The Odds of a City-Killing Asteroid Hitting Earth Keep Rising

WIRED

20-02-2025

Eric Berger, Ars Technica Feb 20, 2025 10:00 AM The likelihood of 2024 YR4 colliding with the our planet in 2032 have ticked up to over 3 percent. Is it time to start worrying? ILLUSTRATION: NASA/JOHNS HOPKINS APL
An asteroid discovered late last year is continuing to stir public interest as its odds of striking planet Earth less than eight years from now continue to increase.
Two weeks ago, when Ars first wrote about the asteroid, designated 2024 YR4, NASA's Center for Near Earth Object Studies estimated a 1.9 percent chance of an impact with Earth in 2032. NASA's most recent estimate has the likelihood of a strike increasing to 3.2 percent. Now that's not particularly high, but it's also not zero.
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Naturally the prospect of a large ball of rock tens of meters across striking the planet is a little worrisome. This is large enough to cause localized devastation near its impact site, likely on the order of the Tunguska event of 1908, which leveled some 500 square miles (1,295 square kilometers) of forest in remote Siberia.
To understand why the odds from NASA are changing and whether we should be concerned about 2024 YR4, Ars connected with Robin George Andrews, author of the recently published book How to Kill an Asteroid . Good timing with the publication date, eh?
Ars: Why are the impact odds increasing?
Robin George Andrews: The asteroid's orbit is not known to a great deal of precision right now, as we only have a limited number of telescopic observations of it. However, even as the rock zips farther away from Earth, certain telescopes are still managing to spy it and extend our knowledge of the asteroid's orbital arc around the sun. The odds have fluctuated in both directions over the last few weeks, but overall, they have risen; that's because the amount of uncertainty astronomers have as to its true orbit has shrunk, but Earth has yet to completely fall out of that zone of uncertainty. As a proportion of the remaining uncertainty, Earth is taking up more space, so for now, its odds are rising.
Think of it like a beam of light coming out of the front of that asteroid. That beam of light shrinks as we get to know its orbit better, but if Earth is yet to fall out of that beam, it takes up proportionally more space. So, for a while, the asteroid's impact odds rise. It's very likely that, with sufficient observations, Earth will fall out of that shrinking beam of light eventually, and the impact odds will suddenly fall to zero. The alternative, of course, is that they'll rise close to 100 percent.
What are we learning about the asteroid's destructive potential?
The damage it could cause would be localized to a roughly city-sized area, so if it hits the middle of the ocean or a vast desert, nothing would happen. But it could trash a city, or completely destroy much of one, with a direct hit.
The key factor here (if you had to pick one) is the asteroid's mass. Each time the asteroid gets twice as long (presuming it's roughly spherical), it brings with it 8 times more kinetic energy. So if the asteroid is on the smaller end of the estimated size range—40 meters—then it will be as if a small nuclear bomb exploded in the sky. At that size, unless it's very iron-rich, it wouldn't survive its atmospheric plunge, so it would explode in mid-air. There would be modest-to-severe structural damage right below the blast, and minor to moderate structural damage over tens of miles. A 90-meter asteroid would, whether it makes it to the ground or not, be more than 10 times more energetic; a large nuclear weapon blast, then. A large city would be severely damaged, and the area below the blast would be annihilated.
Do we have any idea where the asteroid might strike on Earth?
The 'risk corridor' is currently spread over parts of the eastern Pacific Ocean, northern South America, the Atlantic Ocean, parts of Africa, the Arabian Sea, and South Asia. Additional observations will ultimately narrow this down, if an impact remains possible.
What key observations are we still waiting for that might clarify the threat?
Most telescopes will lose sight of this 'small' asteroid in the coming weeks. But the James Webb Space Telescope will be able to track it until May. For the first time, it's been authorized for planetary defense purposes, largely because its infrared eye allows it to track the asteroid further out than optical light telescopes. JWST will not only improve our understanding of its orbit, but also constrain its size. First observations should appear by the end of March.
JWST may rule out an impact in 2032. But there's a chance we may be stuck with a few-percentage impact probability until 2028, when the asteroid makes its next Earth flyby. Bit awkward, if so.
NASA's DART mission successfully shifted an asteroid's orbit in 2022. Could this technology be used?
Not necessarily. DART—a type of spacecraft called a kinetic impactor—was a great success. But it still only changed Dimorphos' orbit by a small amount. Ideally, you want many years of advance notice to deflect an asteroid with something like DART to ensure the asteroid has moved out of Earth's way. I've often been told that at least 10 years prior to impact is best if you want to be sure to deflect a city-killing-size asteroid. That's not to say deflection is impossible; it just becomes trickier to pull off. You can't just hit it with a colossal spacecraft, because you may fragment it into several still-dangerously sized pieces. Hit it too softly, and it will still hit Earth, but somewhere that wasn't originally going to be hit. You have to be super careful here.
Some rather clever scientists at the Lawrence Livermore National Laboratory (which has a superb planetary defense contingent) worked out that, for a 90-meter asteroid, you need 10 years to confidently deflect it with a kinetic impactor to prevent an Earth impact. So, to deflect 2024 YR4, if it's 90 meters long and we have just a few years of time, we'd probably need a bigger impactor spacecraft (but don't break it!)—or we'd need several kinetic impactors to deflect it (but each has to work perfectly).
Eight years until impact is a little tight. It's not impossible that the choice would be made to use a nuclear weapon to deflect it; this could be very awkward geopolitically, but a nuke would impart a bigger deflection than an equivalent DART-like spacecraft. Or, maybe, they'd opt to try and vaporize the asteroid with something like a 1-megaton nuke, which LLNL says would work with an asteroid this size.
So it's kind of late in the game to be planning an impact mission?
This isn't an ideal situation. And humanity has never tried to stop an asteroid impact for real. I imagine that if 2024 YR4 does become an agreed-upon emergency, the DART team (JHUAP plus NASA, mostly) would join forces with SpaceX (and other space agencies, particularly ESA but probably others) to quickly build the right-mass kinetic impactor (or impactors) and get ready for a deflection attempt close to 2028, when the asteroid makes its next Earth flyby. But yeah, eight years is not too much time.
A deflection could work! But it won't be as simple as just hitting the asteroid really hard in 2028.
How important is NASA to planetary defense?
Planetary defense is an international security concern. But right now, NASA (and America, by extension) is the vanguard. Its planetary defenders are the watchers on the wall, the people most responsible for not just finding these potentially hazardous asteroids before they find us, but also those most capable of developing and deploying tech to prevent any impacts. America is the only nation with (for now!) a well-funded near-Earth object-hunting program, and is the only nation to have tested out a planetary defense technique. It's a movie cliché that America is the only nation capable of saving the world from cosmic threats. But, for the time being—even with amazing planetary defense mission contributions from ESA and JAXA—that cliché remains absolutely true.
This story originally appeared on Ars Technica.