The US National Aeronautics and Space Administration (NASA) says it's aware of an asteroid with a "very small chance" of hitting Earth in seven years.
The likelihood of it that happening could very well drop down to zero as scientists learn more about it.
But it raises questions about what scientists could do to divert a threatening asteroid in the future, and whose responsibility it would be to defend our planet.
Here are the basics about Asteroid 2024 YR4 and an look at what options we humans have to protect our planet from cosmic threats.
What's the chance Asteroid 2024 YR4 could hit Earth?
NASA says there's a 2.2 per cent chance it will hit Earth.
On the flip side, that's a 97.8 per cent chance that it will miss our planet.
So the odds of it hitting Earth are one in 45.
NASA says that's a "very small chance".
We first heard about Asteroid 2024 YR4 in late January, when NASA estimated it had a more than 1 per cent chance of hitting Earth.
But that percentage increased over the weekend after new observations were collected.
However, even then, NASA emphasised that an impact was still " an extremely low possibility".
When could the asteroid hit earth?
NASA said there was a very small chance Asteroid 2024 YR4 would hit Earth on December 22, 2032.
What could we do if an asteroid was definitely coming at us?
There are a few options that scientists put forward, including:
- Deflecting the asteroid
- Destroying the asteroid
- Working out where it'll land and clearing the area
"Which of those outcomes is more likely would probably depend on [the predicted impact zone], and on the decisions of the world leaders in charge at the time," University of Southern Queensland astrophysics professor Jonti Horner said.
"The best option would be to deflect it," Professor Horner said.
He pointed to NASA's DART (Double Asteroid Redirection Test) back in 2022, when scientists smashed an unmanned spacecraft into an asteroid in a bid to change its course.
Measurements of the asteroid taken two weeks later suggested the mission reduced the asteroid's orbit by about 33 minutes — which researchers said was "highly effective". .
And Professor Horner reckons we could something like that again.
"That would be the gold standard — especially with the amount of prior warning we have right now," he said.
There other options too, Evie Kendal from Swinburne University of Technology's Ethical, Legal and Social Implications of Emerging Technologies research group said.
"Older technologies that were being suggested for planetary defence include gravity tractors — large vessels that could drag an asteroid off its path using gravitational force," Dr Kendal said.
"This method would require a lot of lead time.
"Lasers are also theorised that could ablate part of the asteroid, changing its mass and direction."
"Nuclear explosive devices (NEDs) could destroy an asteroid with much shorter notice," Dr Kendal said.
"However, it is not legal to test nuclear weapons in space and there are many who are concerned that supporting this option would undermine nuclear non-proliferation efforts around the world."
And Professor Horner said a Hollywood-style destruction plan could backfire on us.
"The one thing you absolutely don't want to do, though, is follow the example set in the movie Armageddon — one of the least scientifically accurate films of all time," he says.
"If you were to do what they did in that film — to blow the asteroid up — all you'd do is turn a single bullet into a shotgun blast.
"In other words, you'd turn one impact into many slightly smaller impacts — as all the fragments of the asteroid would continue along the same path to hit the Earth, one after the other.
"That would be a really bad idea."
Scientists are still trying to work out the trajectory of the asteroid (more on that later) but will get a better idea of that when it nears Earth again in 2028.
And by that point, scientists could work out where on the planet the asteroid would land in the unlikely chance it actually was headed towards our planet.
"There is also an argument that if it was predicted to impact over the ocean and not affect a populated place, we could safely let it impact and study it," Hadrien Devillepoix of Curtin University's Desert Fireball Network said.
"Being able to safely study an impact of this class with modern instruments could advance the field of research massively."
Professor Horner said having four years of warning would be plenty of time to evacuate the impact area.
Who would make planetary defence decisions?
That's not clear.
"At present we lack a universally agreed upon international governance system for planetary defence," Dr Kendal said.
There some bodies out there that focus on is, like NASA's Planetary Defense Coordination Office or the Eurpoean Space Agency's (ESA) Planetary Defence Office.
But they're organisations based in the US or the EU and that complicates things.
"Planetary defence technologies are politically sensitive as many of them could also be used for other purposes," Dr Kendal said.
"In the absence of clear guidelines regarding how they should be developed and deployed, some countries may be suspicious of the motivations of other countries conducting research and testing of these technologies."
Dr Kendal said the discussion about Asteroid 2024 YR4 highlights the need to make come decisions about this.
"As planetary defence matters potentially affect everyone on Earth, it is essential to have global collaboration and cooperation on this issue before a threat arises."
Could the odds of Asteroid 2024 YR4 hitting Earth change?
Yes.
It's possible the odds of an impact may get higher or lower as we know more about it, NASA said on its Asteroid 2024 YR4 webpage.
"It is possible that 2024 YR4 will be ruled out as an impact hazard, as has happened with many other objects that have previously appeared on the NASA JPL asteroid risk list," NASA said.
"It is also possible its impact probability will continue to rise."
ESA is also tracking the probability of the asteroid's impact on a dedicated webpage it says will be updated daily.
"As new observations of the asteroid are made and we continue to study its trajectory, we gain a clearer understanding of how close it will get to our planet," ESA said.
Its website said the probability of impact is likely to go down to zero.
When will we have more certainty about the odds of an impact?
When scientists have more information about the asteroid.
It's thought it made a close approach in 2016, and telescope archival images from that time could tells us more about its trajectory.
But we may have to wait until it next gets close to Earth in 2028.
"The problem is that small asteroids like this are bright enough only when they come close to the Earth and this one is moving away from us, fast.
"In two months' time, even the biggest telescopes available to us will struggle to see it.
"So we need more observations, but crucially we need more observations spaced in time to precisely predict the asteroid's course.
"If we do not find archival images, then it is possible that we cannot sufficiently refine its orbit until 2028, the next time it comes close to the Earth."
How big is Asteroid 2024 YR4?
We don't know exactly how big it is yet.
But NASA estimated that it was somewhere between 40 and 90 metres wide, based on the asteroid’s brightness.
ESA said astronomers would be using the James Webb Space Telescope to get a better idea of its size in March.
"The asteroid is probably comparable in size to that which hit Earth back in June 1908, over Tunguska," Professor Horner said.
"That impact levelled a huge area (around 2,200 square kilometres) of forest — but that was the extent of the damage it caused."
That part of Tunguska, which is in Russia, was very sparsely populated.
"Those closest to the event reported being blown into the air and knocked unconscious, and their dwellings damaged or destroyed," Jon Uri wrote in an article for NASA's Johnson Space Center.
"Fortunately, because of the low population density, very few human casualties resulted, but many herds of reindeer perished."
But Professor Horner pointed out that even in the off chance the asteroid did hit Earth, it would probably land in the sea.
"So the most likely outcome of an impact would that it produces a spectacular light show but no-one gets hurt," he said.
The Tunguska impact caused a light so bright it was visible more than 5,500 kilometres away in Northern Ireland, according to the UK's Royal Museums Greenwich.
