Posted on 16 January 2018 by Daniel
Tiangong-1 frequently asked questions
FAQ on Tiangong-1 reentry
FAQ prepared and updated by the Space Debris Office, ESA/ESOC, Darmstadt, Germany.
Tiangong-1 (天宫一号, Heavenly Palace 1) is China’s first space station and an experimental space laboratory. Its major goal was to test and master technologies related to orbital rendezvous and docking. It is identified by its UN COSPAR ID 2011-053A. It was launched on 30 September 2011 at 03:16:03.507 UTC by a Long March 2F/G rocket from the Jiuquan Satellite Launch Centre in the Gobi desert, Inner Mongolia, China. One uncrewed and two crewed missions, executed by the Shenzhou (神舟, Devine Craft) spacecraft, took place during its operational lifetime.
Q. What’s happening?
The Tiangong-1 space station will reenter Earth’s atmosphere and substantially burn up in the March–April 2018 timeframe.
As of mid-January 2018, the spacecraft was at about 280 km altitude in an orbit that will inevitably decay; it will mostly burn up due to the extreme heat generated by its high-speed passage through the atmosphere (some spacecraft, like Soyuz capsules, are designed to withstand reentry).
Following launch in 2011, the Tiangong-1 orbit began steadily decaying due to the faint, yet not-zero, atmospheric drag present even at 300 or 400 km altitude. This affects all satellites and spacecraft in low-Earth orbit, like the International Space Station (ISS), for example.
As a result, such craft must conduct regular ‘reboost manoeuvres’ to maintain their orbit – typically, ground controllers command the craft’s engines or thrusters to fire for a certain amount of time, speeding it up so that it gains altitude.
During its operational life from launch through to December 2015, regular orbital maintenance manoeuvres were executed by Tiangong-1 in order to maintain an operational altitude of between 330 and 390 km above the Earth’s surface.
Q. What was the original disposal plan?
Initially, a ‘controlled reentry’ was planned for the spacecraft at the end of its life.
This means that ground controllers would have commanded the engines to fire, slowing the craft by a significant amount so that it would fall toward the surface. Firing the engines would have been done at a specific moment so that it would reenter the atmosphere and substantially burn up over a large, unpopulated region of the South Pacific ocean. Any surviving pieces would fall into the ocean, far from any populated areas. This is precisely what ESA did, for example, for the Agency’s series of five ATV cargo spacecraft between 2008 and 2015.
However, in March 2016 the Tiangong-1 space station ceased functioning but maintained its structural integrity. In so far as can be fully confirmed, ground teams lost control with the craft, and it can no longer be commanded to fire its engines. It is, therefore, expected to make an ‘uncontrolled reentry.’
Q: How big is Tiangong-1? What shape is it?
The spacecraft’s 10.4 m-long main body is made up of two cylinders of approximately equal length: a service module and an experiment module. The thinner service module provides power and orbit control capabilities for the station. It has two solar panels, each approximately 3 x 7 m in size. The thicker experimental module comprises an enclosed front conical section, which include a docking port, a cylindrical section, and a rear conical section. The experimental module is habitable.
The overall mass of the spacecraft was reported to have been approximately 8.5 tonnes including fuel at launch. Given that the space station exceeded its originally planned operational lifetime of two years and continued operating successfully for two more years after that, a considerable amount of fuel must have been consumed to sustain the orbit and the habitable environment conditions inside.
This means that a significantly lower mass on reentry is likely, comparable to the mass of defunct satellites that make uncontrolled reentries typically a couple times per month.
Q. To date, who’s done or is doing what?
China notified the United Nations Office for Outer Space Affairs (UNOOSA) of the upcoming re-entry and committed to enhanced monitoring and forecasting of the orbital decay, including requesting an international joint monitoring and information dissemination campaign under the framework of the Inter-Agency Space Debris Coordination Committee (IADC).
IADC comprises space debris and other experts from 13 space agencies/organisations, including NASA, ESA, European national space agencies, JAXA, ISRO, KARI, Roscosmos and the China National Space Administration.
IADC members will use this event to conduct their annual reentry test campaign, during which participants will pool their predictions of the time window, as well as their respective tracking datasets obtained from radar and other sources. The aim is to cross-verify, cross-analyse and improve the prediction accuracy for all members.
ESA is acting as host and administrator for the campaign, as it has done for the twenty previous IADC test campaigns since 1998. A special case for ESA was the campaign in 2013 during the uncontrolled reentry of ESA’s own GOCE satellite.
Regular updates are being provided via the website of the China Manned Space Agency in both Chinese and English.
As of January 2018, the mean altitude of the space station is 280 km. The further decay, and hence re-entry, is assumed to be uncontrolled in the sense of orbit maintenance. This has, however, not been unambiguously confirmed by the Chinese authorities. It has however been reported that the attitude, i.e. the orientation, of Tiangong-1 is stabilised.
Q. Over which parts of Earth will it burn up?
Due to the orbital inclination of the Tiangong-1, approximately 42.8 degrees, and the likely uncontrolled nature of the reentry, the final impact point can be anywhere on Earth between 42.8 degrees North and 42.8 degrees South in latitude.
As you can see in the chart at right in the map above, the re-entry location itself is not uniformly distributed. Due to the geometry of the craft’s circular orbit, the probability of reentry happening at the maximum (42.8 degrees N) and minimum (42.8 degrees S) latitude are higher than at the equator.
Why is this?
Because of the low eccentricity and non-polar inclination of the orbit (in other words, because the orbit of the space station around the Earth is circular and at an angle with respect to the equator), the space station spends more time near the edges of the band then it spends crossing the equatorial region of Earth. This leads to a higher likelihood of reentry occurring near the edges of the latitude band, i.e., the top and bottom of the band in the map above.
Q. Will anyone know the precise location and time of reentry in advance?
Only from one day before the actual reentry will it become possible to roughly predict which ground tracks, and hence which regions on Earth, might witness the reentry.
But even then, an impact location prediction on the order kilometres is, for an uncontrolled reentry, beyond current technical capabilities due to complexities of modelling the atmosphere, the dynamics of the reentering object and limitations in observing the spacecraft.
In general, the uncertainty associated with an uncontrolled reentry prediction is on the order of 20% of the remaining orbital lifetime. Practically, this means that even 7 hours before the actual reentry, the uncertainty on the break-up location is a full orbital revolution – meaning plus or minus thousands of km!
The current reentry uncertainty window is shown below (the latest version will be posted in the home page of this blog).
If the spacecraft does have a functioning attitude control system now, this could stop working under the higher dynamic pressure loads (due to falling lower into the atmosphere) closer to reentry and the uncertainty in the final reentry time window could rise (this was the case, for example, with ESA’s GOCE reentry).
Q. Once it reenters and breaks up, what is the risk that any pieces reach ground?
Tiangong-1 is a large spacecraft comparable in size and mass to other, frequently used space stations and cargo vessels such as ESA’s ATV, the Japanese HTV, Russian Progress and American Dragon or Cygnus.
From monitoring the controlled reentries of those types of spacecraft, it can be surmised that Tiangong-1 will break up during its atmospheric re-entry and that some parts will survive the process and reach the surface of Earth.
Video of ESA’s ATV 1 breaking up during its controlled reentry in September 2008
Given the uncontrolled nature of this reentry event, the zone over which fragments might fall stretches over a curved ellipsoid that is thousands of kilometres in length and tens of kilometres wide. While a wide area could be affected, it is important to point out that a large part of the Earth is covered by water or is uninhabited.
Hence the personal probability of being hit by a piece of debris from the Tiangong-1 is actually 10 million times smaller than the yearly chance of being hit by lightning.
In the history of spaceflight, no casualties due to falling space debris have ever been confirmed.
Q. How does Tiangong-1 reentry compare to the reentries of similar-size craft in the past?
With its 8.5 metric tonnes of (initial) mass, Tiangong-1 is definitely not the largest uncontrolled reentry in spaceflight history. That would be Skylab with 74 metric tonnes.
Tiangong-1 falls within the category of modern space freighters (crewed and uncrewed) such as the already mentioned ATV (12 t), Japan’s HTV (10 t), Russia’s Progress (7 t) and Soyuz (7 t), the US Dragon (7 t) or Cygnus (5 t) and the Chinese Tianzhou (13 t). These masses are for the loaded craft; in the table below, they are shown at reentry.
Q. How will ESA share news or updates on the reentry?
In addition to news posted here in the blog, ESA will regularly update authorities of ESA Member States with detailed information on the reentry, as it does during all such events.
Media and press seeking more information can contact the ESA Communication team as follows:
ESA/ESOC Communication Office +49 6151 90 2516