Dubai: What are the chances of a molten metal, a piece of satellite or space junk falling on your head at any moment?
Short answer: Better get used to it.
What about the out-of-control Chinese sky lab that's about to fall?
On October 16, the disintegration of the Russian-made Progress module as it broke up into smaller chunks and burned up like fireworks created a night-time spectacle in the UAE.
Now, space-tracking site Satview states that China's defunct Tiangong-1 space lab is falling out of orbit.
China launched the 8.5-tonne Tiangong-1 ("Heavenly Palace") in 2011. It has a Norad ID No. 87820 and was hovering at an altitude of about 300km above the North Pacific, as of 8.32am on Tuesday, October 24, 2017.
The satellite, 12 metres long and 3.3 metres in diameter, was used by Chinese astronauts for a series of spacecraft docking tests and visits.
In March 2016, the space lab broke down and in May 2017, Chinese officials told the UN that Tiangong-1 was expected to re-enter Earth's atmosphere between October 2017 and April 2018.
But as of Oct. 24, the satellite was still orbiting at a high altitude (about 300km, which varies depending on which part of the globe it floats over) and wasn't expected to come down immediately, according to the tracking site SatFlare.
This was confirmed in a tweet by a popular astrophysicist, Dr Jonathan McDowell, who said that Tiangong-1 poses no threat to the earth's inhabitants.
Dr. McDowell, who works at the Harvard-Smithsonian Center for Astrophysics, says there's nothing to fear about the Chinese station that's about to fall. "It's big, but our planet is bigger," he twitted on October 20.
China said it would monitor Tiangong-1's descent and post its orbital status on the China Manned Space Agency website.
Once a final forecast for the time and region of re-entry is obtained, China's space agency says it will issue an "early warning in a timely manner" — by informing the UN Office for Outer Space Affairs and the world body's secretary general.
4 more falling
However, between October 26 and November 13, 2017 — four more satellites or space junk hovering overhead are about to burn up as they fall, according to the satview.com, a popular satellite tracking site.
The ISS DEB, technically known as 1998-067BA (or Norad code 31928), is a piece of fabric shield lost in space from the International Space Station on March 30, 2017.
Here's how a spacewalk looks like, uploaded by Nasa on October 20, taken using an enhanced high-definition camera.
Nasa astronauts on a spacewalk accidentally lost a fabric shield needed for the space station.
ISS DEB is expected to reenter at about 5.20am UTC (9:30 am in Dubai) on October 26.
Firefly (Norad code: 39404), a satellite launched on November 20, 2013, currently has a perigee (pass closest to Earth) of 248.8km according to another tracking site.
Satview estimates that Firefly will re-enter the earth's atmosphere on Thursday, November 2 at 17hr45 (9.45pm in Dubai).
Flock 2E'-4 is expected on November 13, at 22h47 (2.47am in Dubai, on November 14) while and CZ-4B DEB is expected to fall on November 14 at 16h27 (8.27pm in Dubai). These timings are all tentative — the sites usually update their telemetry data as needed.
But what's the risk that it will cause any damage or injury to us or an aircraft?
"Very low", says Dr McDowell, since most of it will burn up in the atmosphere as it falls. He did confirm that Tiangong-1 has begun dropping more quickly as it reaches denser parts of the Earth's atmosphere.
However, because of the size of Tiangong-1, he said that pieces of up to 100 kilograms could crash down on the Earth's surface.
By comparison, Tiangong-1 is similar in size to other objects that fell to Earth from orbit in 2012 and 2015.
But but is just a fraction of the size of US Skylab and Russia's Mir. Nasa's 77-tonne Skylab space station went down in July, 1979, and debris landed southeast of Perth, in western Australia. Russia's Mir, which weighed 130 tonnes was deorbited in March 2001 through a controlled atmospheric re-entry and fell near the Pacific island of Fiji.
Space junk: How many?
These are just four of the many disabled satellites or debris in space.
Experts have said that there are tens of thousands of pieces of space junk larger than 10cm — moving at up to 30,000 kph.
There are 500,000 pieces bigger than a marble, and several million smaller ones. These small space junk floating above the earth's are moving so fast could punch a hole in satellites or break the International Space Station's window.
Dr McDowell gave an updated (sort of) account of the amount of things floating in outer space and who sent them.
As for disabled satellites, their altitude "decays" — reduced by a few hundred metres — as the earth's gravity pulls them slowly while circling our planet.
When they reach a certain altitude, they get pulled by the earth's gravity faster and burn up as they hurtle back to the ground. The friction creates a streak of light for a few seconds upon re-entry.
4 conditions for observing a burn-up
Observing a satellite burn-up takes a lot of luck as several conditions must be met, said experts.
For a burn-up to be observed directly, the sunshine must reach the its structure and reflected into the people observing the event on the ground.
For that to take place, it is necessary that the following factors are present — all at the same time, according to SatView:
First, the sky must be dark: it should be night on the observation location.
Second, the Sun's height (or the "solar disk") should be between 10 and 25 degrees below the line of the horizon.
Third, the sun rays should be reaching the satellite directly.
Finally: the satellite should be at least 25 degrees above the horizon.
It then appears that the spectacular burn-up of the Progress spacecraft observed over Dubai at about 7.30pm on October 16 was an extremely rare occasion that met all these conditions.
The Soyuz-2-1a third stage rocket from the Progress MS-07 launch reentered over Dubai at 1528 UTC Oct 16; the reentry was widely observed— Jonathan McDowell (@planet4589) October 17, 2017
Nasa's Skylab space: 77 tonnes (went down in 1979).
Russia's Mir space station: 130 tonnes (went down in 2001).
China's Tiangong-1: 8.5 tonnes (Chinese space agency to inform UN about final re-entry)
Apogee: Apo is a Greek term for ‘away,’ while gaia is ‘earth. The point in an elliptical orbit at which a satellite is at its furthest from the Earth. Whenever a satellite is at its apogee, its travel around the body is also at its slowest.
Perigee: The opposite of apogee, which refers to the point at which a satellite is at its nearest to the Earth when following an elliptical path.