How NASA found ‘city killer' asteroid's new potential target: the moon

The Hill

a day ago

(KTLA) – Although it won't hit Earth, an asteroid once labeled a ' city killer ' is now back in the spotlight — this time, because it might hit the moon.
Nexstar's KTLA spoke with Davide Farnocchia, a navigation engineer at NASA's Jet Propulsion Laboratory who works in the agency's Center for Near-Earth Object Studies. He shared how scientists tracked the 200-foot-long asteroid's shifting path, right up until it slipped out of view — for now.
'You can call it a city killer, but there's no city at risk. The Earth is out of the way,' Farnocchia said. 'But the moon is not.'
Asteroid 2024 YR4, first spotted in December 2024, is now being closely monitored as a potential lunar impactor in 2032.
2024 YR4: The 'city killer' that threatened to hit Earth in 2032
Just weeks after its discovery, YR4 made headlines when its odds of hitting Earth spiked to 3.1 percent, making it the most threatening asteroid observed in over two decades. That risk earned it a 3 out of 10 on the Torino scale, a rarely used classification signaling a credible, though uncertain, threat.
Farnocchia recalled the moment his team had to alert astronomers of the impact probability.
'We had to notify them and say, 'Look, this object should get higher priority in your schedule for observing asteroids tonight,'' he said. 'We needed more data to hopefully prove that it would move away from the Earth, which is what almost always happens.'
But instead of fading from concern, YR4's risk level climbed.
By mid-February 2025, the asteroid had a 1 in 32 chance of impacting Earth. An impact of its size could have devastated a metropolitan area if it had struck land.
'YR4 represented the most significant impact risk that we had over the last 20 years,' said Farnocchia. 'However, we communicated early on that the probability would likely drop as we got more data, and that's exactly what happened.'
By the end of February, improved measurements helped rule out an Earth impact, but the story didn't end there.
Not a threat to Earth. But what about the moon?
While Earth was taken off the risk list, the moon quietly took its place.
In May, scientists captured one final look at YR4 with NASA's James Webb Space Telescope before it slipped too far from view. That data, released in June, refined its projected path for Dec. 22, 2032, and raised the odds of a lunar impact to 4.3 percent.
That's up from 3.8 percent in April, and just 1.7 percent earlier in the year.
Even so, Farnocchia emphasized: 'The moon is smaller than the Earth, but it's a large body, so even an asteroid of 200 feet is not going to do much to it.' He noted that it wouldn't alter the motion of the moon.
And while a 4.3 percent chance may sound notable, it's still considered low by risk standards. Still, Farnocchia said, it's unusual to see the moon become a potential target at all.
'It should be less likely,' he said. 'In fact, you have a lot more impacts on Earth than on the moon; that checks out. It's much easier to hit the Earth than the moon … but now the moon happens to be in the realm of possibilities.'
2024 YR4 might not dazzle, but Apophis will
If 2024 YR4 were to hit the moon, don't expect a light show.
'If it hits the moon, the impact wouldn't be visible to the naked eye,' the NASA engineer said. 'You'd need a pretty big telescope, and even then, it might be hard to detect from Earth.'
But skywatchers won't have to wait long for something more dramatic.
On April 13, 2029, the asteroid Apophis — about 1,100 feet wide — will fly closer to Earth than many satellites, putting on a rare and safe spectacle.
'It's going to be so close, and given its size, it's going to be bright enough to be visible to the naked eye. It's going to be as bright as the stars in the Little Dipper,' Farnocchia said.
Apophis' flyby is one of the closest ever recorded for an object of its size. NASA says it may be the first such event humans have witnessed with modern instruments.
'An asteroid this large passes this close to Earth only every few thousand years on average, so it's likely that an event like this has not happened at any time in recorded human history,' the organization stated. 'Without a doubt this is the first time it's happened when humans have had the technology to observe it.'
How good is NASA at predicting impacts?
Very good — and getting better.
'We had a few very small objects discovered before reaching Earth. They burn up harmlessly, but predicting their impacts and locations felt like a big accomplishment,' Farnocchia said.
New observation tools and modeling techniques are making it possible to predict not only impact locations, but even recover fragments from impacts.
'Recently, we were able to predict impact locations and recover meteorites, which is kind of interesting because that's a final validation that you predicted the right impact point,' he said. 'One colleague was notified of an impact predicted in Canada, so he just went outside and set up a camera to observe it. That was really cool.'
Why it matters
While the moon isn't in any real danger, 2024 YR4 has become a key case study for NASA's planetary defense systems. It's the first object since Apophis to climb to a 3 on the Torino Scale, and the first ever tracked with the James Webb Space Telescope, which improved impact projections by nearly 20 percent.
'There is so much I could say about YR4 — it was really interesting to see the orbit behave exactly as we expected,' Farnocchia said. 'I like to see how the predictions evolve with each new set of observations. It's a sign that our models are working well.'
NASA expects YR4 to reenter view in 2028 for another round of observations — and possibly another plot twist.