Finding asteroids that could hit Earth — early enough for humanity to act

Amy Mainzer likes to quote a mentor who taught her the three basic steps to prevent asteroids from crashing into Earth: "Find them early, find them early, and find them early."

"We can't do anything about an incoming asteroid if we don't know it's there," says Mainzer, a UCLA astronomer who leads planetary defense missions for NASA. "But if we have years, or ideally decades, before an object can make a close approach to the Earth, now we have the benefit of time." Time to send probes up to study the object and learn what it's made of. And maybe even time to shoot something at it that could break it up or knock it off course. (That was the goal of NASA's successful DART mission, which rammed a spacecraft into an asteroid 6.8 million miles away from Earth in 2023.)

Since NASA's Double Asteroid Redirection Test (DART) successfully impacted its target on Sept. 26, 2022 - altering the orbit of the asteroid moonlet Dimorphos by a whopping 33 minutes - the DART team has determined that the mission's kinetic impactor technique can be an effective way to change the trajectory of an asteroid.

Mainzer's goal is to give humanity that time. For over a decade, she led NASA's NEOWISE mission (short for Near-Earth Object Wide-field Infrared Survey Explorer), which repurposed a small space telescope to identify and study comets and asteroids in our celestial neighborhood. NEOWISE went offline in 2024 after spotting over a hundred thousand asteroids and comets in space, including several thousand whose orbits bring them to within 28 million miles of our planet's path around the sun. Over 200 of these near-earth objects were new to science.

Now Mainzer is leading the design of NEOWISE's successor, the first-ever space telescope designed specifically to spot objects that pose a threat to our planet. But as Congress considers a budget proposal that would slash funding for the federal agencies that have long propelled U.S. space science, Mainzer says the future of this critical work could be in jeopardy.

UCLA astronomer Amy Mainzer at the red carpet premier of the 2021 film "Don't Look Up." Mainzer served as a scientific consultant for the film, a dark comedy exploring humanity's attempts to fend off an incoming asteroid. Credit: Getty Images

The high-stakes world of planetary defense science

Thanks in part to NEOWISE, astronomers reckon they're tracking more than 95 percent of the near-earth asteroids that are at least a kilometer across. "That sounds great until you realize that's about the size of the object that killed the dinosaurs," Mainzer says. The consequences of such a collision are obviously "very bad," Mainzer notes. But considering the last such asteroid struck 66 million years ago, scientists believe the likelihood of an extinction-level asteroid strike in the next century is (thankfully!) quite low.

On the other hand, we know that smaller asteroids do crash into our planet with unsettling regularity - and when something's hurtling through the atmosphere at 40,000 miles per hour, it doesn't have to be very big to cause a lot of trouble. In 2013, dash cams in the Russian city of Chelyabinsk captured the flaming trail of a meteorite streaking through the sky before it crashed down outside of town. Shock waves from the rock's fiery passage through the atmosphere were strong enough to bring down the wall of a factory and shatter windows across the city, sending over a thousand people to the hospital.

In the aftermath of the Chelyabinsk impact in 2013, ABC News called up Amy Mainzer to help make sense of what happened.

Before it broke up in the atmosphere, the Chelyabinsk meteorite was less than 20 meters across. (A baseball-sized fragment of that object eventually made its way to the UCLA Meteorite Museum, which is the largest such collection on the West Coast and the seventh largest on Earth.) It was the most powerful known impact since 1908, when a rock about 50 meters across struck Siberia, instantly flattening over 800 square miles of forest.

"So there are lots of smaller asteroids out there that are still capable of causing a lot of damage," Mainzer says. "And as it turns out, we don't know where most of them are right now. That was a bit surprising to me. And I thought, wow, we should work on that."

The UCLA Meteorite Gallery displays this 357-pound iron chunk of an asteroid that crashed into Arizona nearly 50,000 years ago, creating a mile-wide crater. Credit: UCLA/Christelle Nahas

Now, she's leading the not-so-small army of experts building NEOWISE's successor, NEO Surveyor. "We're taking all the things we've learned from the first telescope and we're making its bigger, badder cousin," Mainzer says - with a taller sun shield, a higher-resolution camera and an orbit that's optimized for detecting incredibly small, dark objects hurting through the vastness of space. NEO Surveyor is set to launch no earlier than September 2027.

"Yes, it's rocket science, but it's doable rocket science," Mainzer says. "We can build better space telescopes that will better enable us to spot these objects. It's a problem we can absolutely solve."

But to this point in human history, no other entity but the U.S. federal government has invested the resources, focus or ambition to pursue science at this scale. America's history of robust government funding for science is why over 98 percent of known near-earth objects have been spotted by NASA-funded projects.

What it takes to keep eyes on the skies

For Mainzer's current mission and for the future of planetary defense, the federal government must continue to invest in space science. Building a telescope like NEO Surveyor takes several years and lots of money. And that's to say nothing of the thousands of people whose specialized skills equip them to actually do the work.

A part of the NEO Surveyor space telescope undergoing environmental testing at Johnson Space Center in Houston in December 2024. Credit: NASA

"It takes decades to train a scientist like me," says Mainzer, whose graduate work and training were funded by both NASA and the National Science Foundation. Each agency faces a sizable budget cut in the Trump administration's proposed budget for 2026. If passed by Congress, that could mean both less support for planetary defense, as well as fewer opportunities for America to train the astronomers and engineers who will carry this work into the future.

"If we lose that continuous supply of well-trained scientists and engineers, that loss will reverberate for decades afterwards," Mainzer says.

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