not give an exact date for the space module’s controlled re-entry. After the ending of the last expedition mission Shenzhou 10 in June 2013, the space module was put into a sleep mode to continue flying in orbit to allow the ground control to collect data on the longevity of its key components.
On 21 March 2016, the China Manned Space Agency (CMSA) announced that the ground mission control had lost all telemetry and communications with Tiangong 1, leaving no ability to safely control its descent. By then, the space module had been flying in orbit for 1,630 days since its launch, or 2 years and 9 months since the ending of its final expedition mission.
During the press conference for the Tiangong 2 launch on 14 September 2016, the spokeswoman of the CMSA, Wu Ping, confirmed that Tiangong 1 was intact and operating on a 370 km orbit, with an orbital depletion rate of about 100 m daily. The space module is expected to burn up during an uncontrolled atmospheric re-entry sometime in late 2017.
Although the CMSA did not give a reason for the abrupt ending of the Tiangong 1 mission, it is understood that the mission had ended unexpectedly due to a dysfunctional battery charger, leaving the space module unable to recharge its batteries from its solar panels. Tiangong 1 has two four-panel solar wings attached to its service compartment, each about 3.1 m x 10 m in size. These can be rotated to obtain maximum solar insolation regardless of spacecraft attitude. Sun sensors between the panels measure the sunlight incidence angle which allows the panels to be automatically commanded to an optimum angle.
The space module also carries a number of silver-zinc batteries in its service module, which can provide emergency power for about 6 hours in case of failure of the solar arrays. However, without a functioning batter charger to recharge, these batteries eventually ran out. Without power supply, the space module lost all communications and telemetry transmissions with the earth, so the ground control could no longer to command the space module for a controlled re-entry.
Despite some concerns, the majority of the Tiangong 1 module would burn up during the atmospheric re-entry, though It is probable that some large pieces will survive re-entry and hit the Earth’s surface. While it is impossible to accurately predict the time and location of the re-entry, the likelihood of this taking place over a populated area is minimal.