That "city-killer" asteroid may no longer be projected to strike Earth any time soon, but scientists are still keeping close tabs on it.

Using the mighty James Webb Space Telescope, astronomers have imaged the once-menacing space rock, dubbed 2024 YR4, in infrared light, providing our best estimate yet of its true size. It's also a convincing demonstration of the telescope's usefulness in assessing the hazards of near-Earth asteroids in the future.

Previously, our ground-based glances at 2024 YR4 in visible light had suggested that it could be as long as nearly 300 feet. Now, the latest observations reveal that it's somewhat smaller, between 174 to 220 feet in stature — or about the size of a 10-story building, according to NASA.

"This is the smallest object targeted by the mission to date, and one of the smallest objects to have its size directly measured," Andy Rivkin, principal investigator of the Webb telescope's Director's Discretionary Time program, said in a statement. 

"While we are confident that 2024 YR4 will not hit Earth in 2032, there is still great value in making these observations and analyzing the results," Rivkin added. "This will help us determine the best approach to use during a more urgent observing program should another asteroid pose a potential impact threat in the future."

2024 YR4 was first detected in December. By February, the chances it would strike Earth had exceeded 3 percent. Those were by no means guaranteed odds, but as the asteroid was deemed large enough to wipe out an entire metropolis, they were still way too high for comfort. But just as astronomers had anticipated, those threat estimates were quickly downgraded to no threat at all. 

Nevertheless, the episode rekindled both the public's and the scientific community's interest in planetary defense. There are over a million asteroids in the inner Solar System that we know about, and close to 40,000 near-Earth ones. If one suddenly pops up out of the blue, how can we quickly assess it?

Rivkin explains how the James Webb can help in that scenario.

"Most telescopes observe asteroids by measuring sunlight reflected from their surfaces, and it's hard to precisely determine their sizes from this information," he said in the statement. With the Mid-Infrared Insturment (MIRI), however, "the heat given off by asteroids themselves can be measured and used to directly give the asteroid's size."

In any case, the 2024 YR4 saga ain't over yet. The latest Webb data indicates that there's still a 3.8 percent chance it could smash into the Moon, according to NASA, which is even higher than what was projected a month ago. Let's see if hypocritical humans will show the same levels of horror and urgency now that it's not their world in the crosshairs.

More Webb chronicles: James Webb Space Telescope Captures Images of Individual Planets in Distant Star System


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