After YR4 was first discovered in late-2024, its odds of impact with Earth briefly rose to historically high levels before it was eventually ruled out as a threat during an upcoming flyby.

Because the asteroid has now escaped from our view in its orbit around the sun, Webb’s recent observations were also the last chance for humanity to observe YR4 until it reemerges in 2028.

Earth is safe from space rocks for now, but the world’s space agencies are preparing to mount a planetary defense if the need ever arose.

Earth is perfectly safe from a menacing asteroid known as 2024 YR4 that is big enough to level a city, but the moon? Not so much.

The odds that the asteroid could crash into our celestial neighbor only continue to climb every time astronomers have the chance to better study the massive space rock.

NASA’s James Webb Space Telescope caught the latest glimpse in May of YR4. The resulting data led the U.S. space agency to conclude that a 4.3% chance exists that YR4 is on a doomed collision course with the moon in 2032, NASA said in a June 5 blog post.

That may not sound high, but keep this in mind: After YR4 was first discovered in late-2024, its odds of impact with Earth briefly rose to historically high levels – of just 3.1%. That was all it took for the space rock to grab headlines and warrant close attention from astronomers before it was eventually ruled out as a threat to our planet during an upcoming flyby in seven years.

Because the asteroid has now escaped from our view in its orbit around the sun, Webb’s recent observations were also the last chance for humanity to observe YR4 until it reemerges in 2028. NASA, though, is already making plans to study it again in three years when the asteroid is back in Earth’s cosmic neighborhood.

In the meantime, here’s a refresher on asteroid 2024 YR4, and what to know about its potential upcoming encounter with the moon.

What is asteroid 2024 YR4?

Because it’s big enough to be deemed a “city killer,” asteroid 2024 YR4 became a source of alarm due to the uncommonly high risk it had of colliding with Earth on Dec. 22, 2032.

The space rock was reported on Dec. 27, 2024, to the Minor Planet Center, the official authority for observing and reporting new asteroids, comets and other small bodies in the solar system. The object eventually caught the attention of NASA and other astronomers when it rose on the U.S. Space Agency’s Sentry Impact Risk Table, which tracks any known asteroids with a non-zero probability of hitting Earth.

For a time, it was the only object among more than 37,000 known large space rocks with any chance of hitting Earth anytime soon – with its probability of impact even rising to a record level of 3.1%.

That began to change in late February as more precise observations allowed scientists to effectively winnow down the asteroid’s odds of impact to a number so low, it may as well be zero.

NASA’s Webb telescope observes ‘city killer’ asteroid

Since YR4 was dismissed as a threat, NASA’s Webb telescope has twice turned its eye toward the asteroid to gather some data.

The first opportunity came March 8 when Webb – an advanced telescope launched in 2021 with powerful infrared instruments – gathered images allowing NASA and the European Space Agency to get a better idea of its size. According to the agencies’ conclusion, 2024 YR4 measures anywhere from 174-220 feet, or about the size of a 10-story building.

Earth safe from YR4, but impact odds for moon keep rising

Earth may no longer be at risk of a calamitous collision with the asteroid, but the moon isn’t so lucky.

Webb’s initial observations in March saw the odds of YR4 crashing into the moon rising from the 1.7% figure calculated in February to 3.8%, according to NASA’s Center for Near Earth Object Studies, which tracks objects like asteroids at the agency’s Jet Propulsion Laboratory in California.

Those odds rose once again to 4.3% after Webb’s latest observations were made in May. According to NASA, the data helped improve scientists’ understanding of where the asteroid will be on Dec. 22, 2032, by nearly 20%

But rest assured: If our cosmic neighbor were to take such a hit from an asteroid of that size, NASA assures that the moon’s orbit around Earth would not be altered.

How might NASA, other agencies mount a planetary defense?

YR4 and its much larger cousin, the equally infamous Apophis, were the most alarming space rocks astronomers have discovered and studied for decades.

Now that the two asteroids have been ruled out as threats to Earth, astronomers still studying such imposing space rocks could help the world’s space agencies prepare to mount a planetary defense if the need ever arose.

NASA and the European Space Agency had plans to send uncrewed spacecraft to observe Apophis in the years ahead to map and study its surface to gain further insights into near-Earth asteroids. But NASA’s mission is in jeopardy under President Donald Trump’s budget.

Protecting Earth from space rocks could look a little like the test NASA pulled off in 2022 when it demonstrated it was possible to nudge an incoming asteroid out of harm’s way by slamming a spacecraft into one as part of its Double Asteroid Redirection Test, or DART. As of October, a craft from the European Space Agency is on the way to get an up-close look at the asteroid’s remnants.

NASA also is working on an asteroid-hunting telescope known as the NEO Surveyor to find near-Earth objects capable of causing significant damage. Now set to launch no earlier than 2027, the telescope is designed to discover 90% of asteroids and comets that are 460 feet in diameter or larger and come within 30 million miles of Earth’s orbit.

Eric Lagatta is the Space Connect reporter for the USA TODAY Network. Reach him at elagatta@gannett.com