The first hours after seeing the fireball are reminiscent of a detective mystery. Around midnight last night, people across Melbourne took to social media to report a bright light slowly moving across the sky. The video clearly shows how the fireball disintegrated and these pieces burned one by one, so this object was large.

Unexpected space junk

There are reports of a violent eruption in Victoria. These kinds of sounds, known as sonic booms, mean that the fragments survive long enough to enter the lower atmosphere – otherwise they would not be heard from the ground. In turn, this tells us that at least some of this fireball is dense.

Also, in some videos, the glow of the fireball had clearly visible colors, especially orange. This tells us that this object is not a space rock, but rather man-made, with significant combustion of plastic or metal (familiar to anyone who burned materials in a Bunsen stove in high school chemistry class).

For this reason, we’ve probably seen several tons of space junk—everything that humans put into orbit and that is no longer under our control—re-entering Earth’s atmosphere. However, nothing was provided for re-entry to the global space debris monitoring site SatView.

The fireball may be the third stage of the Soyuz 2 rocket carrying the GLONASS-K2 navigation satellite, according to an early analysis by American astronomer Jonathan McDowell. It was launched by Roskosmos (Russian space agency) on 7 August from the Plesetsk Cosmodrome, about 800km north of Moscow.

The incredible luminosity of the fireball is explained by the rate at which objects re-enter the thin upper layers of Earth’s atmosphere at 25,000 kilometers per hour or more.

When you rub your hands together, they rub against each other and get hot. Do this a thousand times faster and you’ll begin to imagine them glowing white from the heat. If friction occurs between the metal of space debris at an altitude of 100 km and the thin atmosphere of the Earth, we can get a very bright glow.

You can help astronomers with details

To help us confirm what the fireball is and where it came from, we need witnesses to download the Fireballs in Sky app and recreate the trail as best they can.

From all these observations we can triangulate the trajectory and determine where the surviving fragments might have landed and try to collect them. So far the reports are conflicting and we need more data. It appears to have entered the atmosphere from the northwest via Victoria to southeast Tasmania, but it’s too early to tell its exact path.

Most space junk does not reach Earth. The incredible heat of 5,000 Kelvin or more during re-entry burns almost all such parts. However, some of the more robust engine units can reach the ground, so warnings of space debris re-entering the atmosphere are sent specifically to aircraft. But space debris moves so fast that even a minor error in the reentry calculation would make it appear hundreds of kilometers away. For most purposes, such warnings are not as helpful as they could be.

To improve this system, we need better monitoring stations on the ground and advances in modeling the interaction between space debris and the upper atmosphere to improve our predictions. Fortunately, even buildings, let alone humans, are tiny targets compared to vast, desolate areas of land and sea. While coincidences have been reported, luckily they are incredibly rare, so space debris is unlikely to pose a danger to us on Earth.

As astronomers now rush to decipher the details of this beautiful fireball, it marks a spectacular opening for Australia’s National Science Week, with thousands of live performances explaining science as broadly as the event, just like the event.