For a brief moment on Thursday night, a tiny rock approximately the size of a microwave turned up unannounced somewhere near our galaxy.
Sydney’s cloudy evening sky turned white, with surf cams on the eastern suburbs showing the sky turn completely white in a giant flash. Other angles showed a gigantic mass of light coursing through the clouds at breakneck speed.
The abrupt event left anyone lucky enough to be gazing east at 6.50pm in total shock. Social media exploded with people initially fearing a plane crash or missiles. There was even a bit of optimistic UFO chatter.
Normally astronomers are all over this stuff, warning of the tiniest observable movement for space-heads to set their alarms to.
But for this one … nothing.
It served as a lovely reminder that we are just vulnerable little microorganisms hurtling through the abyss, on a relatively small chunk of rock at risk of colliding with other pieces of deadly, intergalactic phenomena.
While all that might sound a bit unnerving, there’s an explanation behind the astonishingly tiny meteor that gatecrashed everyone’s evening without notice.
ANU astrophysicist Dr Brad Tucker said the object may have been just “30 to 50 centimetres in size based on the brightness”, potentially smaller than your TV.
The sheer speed it was travelling was enough to light up the sky from Sydney to Canberra and beyond.
Meteors hit Earth’s atmosphere at tens of thousands of kilometres an hour. At those speeds, even a small object carries enough energy to superheat the air around it, fragment mid-flight and produce a brilliant flash far larger than the rock itself would suggest.
Dr Tucker said the bright flash likely showed the object breaking apart, while the green-blue colour pointed to metals such as iron and nickel.
“People started to see this bright fireball, and then all of a sudden they got this bright flash happening,” he said.
“Enough pressure builds up with all the friction and pressure … it causes it to fracture.”
Why wasn’t it spotted earlier?
The meteor slipped through because objects this small are incredibly hard to spot before they hit the atmosphere.
NASA-backed systems such as ATLAS are designed to detect larger near-Earth objects, with ATLAS saying it can see a roughly 20-metre asteroid several days out and a 100-metre asteroid several weeks out.
Sydney’s meteor, by contrast, was likely too small to register.
That puts it in the very annoying category of space rocks that are too small, too dim and too fast to reliably detect in advance, but still big enough to produce a spectacular atmospheric light show when they finally rock up.
Much larger objects have slipped through before. Sometimes violently.
The Chelyabinsk meteor that exploded over Russia in 2013 was roughly 17 to 20 metres wide, injured about 1500 people mostly through shattered glass, and still arrived without warning.
NASA would later describe it as a “cosmic wake-up call”. Because that meteor came from near the direction of the Sun, ground-based telescopes struggled to see the incoming phenomenon.
That charming little blind spot remains one of the great awkward truths of astronomy.
The European Space Agency openly admits that there are “an unknown number of asteroids on paths we cannot track hidden in the glare of our Sun”.
While it might seem outlandish and sci-fi, global powers actually spend quite a bit on planetary defence.
NASA’s future NEO Surveyor is one of the latest examples. The mission has been specifically designed to help detect asteroids approaching from the Sun’s direction that were once invisible on the ground.
So while Sydney’s meteor looked enormous, it was certainly nowhere near a city-killer
