Tiangong-1 uncontrolled return – disposal corridor was over Pacific Ocean

The Chinese space station module Tiangong-1 completed a destructive uncontrolled re-entry on Sunday. The event was tracked by numerous global tracking assets – many predicting different entry points – as they fine-tuned the entry corridor likely resulted in some of the vehicle’s hardware surviving the fiery return.

The first key element of China’s space station ambitions, Tiangong-1 was launched by a Long March 2F/G in late September 2011.

During its mission, Tiangong-1 was visited by three Shenzhou spacecraft during its first two years of planned operations.

This included the uncrewed Shenzhou 8, which successfully tested docking procedures, a major milestone for the Chinese and their future goals of sending crewmembers to the Station.

With that milestone achieved, Shenzhou 9 then docked in June 2012 with its crew of two taikonauts. A third Tiangong-1 rendezvous and docking mission occurred with Shenzhou 10, which docked in June 2013 – including China’s first female astronauts, Liu Yang and Wang Yaping.

Such was the success of Tiangong-1, officials opted to extend its mission by two years – while they launched a second module, Tiangong-2, in 2016.

This allowed for docking and refueling objectives to be conducted, while the second module – still in orbit – is an advancement on the first, all building towards the launch of Tianhe core module in the coming years, via the new Long March 5B rocket, which will allow for the construction of an advanced Chinese space station.

However, the decision to extend Tiangong-1’s lifetime may have contributed to what is now an uncontrolled return for the module, after the required telemetry link was lost and Chinese controllers were left without the ability to conduct a planned disposal burn to precisely send the craft into a designated corridor over an unpopulated area of the Pacific ocean.

Only until the final hours – possibly with just one to two orbits remaining – was tracking assets be able to work out where the spacecraft will re-enter, which allowed for the calculation of the debris footprint.

Amazingly, even with mintues remaining, officials were predicting the Atlantic Ocean, only for the Chinese to then say it was the Pacific Ocean, which was later confirmed by US officials who had initially got the information wrong.

UPDATE: #JFSCC confirmed #Tiangong1 reentered the atmosphere over the southern Pacific Ocean at ~5:16 p.m. (PST) April 1. For details see https://t.co/OzZXgaEX0W @US_Stratcom @usairforce @AFSpaceCC @30thSpaceWing @PeteAFB @SpaceTrackOrg pic.twitter.com/KVljDALqzi

— 18 SPCS (@18SPCS) April 2, 2018

While numerous spacecraft at the end of their lives complete their death plunge via a destructive re-entry, the majority of returning spacecraft completely dispose of their hardware during the event, with no debris of any note reaching the ground.

Most are controlled entries, which allows for any surviving hardware to safely fall into the ocean.

However, TianGong-1 launch mass was documented at 8,506 kg and around 20 percent of the hardware could have survived re-entry. It was hoped, but not certain, this hardware will fall over the ocean, as opposed to a city. The resulting Pacific Ocean return could not have been more appropriate, as it would have likely been the target for a controlled re-entry.

The spacecraft was composed of two cylindrical sections with a docking port on its front-end. The two modules are known as the experimental module and the resource module.

The 3.35 meter experimental module was composed of an enclosed front cone shaped section, a cylindrical section and rear cone-shaped section. On the front end of the experiment module was the heavy docking mechanism and communication equipment – used to support the rendezvous and docking objectives.

The experimental module was where the taikonauts lived and worked, with 15 cubic meters of pressurized space. This section was also equipped with two sleeping sections with adjustable lighting systems, exercise equipment, entertainment systems, visual communications devices and control systems. All of this hardware remained on board after the final crew departed.

The lack of panic over the spacecraft’s return was mainly based on historical data. To date, no one has been injured by a piece of hardware that has survived re-entry to impact on the ground, despite some much larger spacecraft ending their lives via the destructive plunge.

However, NASA takes the risks involved very seriously, as seen via a study from NASA’s Orbital Debris Program Office which reviewed the hazards posed by space hardware fragmentation during re-entry, with the aim to apply mitigation to any potential risks from hardware breaking up and surviving entry – in turn threatening human life on the ground.

“As we progress forward with future programs, we should keep in mind that in systems engineering the definition of a System Life Cycle includes not just deployment and operation but also retirement and disposal,” an expansive presentation from the meeting (available on L2) noted.

Several examples of hardware entering and surviving the extreme heat and aerodynamic stresses – which usually destroys returning hardware – are cited, including some very interesting case study examples, not least Apollo 13’s Lunar Module in April of 1970.

“The Aquarius LEM re-entered Earth’s atmosphere after having served as a lifeboat for the Apollo 13 crew. Mounted on the LEM descent stage, was a SNAP-27 (System For Nuclear Auxiliary Power) RTG (Radioisotope Thermoelectric Generator) which contained 8.3 lbs (3.9 kg) of Plutonium-238,” noted the presentation.

“Re-entry was at 122 km above the South Pacific Ocean near the Fiji Islands. High and low altitude atmospheric sampling in the area indicated there was no release of plutonium, so the graphite fuel cask is assumed to have survived re-entry and now resides on the bottom of the Tonga Trench in 6 to 9 km of water.”

NASA is also working with its international partners on how to deorbit the ISS when the Station is eventually retired and deemed unsafe to keep in orbit.

Although the disposal corridor over an uninhabited ocean expanse would be refined nearer the time, the deorbit and destructive re-entry of the Station would be by far the largest man-made object to make the fiery plunge back to Earth. A large amount of hardware would likely survive re-entry.

The use of two Progress vehicles is deemed as the current preference for the Station’s execution, per its natural EOL (End Of Life). This will help ensure the re-entry of the ISS will be over an unpopulated disposal corridor.

NASA is still working on the plan, with at least 10 years before it is required. However, the work began early in case of an emergency situation where the ISS had to be brought down ahead of schedule.

(Lead image via ESA)