A bright, green fireball was spotted over Western Australia’s night skies on Sunday but its origin remains a bit of a mystery to scientists.
The phenomenon was captured in a video showing the bright light lasting for around 30 seconds, per ABC Pilbara.
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Most meteor sightings are momentary sparkles in the vast night sky so rare sightings such as these remain a bit of a spectacle for anyone lucky enough to catch it.
A fireball seems the most likely culprit
Dr Ellie Sansom is a part of Curtin University’s Desert Fireball Network (DFN) — a team that observes fireballs throughout South and Western Australia — and suspects the culprit was likely a fireball.
She explained to Gizmodo Australia that usually when descriptions like these come in and the network is tasked with identifying them, it’s assumed to be space junk.
“When someone describes something really, really long like that — long enough you can actually get your phone out and take a video of it — normally we think that’s going to be space junk,” Dr Sansom told us on a call.
“That is definitely still an option, but for us … this one was very different, it was very green, it was very bright.
“We’re pretty sure, obviously we can’t say 100 per cent, but we’re pretty sure that it was a rock from outer space.”
Meteor over Cape Lambert, Pilbara region, WA last night! https://t.co/kG9srYqUEg
— Damian Hicks (@DamianHicksAus) June 15, 2020
It’s the green colour that makes Dr Sansom and the network believe it’s not just your average space junk. While the space rock itself is unlikely to be green, the green light it emits upon hitting the Earth’s atmosphere was a bit of a giveaway.
“[Man-made space objects are] mostly aluminium, which is a more red-y, orange,” Dr Sansom said. While about 95 per cent of the light we see is from the atmosphere burning, Dr Sansom explained, space rocks are full of iron and sodium, which can give off a green light too as it burns.
The fireball was at least three times higher up than your international flight
It’s hard to tell how close the mysterious object came to Earth based off mobile footage, but Dr Sansom explained it was at least 30 kilometres away from Earth — about three times above your standard international flight.
“When we observe fireballs, they start burning probably around 80 kilometres up and then they’ll stop visibly burning at probably about 30 kilometres,” Dr Sansom explained.
As pointed out in a published 2019 study Dr Sansom worked on, once the light stops burning, it’s usually due to three outcomes. First, if it’s a bigger object, it could slow down enough to drop off a rock at the end of it that scientists could trace on Earth. Those rocks tend to be magnetic but can be extremely tough to find in areas like the the Pilbara, where this one was spotted, as the natural magnetic rock type of the region interferes with the network’s tools.
“That’s why we don’t actually have any DFN cameras up in that direction just because we can’t find the rocks afterwards,” Dr Sansom said.
It could also completely burn up in the atmosphere leaving little or nothing of the original rock. The other option is a new development in our understanding of what happens to meteors once they leave our sight — they can actually be launched back into space.
Not the first space rock to say ‘yeah, nah’ to Earth
Unlike other meteors seen by the naked eye, this one lasted for around 30 seconds — just over 20 seconds longer than your typical meteor. It’s not quite the longest one Dr Sansom has ever seen but it certainly comes close.
“The longest one we’ve ever seen was on July 7 in 2017, and that was a minute and a half and it pretty much went from South Australia all the way across into WA,” Dr Sansom said.
That one was estimated to have visibly travelled 1,300 kilometres across the sky. It was the subject of a paper, published in April 2020, that explained the fireball was slingshotted back into space after it made its foray into the Earth’s atmosphere. It’s expected to reach Jupiter by 2025.
Still, it’s rare for any fireball to last longer than 10 seconds with four-to-six seconds being the average viewing time. The DFN has 52 autonomous cameras set up in South and Western Australia, which covers around a third of Australia’s night skies.
While the network sees an average of one fireball a night, meteors that drop space rocks on Earth tend to only happen twice a year. Dr Sansom said the ones that do tend to disintegrate to about 80 per cent of its original size when they finally hit Earth. A previous space rock the network found was estimated to be the size of washing machine but once it landed on firm ground, only a rock the size of two fists remained.
It’s difficult to know how big a rock, like the one spotted over Pilbara, truly is but Dr Sansom explained the speed often gives a good indication.
“A lot of people try and do it based on the brightness, but because most of that brightness is actually the atmosphere, it’s quite hard to figure out how much of that is the rock,” Dr Sansom said.
“Manmade things are in orbit and the fastest they can come back [to Earth] in is about 11 kilometres per second. Space rocks that come up from the solar system can come in between 11 and 72 kilometres a second, which is pretty damn fast.
“The ones that come from the asteroid belt that [the DFN] normally sees come in at about 20 to 50 kilometres a second, usually.”
The space rock’s true origins, like many others, will remain a mystery for now but with the DFN setting up shop in Australia’s fireball hotspots, the hope is it won’t remain one for long.
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