The odds of a 'city-killer' asteroid hitting Earth have fallen to zero

The astronomers told us this was the most likely outcome: a "city killer" asteroid that once had a better than 3% chance of striking Earth in 2032 is no longer a concern.

The near-Earth object 2024 YR4 was found in December by a telescope in Chile. It crossed an important threshold last month when the International Asteroid Warning Network (IAWN) — a global collaboration started in 2013 to monitor and track potential Earth impactors — issued a notification alerting the astronomical community that it had a greater than 1% chance of hitting us in seven years.

Now, "it's down to about one in a few hundred thousand … and that's below the background of things we're very interested in," IAWN Manager Tim Spahr says.

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"It went from 1% to 3% in a couple of weeks," Spahr says. "I was a little surprised, but also not shocked, really."

When Paul Chodas, director of the NASA JPL Center for NEO Studies (CNEOS), spoke to NPR earlier this month, he stressed that the probability that 2024 YR4 would hit the Earth would probably go up and then at some point rapidly fall to zero as new observations came in. That is precisely what happened.

"This is the pattern that we expected to see," Chodas says.

"If it were an impactor, the probability would have continued to rise," he says, however, "we're lucky... it's going to miss."

The observations used to rule out a strike were challenging, as the object was moving out of range for even the most powerful ground-based telescopes to see. Nonetheless, astronomers were able to get the data they needed before 2024 YR4 dimmed too much, according to Spahr.

"The real trick of this is that the asteroid doesn't tell us where it is," he says. "We're measuring the projection of it on the sky ... we're literally estimating every night we see where it is, and then we adjust that when more observations come in."

After weeks at the top of the CNEOS' Sentry list of objects of concern, 2024 YR4 has seen a steady drop in recent days. It once registered an attention-grabbing three on the Torino Scale – a level that suggests "attention by the public and by public officials is merited if the encounter is less than a decade away." The asteroid now rates zero on the scale.

Chodas says most of the really big potential Earth impactors have already been identified. But many of the smaller ones have not.

In 2005, Congress directed NASA to track 90% of all near-Earth objects 140 meters or larger by 2020.

"We're at something like 45% right now. Less than half," Chodas says.

NASA is planning as soon as 2027 to launch NEO Surveyor, an infrared space telescope designed to hunt these objects.

"Asteroids are bright in the infrared wavelengths," Chodas says. "Brighter than stars, typically." As a result, NEO Surveyor "will detect a lot more asteroids than we know of now."

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