Out-of-control space station ends in fiery ocean plunge

What's this all about?

Anyone who's anyone in the field of monitoring space junk has been keeping a close eye on Tiangong-1, or "Celestial Palace 1", China's first space station.

After the Chinese government told the United Nations they had lost control of the station, back in September of 2016, knowing exactly where the station was in orbit, and how high up in space it was, became very important. The controlled re-entry of a spacecraft or defunct satellite is planned to happen within a specific window of time, so that it splashes down in the ocean, guaranteeing that it won't fall on a populated area. With Tiangong-1 out of control, though, there was and is now no telling exactly when it would fall out of the sky, nor is there a way to predict where it will fall.

Relying on modelling techniques of the space station's orbit, and rate of decent, organizations such as the European Space Agency's Space Debris Office, The Aerospace Corporation, and sites such as Satview.org, have been gradually narrowing down the timing of Tiangong-1's re-entry into Earth's atmosphere.

As of April 1, 2018, The Aerospace Corporation was saying that the station re-entered Earth's atmosphere at April 1st, 2018, 8:18 p.m. ET, plus or minus 2 hours. That put the re-entry between 6:18 p.m. and 10:18 p.m. ET, on April 1. The ESA's prediction agreed with this.

Tiangong-1 was just over 155 km above Earth's surface at around noon, ET, on April 1, 2018, with its orbit predicted to decay very quickly in the hours ahead. Credit: ESA

The exact date and time of re-entry were being refined on a daily basis, but even on the day of re-entry, a 2-hour window is as good as the prediction was going to get.

According to Satview, the map below provided a forecast for Tiangong-1's re-entry location.

Credit: Satview.org

It's fairly easy to tell the location of large pieces of space junk, and their altitude, but there are specific factors that weigh against a more exact prediction of when they will fall to Earth.

Even with it below an altitude of 200 km, when its orbit is supposed to decay very quickly, according to The Aerospace Corporation report, the exact timing will come down to the density of the upper layers of the atmosphere, the exact orientation of the station and its solar panels, the exact mass of the station and how well the station materials hold up to the forces it encounters, and the station's exact location and speed.

Max Fagin, an aerospace engineer with Made In Space, goes through a very detailed explanation of these uncertainties in the video below.

Even refining the window down to a few hours still meant that the space station could really fall anywhere. This is because the station is travelling at a speed of over 7 kilometres per second.

A report by The Aerospace Corporation does highlight specific regions of the world that are more likely to see Tiangong-1's fiery demise, though.

A plot of Tiangong-1's orbit shows that some areas of the world are more likely to see it re-enter Earth's atmosphere. The two yellow bands have a higher probability. The green band between them has lower probability. The blue regions will not see it. The higher probability along the yellow bands is due to the space station spending more time during its orbit at those latitudes than at any other latitude. Credit: The Aerospace Corporation

Watch below as a Chinese rocket burns up on re-entry, over the western United States and Canada, in February of 2015.

The Tiangong-1 space station was launched into orbit on September 29, 2011, as a test platform for the future of China's space program. During its time in service, it supported three missions - one robotic, Shenzhou 8, which performed a docking test one month after the station was deployed, and two crewed, Shenzhou 9 in March 2012 and Shenzhou 10 in June 2013.

Since June 25, 2013, Tiangong-1 has simply been orbiting the planet on its own, with no more visits from any spacecraft, crewed or otherwise, and as of March of 2016, China hasn't even been in contact with the station, as it had "ceased functioning."

Reduced to a 10 metre long, 8 metric ton piece of space debris, Tiangong-1's orbit has slowly but steadily decayed since, from a distance of over 380 km, down to less than 300 km as of October 2017, to under 200 km above Earth's surface at the end of March of 2018.

In a statement delivered to the United Nations back in May of 2017, China's Permanent Mission to the UN said that the spacecraft's orbit is "decaying at a daily rate of approximately 160 metres" and that its reentry into Earth's atmosphere "is expected between October 2017 and April 2018."

Chinese officials have said that there is no danger from the station's impending breakup, however there are some larger portions of the station, which are made of denser materials, which could survive all the way to the ground. While the most probable location for debris to fall is into the oceans, any pieces falling on land should be avoided and reported immediately, as they may be contaminated with hydrazine, a highly toxic and corrosive chemical that is used as spacecraft fuel.

Sources: ESA | The Aerospace Corporation | Satview.org | Max Fagin | With files from The Weather Network

Watch Below: Space Debris in Motion - NASA presents a look at just how much junk is floating around above the Earth