Astronomers have raced to observe asteroid 2024 YR4
NASA/Magdalena Ridge 2.4m telescope/New Mexico Institute of Technology/Ryan
The world’s space agencies have reduced the chances of asteroid 2024 YR4 impacting Earth to below 1 per cent, which strongly suggests that a potentially devastating collision will be avoided. However, the asteroid will still likely pass extraordinarily near to Earth, giving astronomers a rare opportunity to observe an asteroid up close.
“We are not expecting the impact probability to rise back above 1 per cent for the close approach with Earth in 2032,” says Richard Moissl at the European Space Agency (ESA). “The most likely further development is a further drop in the impact probability, likely even to 0.”
Alarms about asteroid 2024 YR4 were first raised in December, when astronomers found it might be on a collision course for Earth in 2032. The asteroid appears to be between 40 and 90 metres wide, and could generate a deadly blast should it hit a city. In the following weeks, the world’s telescopes and space agencies closely tracked its trajectory, honing its path with greater precision. It reached its highest impact risk on 17 February, with a 1-in-32 chance, but in the days after, this fell to 1-in-67, or a 1.5 per cent risk.
On 20 February, new observations led to a sharp downgrade of this risk, with NASA putting it at a 0.27 per cent chance, or 1-in-360, and ESA even lower, at 0.16 per cent, or a 1-in-625 chance. These ratings put it at a 1 on the 10-point Torino scale, a gauge to assess the likely consequences of an asteroid impact event. That score is down from 3, making 2024 YR4 one of many low-risk asteroids that are discovered each year but that ultimately miss Earth.
This is good news, says Gareth Collins at Imperial College London, but the asteroid will still make a useful dry-run for both our planetary defense systems and for scientific purposes. “This is still something that will make a spectacularly close approach. If the risk of hitting was as high as it was, it must be coming very close to us,” he says.
NASA, ESA and space companies that were sketching out possible missions to deflect the asteroid will likely continue planning, says Niklas Voigt at OHB, a German space company. Voigt and his team had begun thinking about a mission to deflect 2024 YR4, but the new risk doesn’t change that, he says. “The risk decreased, but for the time being we are still proceeding with work on the topic,” says Voigt.
The close approach could still be a good opportunity to test our ability to deflect asteroids, says Voigt – the only previous attempt was NASA’s Double Asteroid Redirection Test, which successfully changed the trajectory of the 160-metre-wide asteroid Dimorphos in 2022. Or, we could build a satellite to send to the asteroid, he says, similar to ESA’s Ramses satellite, which will travel to observe the asteroid Apophis, which is due to pass close to Earth in 2029.
A final decision on what to do about 2024 YR4 likely won’t be made until planned observations in March from the James Webb Space Telescope which, as well as gathering trajectory data, will better assess the size and composition of the asteroid. This will be then fed to the United Nations-backed Space Mission Planning Advisory Group, which will decide on a best course of action around the end of April. “These are incredibly useful exercises for finding out the pinch points to make decisions, in order to still have time to do something sensible in advance,” says Collins. “Absolutely, those committees will still be meeting, but they’ll probably be less stressful.”
While the chances of an Earth impact have plummeted, the chances of an impact with the moon have risen to 1.2 per cent, up from a previous 0.3 per cent chance. There is a “distinct possibility of that number rising further,” says Moissl. “The exact effects for an impact on the moon from an object of this size are still under evaluation.”
The response to 2024 YR4 has also been a useful rehearsal for any future concerning asteroids, says Collins. “We want to avoid, in future, a cry wolf situation where the public gets so used to this threat that they think, ‘oh, it never happens’.”
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