Chinese Space Station Reentry Expected April 1-2

Experts are predicting that China’s small space station, Tiangong-1, will reenter Earth’s atmosphere sometime tomorrow or Monday (April 1-2 Eastern Daylight Time).  Most of the 8.5 metric ton (MT) space station, about the size of a schoolbus, will burn up during its fiery descent, but there is a tiny chance something could make it all the way to Earth.

China’s most recent public statement is that it will reenter on April 2 Beijing Time plus or minus one day, or April 1-3.  It has a website in English where it is posting its most recent predictions.  Beijing Time is Eastern Daylight Time (EDT) + 12.

Tiangong-1 is expected to make an uncontrolled reentry, meaning China cannot command it to reenter at a certain time and place.  That is based on a Chinese statement in March 2016 that Tiangong-1 had “terminated its data service.”  China repeated that on Thursday, but did not explicitly state that it cannot control the spacecraft.  It did, however, offer assurances that it is “unlikely to cause any damage.”

The fact that it is a space station and making an uncontrolled reentry has heightened interest even though space objects make uncontrolled reentries virtually every day and Tiangong-1 is relatively small.

The Aerospace Corporation, a federally funded research and development center (FFRDC) that supports the U.S. Air Force, estimates that two to three objects the same size as Tiangong-1 reenter every year. During a media briefing on Wednesday at its Arlington, VA offices, Roger Thompson and Andrew Abraham of its Center for Orbital Reentry and Debris Studies presented a table showing the top 16 space objects in terms of mass that have reentered — either controlled or uncontrolled — to demonstrate how small Tiangong-1 is by comparison.

Thompson later told SpacePolicyOnline.com that Tiangong-1 is number 268 in terms of mass in the list of uncontrolled reentries.

He and Abraham liken its size to that of a schoolbus.

As the table shows, Russia’s Mir space station is the largest space object to reenter, but it was a controlled reentry over the Pacific Ocean.  The largest object to make an uncontrolled reentry so far was the U.S. Skylab space station, which reentered in 1979 and spread debris over the Indian Ocean and western Australia.  There were no injuries.

Seven other space stations have reentered, all launched by the Soviet Union.  Five of the seven (Salyut 1, 3, 4, 5, and 6) were controlled reentries.  Two others, Cosmos 557 (which was to be named Salyut 3, but failed shortly after launch and therefore assigned a generic Cosmos number and the next in the series was designated Salyut 3), and Salyut 7, made uncontrolled reentries.  They were much larger than Tiangong-1.  There were no injuries.

In fact, more than 60 years after the first space object was launched, only one person has ever been hit by falling space debris according to Aerospace.  Lottie Williams of Tulsa, Oklahoma was struck on the shoulder in January 1997 by a piece of a reentering Delta rocket’s second stage.  She was not injured.  She later characterized it as being about as heavy as an empty soda can.

Aerospace estimates the odds of getting hit by a piece of Tiangong-1 as “far less than winning the Powerball jackpot.”

Predicting when and where any uncontrolled space object will reenter is extremely difficult because of atmospheric density changes influenced by the Sun.   In just the past day, predictions of Tiangong-1’s reentry have moved later by several hours because of low solar activity.

Tiangong-1 could reenter anywhere along its flight path around the globe.  It is orbiting at 42.7 degrees inclination to the equator, so could come down anywhere on Earth between 42.7 degrees north latitude and 42.7 degrees south latitude.  There is a 70 percent chance it will come down over an ocean since the oceans cover 70 percent of Earth.

U.S. Strategic Command’s Joint Space Operations Center (JSPoC) is tracking Tiangong-1. The data are available on its public website, Space-Track.org.   A number of organizations and skilled amateurs are using JSPoC data with their own models to generate predictions of Tiangong’-1’s reentry.

As of 2:30 pm EDT March 31, three of those predictions as published on their websites or Twitter feeds are:

• Aerospace Corporation: @AerospaceCorp or http://www.aerospace.org/cords/reentry-predictions/tiangong-1-reentry/
  • April 1 at 23:30 UTC plus or minus 7 hours.  That is April 1 at 16:30 UTC to April 2 at 06:30 UTC.  In Eastern Daylight Time (EDT), it would be from April 1 at 12:30 pm until April 2 at 2:30 am.
• European Space Operations Center:  http://blogs.esa.int/rocketscience/2018/03/26/tiangong-1-reentry-updates/ 
  • April 1 at 23:25 UTC, with a window of April 1 in the afternoon through early morning on April 2 UTC
• Marco Langbroek (Netherlands-based amateur satellite tracker, @Marco_Langbroek)

My latest #reentry predictions for #Tiangong1, based on 2 methods (UT = GMT):
SatAna/SatEvo: 1 Apr, 22:55 UT +- 9h
GMAT: 1 Apr, 22:46 UT +- 9h
Last elset included: 90.515
Perigee now below 165 km@SSC_NL @drspacejunk @cosmos4u @planet4589 pic.twitter.com/LPydH7oeRg

— Dr Marco Langbroek (@Marco_Langbroek) March 31, 2018

JSPoC operates a global system of radars and optical systems that track space objects.  The point at which it will be able to definitively state that Tiangong-1 has reentered will depend on whether it happens in view of those sensors.  If not, it will be a matter of determining that the space station did not pass over the next sensor and then calculating back to where it most likely came down.

China has its own space tracking network and could publicly announce the reentry itself, of course, probably via its official news agency, Xinhua.   Xinhus has an English-language website and Twitter feed (@XHNews).

Waiting for official confirmation from one of those sources is advisable since untrained observers might mistake other objects or phenomena for a reentry, especially if it takes place at night when aircraft or meteors might also be visible.  One must also beware of April Fool’s jokes, tomorrow being April 1.

Tiangong-1 is China’s first space station.  Launched in 2011, it was visited by two three-person crews in 2012 and 2013.  China’s second space station, Tiangong-2, was launched in 2016 and was visited by one crew that year.  It remains in orbit.  No further crew visits are anticipated because China is focused on building a larger, three-module, 60 MT space station that it hopes to complete in the early 2020s.  The launch of the first module has been delayed because of the failure of China’s new Long March 5 rocket last year.  The Long March 5 is needed to send the 20 MT modules into orbit.  It is expected to return to flight this year, but the launch dates for the three space station modules are unclear.  China had planned to complete it by 2022.