Asteroid Hunters

The air was crisp and dry on the top of Mount Lemmon. The peeling white paint of telescope domes turned pink with the setting sun. At an elevation of just over 9,000 feet, we were as close as we could get to the stars outside Tucson. It was a perfect, clear night to hunt for asteroids.

My two colleagues and I, all filmmakers, met David Rankin, the telescope operator and resident asteroid hunter, at the gated entrance to the Catalina Sky Survey, a NASA-funded project at the University of Arizona that involves searching for near-Earth objects. Rankin was wearing jeans and a T-shirt emblazoned with the emblem of the Planetary Defense Coordination Office (PDCO), a small NASA program with a big mission: to find and track near-Earth asteroids that could pose an impact threat to our planet. Rankin was tasked by PDCO to find those asteroids, and we were making a NASA-funded documentary about the search—from discovery, tracking, and characterization of these space rocks to the eventual mitigation of potential threats. What started as a 15-minute video grew into a feature-length documentary called Planetary Defenders, set to come out this April.

Rankin swung the gate open, and we followed him in his pickup truck to the top of the mountain, past a series of small and large domes that sat silent and empty. Rankin’s license plate read “Apophis,” an homage to an asteroid that got everyone’s attention when it was discovered in 2004 and looked like it might one day hit Earth. Now determined not to be a threat, Apophis will make its close approach on April 13, 2029, when it comes less than 38,000 kilometers from our planet.

Asteroid scares are the stuff of science-fiction movies. Think of Bruce Willis, Ben Affleck, and the nuclear bomb in Armageddon; Téa Leoni in Deep Impact; or Meryl Streep as the president scoffing at scientists in the dark comedy Don’t Look Up. But asteroids large enough to damage our planet have actually hit Earth, and it is a statistical certainty that they will do so again. All you have to do is go to Meteor Crater in Arizona to see the evidence: at 1.2 kilometers across and 180 meters deep, the impact crater was formed 50,000 years ago by an iron asteroid. Or if you’re in the mid-Atlantic, take a look at Chesapeake Bay, thought to have been formed by an object three to five kilometers in diameter that hit Earth 35 million years ago. The crater, now buried 300 to 500 meters beneath the surface, was filled in by water. It is the seventh-largest confirmed impact crater on Earth. And, of course, the asteroid that killed the dinosaurs was the Chicxulub impactor, more than 10 kilometers in diameter, which hit 66 million years ago in what is now the northern part of Mexico’s Yucatán Peninsula.

Most asteroids are found far from Earth, between the paths of Mars and Jupiter, in a region fittingly known as the main asteroid belt. But every so often, asteroids make their way into the inner solar system. With both Earth and the asteroids traveling around the sun, orbits will occasionally intersect. The damage potential depends on the size of the asteroid, what it’s made of, its speed, and where it hits. Small asteroids approach Earth all the time but disintegrate in the atmosphere, sometimes leaving behind small pieces of space rock. These are the most frequent impacts, though some asteroids are large enough to make it all the way to the ground and form an impact crater. You may have seen a meteorite collection on a field trip. Perhaps you have held in your hands a cold stone that crashed through the Earth’s atmosphere and made it to the ground intact. Maybe if you’re a space enthusiast, you even have meteorite earrings. It’s possible that the asteroids (and comets) that hit Earth billions of years ago might have brought the raw materials needed to form life. They also brought devastation.

With a larger asteroid, the event could create a blast wave with serious thermal consequences. Alternatively, a fall into the ocean could cause tsunamis that could kill coastal residents and damage coastlines. Emergency management personnel draw the damage levels in circles surrounding an impact site like a bullseye, with “shattering windows” on the outside ring of the circle and “devastation, structures flattened or burned” at the center. But asteroids of the size that could do damage are not nearing Earth every day. As of this writing, we know of just one significant impact threat to Earth in the next century—and that asteroid, 2024 YR4, has a less than 0.002 percent chance of hitting us in 2032. (The orbit calculations are less reliable after 100 years, so all the numbers are couched within that context—which frankly feels like a pretty good buffer to me.) All of this makes asteroid hunting a long game. The asteroid hunters I know don’t lose sleep over impending doom, but that doesn’t make their jobs any less important: what they discover will have obvious consequences for the future of the human race.

In 1998, Congress officially gave NASA a mandate: find at least 90 percent of near-Earth objects a kilometer or larger—the ones that could wipe out human life. In response, NASA formally launched the Near-Earth Object Observations Program, which deploys telescopes to search the skies. The program grew into PDCO in 2016. In the years since, NASA has fulfilled the mandate by finding 873 asteroids in the specified size range. In 2005, Congress sent over a new mandate: to find the asteroid population 140 meters or larger—space rocks that scientists colloquially call “city killers,” referring to the potential of causing extensive regional damage rather than extinction. To date, scientists have found 11,209 of these objects, making up only 45 percent of the total number (estimated using Bayesian statistics). Again, as of this writing, none of these will pose a threat to Earth in the next 100 years.

Of course, astronomers haven’t found them all. This was actually a surprise to me when I started learning about this field. I had assumed that NASA astronomers knew where everything was in our solar system. The problem, it turns out, is complex. And many scientific questions of utmost importance still linger, some more practically significant than others: How many near-Earth asteroids are there? Where are they and where are they going to be in the future? Are they on an impact trajectory with Earth? If yes, when? How big are they? What are they made of? What is their impact potential? And if they did hit Earth, what damage would they do?

There are millions of asteroids in the solar system and probably hundreds of thousands in orbits that enter the inner solar system. Astronomers first learned of the existence of asteroids a little more than two centuries ago. The knowledge that asteroids come near Earth is barely more than 100 years old. Most of these asteroids are small objects and therefore difficult to find, and only in the past few decades has our technology been able to detect them. On top of that, many of them are not visible from the ground, either because the asteroids are not reflective or because current terrestrial telescopes can’t be pointed close to the sun.

In the 1990s, a concerted effort was made to start tracking asteroids that could potentially threaten Earth. Thus the field of planetary defense was born, so named by Lindley Johnson when he was working in space surveillance for the U.S. Air Force. He later moved over to NASA and eventually started PDCO, where his title became, fittingly, planetary defense officer (now, emeritus).

Asteroid detection was made much more efficient by the development of digital photography, but a few events spurred the field along, including the impact of the comet Shoemaker Levy 9 on Jupiter in 1994. Astronomers Eugene and Carolyn Shoemaker and amateur astronomer David Levy discovered a comet that had broken into fragments that were headed straight for the largest planet in our solar system. Asteroids and comets were known to be a fundamental part of the formation of our solar system, but to be able to observe one hitting another planet today—that was a new thing. Scientists went wild. They pointed both ground-based and space-based telescopes at Jupiter, and the whole world watched as the comet’s fragments hit the planet one by one. Big plumes of material rained back down on the upper part of Jupiter’s atmosphere, leaving bruises that were captured in images. It looked as if the planet were having an ultrasound.

“That was my first observing run ever,” Kelly Fast, NASA’s acting planetary defense officer, told me when we met at NASA headquarters in Washington. She grew up in Los Angeles with views of the Hollywood sign and Griffith Observatory out her bedroom window. In 1994, Fast was a young astronomer visiting the NASA Infrared Telescope Facility in Mauna Kea, Hawaii, where all eyes were on the Shoemaker Levy 9 comet. She and other scientists were captured on VHS tapes laughing, hugging, dancing, leaning over the small circular screens of their computers. “We were like kids in a candy store,” Fast said.

Being able to observe the impact was not only scientifically interesting, it was also a reminder, a wake-up call that asteroids and comets are still colliding with planets in our solar system. In one of the 1994 tapes, Fast recorded a TV news anchor covering the event: “Scientists say if a fragment the same size hit Earth, it would leave a crater the size of Rhode Island.”

In recent history, two collisions on Earth have been large enough to do significant damage. On June 30, 1908, a 40- to 60-meter asteroid disintegrated in the atmosphere over the Tunguska River valley in Siberia, releasing a shockwave that devastated more than 2,000 square kilometers of forested land—an area roughly 2.5 times the size of New York City. Then, on February 15, 2013, a previously undetected 18-meter asteroid exploded over Chelyabinsk, Russia, causing an airburst and shock wave that struck six cities in the region. No one was killed, but the blast injured more than 1,600 people, mostly from broken glass.

Those asteroids were considerably smaller than the 140-meter space objects that NASA astronomers are mandated to look for. If an 18-meter asteroid can do that, I certainly don’t want to see anything larger heading our way. Of course, asteroid hunters aren’t looking only for asteroids larger than 140 meters; that is simply the priority. If smaller asteroids come up in their scans of the sky, they log those, too.

“This is the only natural disaster that can be prevented,” Fast likes to say. That and, “We find asteroids before they find us … so we can get them before they get us.”

She is referring to DART, NASA’s Double Asteroid Redirection Test, in which a spacecraft was designed to crash itself into an asteroid to change its orbit. Scientists targeted a binary asteroid system—Didymos and Dimorphos—that would pass close enough to Earth to be observable with ground-based telescopes. Dimorphos, the smaller asteroid, orbits the larger Didymos—the perfect model of an asteroid orbiting the sun.

In 2022, our team received funds to broadcast the DART impact to the public. We watched along with the scientists and engineers from the TV studio at the Johns Hopkins Applied Physics Laboratory (APL) as the first images came in from the spacecraft—first one dot, then two—and watched as the spacecraft, guided by an autonomous navigation system, flew past the larger asteroid and aimed for its target. The mission engineers sat nervously at consoles, with their headsets on, in a room they called the fishbowl—officially, Mission Operations—on the other side of APL’s campus. Mission Systems engineer Elena Adams, who had overseen the construction of the school bus–size spacecraft, counted down the impact live on air. As she called three, two, one, the asteroid took up the whole frame on all of our screens; we could see every bump on its surface until the entire thing flashed red. My colleague sitting next to me held his phone up to film the moment on the livestream that we were feeding to the world. We erupted into cheers as the spacecraft struck.

The goal of this exercise was to demonstrate an asteroid deflection technique called kinetic impact. If a threatening asteroid is found when it is far enough away, its orbit can be changed ever so slightly to make it actually miss Earth. DART changed the orbital period of Dimorphos around Didymos by 33 minutes, far more than what was considered necessary to make the mission successful—proving that the capability to deflect an incoming asteroid exists if needed.

“We have the power to at least look at doing something,” Fast said. “And that might be what could prevent an impact in the future.” But, she is quick to add, DART won’t work for every asteroid. Other techniques could be used to deflect an asteroid—an ion beam from a controlled electric thruster (which would slowly push it), a “gravity-tractor” (which would slowly pull it), or even a nuclear explosive device—but those haven’t been tested yet. Unlike what you see in Hollywood movies, the nuclear explosion would happen next to the asteroid rather than on it, to boil part of it off, changing its speed and therefore its orbit. But for any of these methods to work, the physical properties of the asteroid must be known: a metallic rock will respond differently to DART, for example, than a carbonaceous one.

First, of course, the asteroids must be found. To that end, a space telescope currently being built at NASA’s Jet Propulsion Laboratory (JPL) will search for near-Earth objects in infrared rather than visible light wavelengths, allowing scientists to observe dimly lit asteroids and those closer to the sun. When it launches in 2027, it will help fill in the gap of what is left to be found. And every few years, NASA’S PDCO, the Federal Emergency Management Agency, and other U.S. government agencies get together to walk through simulated asteroid impact exercises. They are flexing the muscles of a system that might never need to be used, at least in our lifetimes, thanks to planetary defenders like Kelly Fast and David Rankin, along with dozens of their compatriots all around the globe who drive up mountains, open telescopes, and look up, night after night.

When our documentary team first contacted Rankin, he suggested that we film his work in the autumn rather than summer. For one thing, the summer monsoon season made for consistently cloudy skies—and the occasional impassable road.

“Do you get excited when you find a new asteroid?” we had asked him.

“In the beginning, I did,” he said. “But I find asteroids every night.”

As sunset approached on Mount Lemmon, Rankin opened the telescope dome with the push of a button, revealing pink sky outside and letting cold air in. The dome rattled loudly as it turned. The observation room was really more of a hallway with a desk and a few very large curved monitors, on which lots of numbers and images of star fields appeared. The hallway was flanked by a kitchenette and a bedroom where observers sleep when the sun comes up. They are often on the mountain for two to three nights at a time, sleeping during the day and observing at night. Rankin told us to make sure to latch the door in case of bears. Tonight, if we weren’t here, he would be alone on the mountain. The neighboring telescope was operated remotely by his colleague in Tucson.

We filmed over Rankin’s shoulder as he showed us what asteroid hunting looks like. The telescope takes four back-to-back images spanning 20 minutes of a single patch of sky, mostly revealing a static star field. If there is a moving dot within the star field, it could very well be an asteroid—since the stars are so much farther away, they don’t move in the image. Four points of moving light will be our very first view of any object. The computer takes the first pass and presents the telescope operator with a series of possible asteroid detections.

If the telescope operator determines that these points of light may in fact be an asteroid, the data from these images will ping-pong across a series of laboratories around the world. The first stop is a small room off an unassuming hallway in an old building on Harvard’s campus: the Minor Planet Center, or MPC, which keeps a comprehensive database of all small bodies in our solar system. It is funded by NASA but falls under the auspices of the International Astronomical Union, a global organization that promotes and coordinates astronomical research (this was the group that reclassified Pluto as a dwarf planet in 2006).

When MPC scientists receive a new observation, they run it through their database to determine whether the asteroid in question is in fact new or if it has been observed before. Observations come in almost daily from all around the world, from professional and amateur astronomers alike. To date, the MPC has received more than 400 million observations. As soon as an asteroid is determined to be new, the data points are updated for the public.

From there, scientists are able to do orbit calculations for all known asteroids or comets. The Center for Near-Earth Object Studies at NASA’s JPL immediately pulls any new information added to the MPC’s public database and calculates all possible orbits for that particular object, along with statistical assessments of the impact probabilities. If any of those orbits show an impact probability—and especially if a potential collision is relatively imminent—those asteroids are flagged for NASA and the planetary defense community. If it’s a new discovery with a high impact probability, asteroid astronomers around the world will get an alert. That way, they can collect more observations of the asteroid and scientists can refine the orbit even further.

So far, scientists have detected 11 small asteroids that later collided with Earth—the latest being an asteroid approximately one meter across that disintegrated harmlessly over Siberia in December 2024. An astronomer first reported the asteroid, the MPC declared it new, JPL’s watchdog calculated its orbit and alerted the community of a potential impact, and follow-up observations refined the orbit. Scientists thus determined ahead of time where it was going to hit and when. Again, the asteroid wasn’t of a size that could have done major damage, but the point is, the system worked.

Back on Mount Lemmon, Rankin received his first batch of images from the telescope around seven p.m. He scanned them quickly, almost as if he were playing a video game. Before my eyes could even register what I was looking at, he had already determined that the object was actually a star. Some of the moving dots were asteroids, but they were already in the known database at the MPC and the computer put a green circle around them with the asteroid’s name next to it. Rankin clicked through the images quickly, flashing them on the screen, comparing them with historical images, and then throwing them out. Not a new asteroid. Not a new asteroid. Click, flash, click, flash, next. Until he stopped.

A yellow circle formed around a white dot, and in each new image, it moved distinctly across the star field.

“Oh, this might be something,” he said. “Look at that.” We held our cameras still to capture the moment—our very first potentially new asteroid. Rankin ran it through the catalog of known asteroids and no match came up, so he sent the observation off to the MPC. Within a couple minutes, it was confirmed: c9v16g2 was a new near-Earth asteroid. And only a few minutes after that came a notice from the Center for Near-Earth Object Studies: not only was this asteroid new, but it was a big one.

“There it is. Bam. It’s live,” Rankin said. “The nominal size is over 100 meters. Maybe even over 150 meters across.”

A city killer, I thought.

I tried to hold my camera as still as possible. Shooting a side glance at my colleagues across the room, I saw that they were doing their best to hold their cameras still, too.

“It has a probability of being a potentially hazardous asteroid of almost 70 percent,” Rankin said. This designation—potentially hazardous asteroid, or PHA—means that the asteroid is larger than 140 meters in diameter and is coming within a certain orbit intersection distance from Earth.

“If you were going to be here for a discovery, a PHA is definitely what you want,” Rankin said, laughing. In five years of observing, he had identified only 25 PHAs among thousands of smaller asteroids—and yes, he was counting. The odds of finding a PHA in one of the first sets of observations were insanely low. And we’d been there to see it.

When we were done filming, we began hammering Rankin with questions. How far away is it now? (“We don’t know. That’s one of the hardest things to constrain early on, but it’s likely many lunar distances away from Earth at this point.”) How soon will you know if it could collide with Earth? (“I don’t think it’s going to hit Earth. If you don’t get any notice right away from JPL’s watchdog, that’s good news. What makes it a PHA is that its orbit comes close to Earth.”) What happens next? (“We’ll get observations from the telescope next door. Once the arc is extended to three or four hours, we’ll have an even better idea if it’s a PHA. If it is, that’s an exciting way to start the night. If this was the only thing I found for the next two months, I’d be happy.”)

Once we’d finally stopped badgering him, he said, “You guys are going to get addicted if you’re not careful.” It was true. We were hooked.

By the time we packed up that night, Rankin had received yet another update with more information about the asteroid—it was likely 230 meters in diameter, with an orbit that could eventually bring it somewhere between Earth and the moon.

At Rankin’s instruction, we drove down the mountain with our headlights off until we were clear of the telescope. We giggled with excitement at what we called “our asteroid discovery,” but then we caught ourselves. Of course we didn’t want this asteroid to be hazardous, but to observe the system working—and so quickly—was cool.

Rankin had skirted that line, too, between scientific fascination and the gravity of the situation. “That’s a nice one,” he’d whispered to his screen. “These are the ones we want.”

That night after we went to sleep, our asteroid—now designated 2023 VS3—was further tracked by telescopes in Chile, Illinois, Kansas, and the telescope next door to Rankin’s. The resulting data were crucial in fine-tuning the asteroid’s orbit—and would prevent the asteroid from getting “lost”—something that makes telescope operators squeamish.

It took 36 more days of observations to collect enough information to calculate an approximate orbit. 2023 VS3’s estimated size is still 230 meters, and its nearest approach will be October 5, 2031, when it could pass between Earth and the moon. One more big asteroid could be checked off the list of the estimated 14,000 left to be found.

Note: Since our Spring 2025 went to press, the asteroid 2024 YR4’s chance of hitting the Earth in the next century briefly went up before swiftly falling to 0.0017 percent as of online publication.