Chelyabinsk meteor: Ten years on from 'wake-up call', how safe are we from a potentially catastrophic strike?
A decade on from the Chelyabinsk meteor, and our planet has come a long way in detecting the threat of interstellar objects potentially hurtling into the atmosphere. But has the risk been completely eliminated - and exactly how prepared are we for another dramatic impact?
It wasn't quite the day the Earth stood still, but those who witnessed a fiery asteroid briefly outshine the sun as it soared towards the Russian city of Chelyabinsk will almost certainly never forget it.
Comparable to the size of a house and travelling at a scintillating 11 miles per second, what was quickly dubbed the Chelyabinsk meteor arrived unannounced in a manner reminiscent of a science-fiction disaster film. It was an unnerving spectacle.
Dashcam footage from the morning of 15 February 2013, in the central Russian city close to the Ural Mountains, shows the small asteroid entering the Earth's atmosphere before it exploded with 30 times more force than the US atom bomb that destroyed Hiroshima in the Second World War.
Windows shattered, buildings were damaged, and hundreds of people were injured - but Chelyabinsk got lucky.
"Had it been directly over the city, the damage would have been worse," warns NASA's planetary defence officer Lindley Johnson. "It was definitely a wake-up call."
'We've never seen anything like it since'
Working with partners like the European Space Agency, Mr Johnson's department warns of any impacts to Earth by comets and asteroids and guides the response.
A standard test case was a "shooting star" asteroid that soared above the English Channel this week, which was tracked and publicised in advance, so people could see it for themselves.
"We've never seen anything like it since we started working in this area," says Mr Johnson, whose office inside the US space agency was only established in 2016.
"It was daylight, clearly visible in the daytime sky, and that doesn't happen very often.
"It came in on the daylight side of Earth, and we had no chance of being able to detect it ahead of time with the ground-based observatories that we used to find these objects at that time."
What are the chances of another Chelyabinsk?
Mr Johnson was in Vienna, Austria, on the day of Chelyabinsk's arrival, attending meetings of UN members of the Committee on the Peaceful Uses of Outer Space.
It didn't take long for recommendations on how to protect the Earth from such events to be endorsed, including an international asteroid warning network.
Professor Alan Fitzsimmons, of Queen's University, Belfast, is an expert in these so-called near-Earth objects, and a committed member of the "planetary defence community".
"We're very open about what we find and our current state of knowledge about potential impact risks," he says. "All asteroids that are detected are announced on public websites.
"Technology has come a long way in terms of how well you can detect asteroids, even as small as Chelyabinsk, but there's still the chance that one could sneak through. And it's quite likely that the next significant asteroid we have would be unannounced."
How are we protecting ourselves?
Chelyabinsk was considered a small asteroid - that and its arrival during daylight is why it was hard to see coming.
"We're still vulnerable to those that are coming from the sun," admits Mr Johnson.
"Most of these objects come from a main belt of asteroids out between Mars and Jupiter, and when they're coming inbound into the inner solar system, we can find them in the night sky. But when they loop around the sun and come back out, that's when we're vulnerable."
The key to being able to expect the unexpected, he says, is space-based observation.
NASA is working on the $1.2bn (£985m) Near-Earth Object (NEO) Surveyor for launch in 2028, which will be the first space telescope specifically designed to hunt asteroids and comets that may be potential hazards to Earth.
Even then, Chelyabinsk was far smaller than the asteroids NEO will focus on. Amy Mainzer, NEO Surveyor's principal investigator, says it will prioritise "finding the one asteroid that could cause a really bad day for a lot of people".
Also in the repertoire is the Double Asteroid Redirection Test (Dart) spacecraft. During testing last year, it was deliberately crashed into an asteroid and successfully altered its orbit.
What if another one gets through?
Chelyabinsk's arrival showcased the importance of quick and effective communication - its arrival was rapidly documented around the world, Russian scientists shared their findings, fragments have been collected, studied, and found new homes, and the event informed international policy.
Prof Fitzsimmons says such transparency and coordination would again be vital, perhaps even more so in an age where misinformation can quickly spread.
Were such an asteroid to break through the atmosphere today, over a populated area, it would do so in a far more fragile geopolitical climate than back in 2013, with Russia's war in Ukraine and an escalating US-China row over the perceived threat of flying objects.
"When these kinds of events are determined to be of natural causes, the flow of information is pretty good even in today's environment," says Mr Johnson. "But there certainly is concern in knowing quickly that it's a natural event versus something that's human caused.
"The entry and detonation of these objects by the heat pressure in the atmosphere, to the human eye, can look very much like an attack, whereas sophisticated instrumentation rapidly discerns the difference."
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