For a brief period earlier this year, the world鈥檚 space agencies were warning that the sizeable asteroid 2024 YR4 had an uncomfortably high 3.1 per cent chance of hitting Earth in 2032. Thankfully, more detailed observations have now dropped those odds to near zero, but fresh analysis using the James Webb Space Telescope (JWST) shows there is still a chance it will impact our moon 鈥� something that astronomers are excited to see.
In February, when concerns of an impact with our planet were still high,听 at Johns Hopkins University in Maryland and his colleagues requested time on JWST to look at the space rock. This 5-hour observation took place on 26 March and has .
For example, previous estimates from Earth-based telescopes put the asteroid somewhere between 40 metres and 90 metres in diameter, but these were based on visible light observations, requiring a guess at how reflective the rock is. Thanks to JWST鈥檚 infrared sensors, we now know that it has a diameter of 60 metres, plus or minus 7 metres.
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If 2024 YR4 was still thought to be heading towards Earth, this would be large enough to trigger a response from the United Nations-backed Space Mission Planning Advisory Group, potentially to work up a rapid mission to deflect the asteroid and avoid possible city-scale devastation.
Instead, astronomers will now be cheering on the prospect of 2024 YR4 hitting the moon in 2032, giving them an unprecedented opportunity to watch an impact of known size, speed and composition and study the results 鈥� probably a crater about a kilometre across. Currently, there is a 2 per cent chance of a lunar impact.
鈥淧art of our motivation to continue observing this asteroid specifically is to figure out, is that number gonna go up or is it also going to go to zero,鈥� says Rivkin. 鈥淏ut a 2 per cent chance of hitting means a 98 per cent chance of not hitting. If you were in a casino, you鈥檇 be crazy to take that bet.鈥�
鈥淲e鈥檝e got our fingers crossed for a moon impact,鈥� says at Queen鈥檚 University Belfast in the UK. 鈥淚t would have no effect on Earth, but would allow us to study the formation of a lunar crater by a known asteroid for the very first time.鈥�
at the University of Kent, UK, says that a predicted asteroid impact would be a fantastic opportunity to study the flashes of light seen at the point of such collisions. 鈥淚t used to be believed that this was impossible to see until about 2000 when some people proved that, by using two telescopes simultaneously, you could see the same flash, and therefore it wasn鈥檛 a glint of sunlight or a glitch in a CCD camera or whatever,鈥� says Burchell. 鈥淪o overnight the paradigm shifted.鈥�
Some artificial objects have been deliberately smashed into the moon and caused a flash, such as the , but while natural asteroids crashing into the moon aren鈥檛 vanishingly rare they are hard to predict and even when spotted are of unknown mass and speed, making scientific observations difficult.
Seeing such a flash involving 2024 YR4 would require not only that it hit the moon, but that it did so on the side facing Earth and in the dark portion 鈥� making it unlikely, even if the impact occurs. It would also require the observer to be in the right spot on Earth and get lucky with the weather.
鈥淭hose conditions, if they鈥檙e all met, you will have a perfect controlled experiment where something of a known size and a known speed hits the moon and you can see what the flash intensity is. So it鈥檚 a great experiment and a perfect opportunity,鈥� says Burchell. 鈥淭elescopes would certainly see it, I would say, and binoculars might see it.鈥�
