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Space junk's journey from Kazakhstan to Cobar

On Tuesday it lifted off from Kazakhstan to deliver satellites into space. By Thursday it had people across south eastern Australia concerned it was a meteor or a plane crashing.

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One person will be hit by a piece of space junk and killed in the next 50 years, scientists say.

That's the answer for anyone left in awe, but slightly worried, by the fireball that streaked across the sky from Hobart to far northern NSW on Thursday night.

But even that statistic is alarmist, said Dr Ben Greene, chief executive of the Cooperative Research Centre at Mount Stromlo Observatory. “That’s what we expect, but there’s a higher chance of winning the lottery without buying a ticket.”

Witnesses called the fireball a meteor, others rang triple-O thinking it was a burning plane. But astronomers, astrophysicists and other scientists agreed on Friday it was a chunk of Russian rocket Soyuz, used on July 8 to launch the Meteor-M weather satellite.

The seven-metre, three-tonne cylindrical object was among more than 300,000 pieces of space junk orbiting Earth, Dr Green said.

"There is so much debris up there that it's colliding. A catastrophic avalanche of collisions could quickly destroy satellites worth trillions of dollars," he said.

Professor Brian Schmidt, an astronomer at the Research School of Astronomy and Astrophysics at the Australian National University, said NASA sent an alert about the re-entry earlier in the day.

“Orbits of these [pieces of space junk] is monitored quite closely," he said. "This one was decaying rapidly and the prediction of the path was confirmed, because everyone saw it."

Dr Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said of the 6500 tonnes of “stuff” hurtling around Earth, about three-quarters was junk.

"Most of this will eventually re-enter, but not for decades to centuries," he said from his home in Massachusetts. 

Ninety tonnes of junk re-entered the earth’s atmosphere in 50 separate re-entries in 2013, according to his calculations.

Dr Alan Duffy, from Swinburne University of Technology, said the chances of anyone being hurt was “tiny”, even though Australia had a track record of being bombarded with other countries' space junk.

“Famously, the United States had their Skylab, the precursor to the International Space Station, fall in Western Australia, and big chunks remained in remote outback country,” he said.

"Just because of the country’s sheer size, it’s most likely we’ll get more debris, but this stuff will burn up and land in the ocean, just because there’s more of the sea to hit."

He said if the object hit Earth's atmosphere two minutes later on Thursday, Australia would have passed the visual feast to south-east Asia.

Even for astronomers, the fireball was a "very rare and exciting” event, said Dr Nick Lomb, curator of astronomy at Sydney Observatory.

Dr McDowell said data from the Space-Track organisation showed the rocket part re-entered over a track from southern NSW to southern Queensland.

"Break-up is over NSW [near] Canberra but subset of densest debris, if any survives, might have made it to south Queensland," he said.