by Bryan Walsh
Humanity’s first real inklings of the peril posed by NEOs came on March 24, 1993, when a team of three astronomers at the Palomar Observatory outside Los Angeles discovered a comet on a collision course. The weather at the Peninsular Ranges was windy and cold that night, with clouds obscuring the sky from an incoming storm. The geologist Eugene Shoemaker; his wife, Carolyn, an astronomer; and their colleague David Levy were carrying out a multiyear NEO survey, painstakingly searching for asteroids, comets, and anything else they might be able to find arcing through our solar system. But hopes that night were as dim as the sky. The best conditions for asteroid hunting are clear and still. March 24 was neither.7 Even today, with the aid of digital cameras and tracking software, the practice of discovering and cataloging asteroids is as much art as it is science, and one that favors the meticulous. In the early 1990s, though, asteroid hunters still used analog film. Aiming their cameras through telescopes, they took multiple minutes-long exposures of specific sections of the night sky, then compared those images in the hopes of locating a heavenly body moving against the known background of stars and planets. As if the bad weather that night weren’t obstacle enough, the Shoemakers and Levy had discovered a couple of nights earlier that a stack of their highly sensitive film—which cost nearly four dollars per shot—had been accidentally exposed to the light, making it unusable except for a portion around the center of the slide. Was it even worth potentially wasting their precious remaining good film on such a cloudy night?
Eugene Shoemaker, known as Gene, was a legend in asteroid studies. He was one of the first scientists to conclude that the craters found throughout the Earth were physical evidence that the planet had been struck by asteroids and comets in the past.8 But between the cost of the film and the bad weather, even the stalwart Gene Shoemaker was ready to pack it in for the night. Levy, though, argued that they might as well try to use the damaged slides—if the conditions remained poor, at least they wouldn’t have squandered any of the unexposed film. Gene eventually agreed. Before the gathering clouds finally drew a curtain on the night, Levy managed to take exposures of three fields of sky they hoped could be compared to existing images they had taken previously.9
Once the exposures had been shot and developed, Carolyn Shoemaker began scanning them, two at a time, using a stereo-microscope that caused any moving images—like asteroids—to rise above the background field of stars. Carolyn’s story is an inspirational one. She had spent decades as a wife and homemaker to Gene before making a later career change to become an astronomer, and at age fifty-one now spent her nights hunting for asteroids at Palomar and Northern Arizona University. As she scanned a pair of images the following day, Carolyn spotted something near the center of the field a few degrees away from the planet Jupiter. “I don’t know what this is,” she told Gene and Levy, “but it looks like a squashed comet.”
Over the course of her second act, Carolyn Shoemaker discovered more than 800 asteroids and 32 comets.10 Yet no find would prove more consequential than the object she spotted that evening. What Gene and Levy photographed and Carolyn picked out was indeed a comet, one that would eventually be designated Shoemaker-Levy 9, as it was the ninth such comet discovered by the team. It appeared squashed because it had fragmented, shattered by the force of Jupiter’s gravitational field when the comet’s orbit had taken it near the planet in 1992. But even a broken comet is dangerous, and now what remained of Shoemaker-Levy 9 was on a direct course for the largest planet in our solar system.
Beginning on July 16, 1994, with the telescopes of the world watching—along with the spacecraft Galileo, then just 150 million miles from Jupiter—the lead fragments of Shoemaker-Levy 9 slammed into the gas giant.11 Shoemaker-Levy 9 marked the first time that astronomers were able to directly observe a collision between two extraterrestrial objects in the solar system, and the show did not disappoint. Most of the cometary fragments were at least as large as 1.25 miles across and were traveling at 125,000 miles per hour, fast enough to cross the width of the continental United States in little more than a minute. A single piece of Shoemaker-Levy 9 delivered the energy equivalent of 6 million megatons of TNT, a force hundreds of times greater than the explosive power of the world’s combined nuclear arsenal at its peak.12 The impacts created fireballs that reached heights of more than 1,800 miles—higher than three hundred Mount Everests—and burned at temperatures as hot as 42,660 degrees.13 The great dark scars from the collisions were as large as 7,400 miles across, and were visible even from a child’s backyard telescope on Earth.14
For astronomers, the Shoemaker-Levy 9 collision with Jupiter was an unprecedented cosmic fireworks spectacle, one so powerful that the thin rings circling the planet were left tilted, like a football player whose helmet had been knocked askew by a blind-side hit.15 When one 2.5-mile-wide fragment struck the planet, the flash generated by the impact was so bright that it temporarily blinded many of the infrared telescopes trained on the event. Yet if Carolyn Shoemaker hadn’t picked out that “squashed comet”—and if David Levy hadn’t urged his team members to keep working through that cold and cloudy night at the Palomar Observatory—scientists might never have witnessed the collision.
But Shoemaker-Levy 9 wasn’t merely a scientific milestone. Jupiter is the biggest planet in the solar system, so large that you could fit 1,300 Earths inside it and still have room to spare. If any planet in the solar system would be capable of shrugging off a blow from a comet, it would be Jupiter. Before the impact event, astronomers wondered if Shoemaker-Levy 9 would simply disappear into the bowels of the gas giant. Yet months after the collision the scars from Shoemaker-Levy 9 were even more pronounced than Jupiter’s Great Red Spot, that eye-shaped spinning storm in the planet’s atmosphere that is twice the size of the entire Earth. It was as if a new inmate had picked the biggest, baddest guy in prison—and beat him to a pulp. If a cosmic collision could do that much damage to Jupiter, what would it do to our little blue planet? Suddenly the solar system seemed like a much more dangerous place.
Expert concern about the threat posed by NEOs had begun to mount in the years before Shoemaker-Levy 9, spurred by the Alvarezes’ discovery and by a close call with a half-mile-wide asteroid that passed within 500,000 miles of the Earth in 1989.16 In 1990 Congress included language in NASA’s budget authorization requiring the agency to prepare a report on NEOs—how to track them and how to stop them. But when then vice president Dan Quayle endorsed an idea that same year for the federal government to buy telescopes to track potentially hazardous asteroids—and use modified Strategic Defense Initiative antimissile weapons in orbit to destroy them—it became a punch line, not policy.17 Planetary defense was plagued by the “giggle factor”18—a real term for when a scientific subject appears too ridiculous on its face to be taken seriously. The highly visual lesson of Shoemaker-Levy 9 helped change that. “We woke up to the fact that asteroid impacts had an effect on the evolution of life on the earth,” said Steve Larson, a coinvestigator at the Catalina Sky Survey at the University of Arizona, one of the first programs dedicated to searching for NEOs.
Hollywood even got in on the act, eager in those immediate post–Cold War years for disaster scenarios on an apocalyptic scale. Two separate films about efforts to prevent an extinction-level collision event, Deep Impact and Armageddon, were released within two months of each other in the summer of 1998. Deep Impact, in the likely event that you’ve blocked it from your mind, is the comet one, and Gene and Carolyn Shoemaker were even credited as “comet advisors” on the film. Armageddon is the one with Ben Affleck and Bruce Willis as oil rig workers drafted by NASA to drill into an incoming asteroid and blow it up with a nuclear bomb. Armageddon was so critically loathed that director Michael Bay—who has made more than his fair share of critically loathed movies—actually apologized for it fifteen years later.19 (In the DVD commentary, Affleck made the astute point that it would probably have been easier to train astronauts to become oil drillers than vice versa.) It still made
$553 million to Deep Impact’s $349 million. I saw them both in the theaters.
The same year those films hit screens, NASA established its NEO Program and, under a congressional directive, dramatically increased its participation in the Spaceguard Survey, which was tasked with discovering and tracking at least 90 percent of potentially hazardous NEOs larger than 1 kilometer (0.62 miles) within the following ten years. Impactors that big could potentially cause continental or even planetary damage. In a field where cost overruns and blown deadlines are common, NASA almost met that target on time, and achieved its goal in 2010. Because the movement of celestial bodies is predictable with sufficient data, scientists could forecast the orbits of identified asteroids decades into the future, and determine whether any have a chance of colliding with the Earth in the decades to come.
The results were a relief. Thanks to the work of Spaceguard and the astronomers around the world who contribute to it, we can be confident that our civilization is unlikely to be ended by any Armageddon-class asteroids for the foreseeable future. But humanity shouldn’t become complacent. While NASA has pinpointed nearly all of the largest NEOs, of which there are some 2,000,20 a new congressional mandate in 2005 to find at least 90 percent of potentially hazardous NEOs larger than 140 meters (460 feet) by 2020 has proven far harder to fulfill. While an impact from a smaller asteroid may not threaten civilization, it could easily wipe out a city, and potentially much more. And there’s a good chance that we might not even see it coming before a collision was imminent, far too late to deflect the asteroid or even evacuate those in harm’s way. “This is the only natural hazard that is predictable and preventable,” said Clemens Rumpf, a research fellow at the University of Southampton who studies asteroid strikes. “But we have to know that the asteroid is coming.”
To do that—to fully “retire the risk,” as asteroid hunters say—we’ll need to spend more on planetary defense. That includes executing a mission that would rehearse tracking and deflecting an asteroid in space. Such an effort would ensure that should the time come to save the world, we’ll at least have a practice run under our belts. It would cost more than the roughly $60 million NASA currently dedicates to planetary defense each year,21 but the math of existential risk proves that it’s worth spending far more to reduce even a tiny risk of planetary catastrophe. The biggest challenge isn’t what we can budget for, but what we can imagine. “There’s no technical obstacle to protecting ourselves,” said Franck Marchis, a senior planetary astronomer and asteroid expert at the SETI Institute in Silicon Valley. “We know how to protect ourselves. We’ve just never tried.”
To understand the risk from NEOs, you need to understand what they are and where they come from. Our solar system was formed some 4.6 billion years ago, as the sun emerged from a cloud of gas and dust known as a solar nebula. Over a hundred million years the matter that remained following the sun’s formation began to clump together. The planets, including our own, grew through a process of accretion, like sand castles built one grain at a time.
Our planet’s youth was a wild one, as the Earth endured a rain of asteroid and comet strikes. Not long after the planet was formed, a celestial object the size of Mars collided with our planet, blasting billions of fragments of rock and molten magma into space. Some of that rock eventually cooled and formed a sphere—our moon.22 That was just one massive impact—between 3.9 and 4.5 billion years ago, the Earth, moon, and many of the other planets of the solar system were struck again and again during what scientists call the Late Heavy Bombardment. Extrapolating from the impact evidence on the pockmarked moon—where there is no air or water to erode the silent craters—the Earth may have been hit by more than 20,000 asteroids or comets capable of leaving craters from 6.2 miles to 620 miles in diameter.23 The Late Heavy Bombardment was, as the author Craig Childs writes in his book Apocalyptic Planet, “what everything else is measured against, an apocalypse unmatched in Earth’s history.”24 Though there was still creation amid the destruction. Some of those comets may have borne water to a young and barren Earth, along with the carbon-based molecules that form the foundation of life. Whatever asteroids and comets may do to us in the future, we might not even be here without them.
Even after the solar system finally settled down into a more sedate middle age, there were still millions of pieces of rock and metal wandering unattached through space like leftover pieces of a jigsaw puzzle. These were asteroids and comets. Comets are composed of rock, frozen gases, and ice—the characteristic tail in a comet is a result of the sun vaporizing some of its material, releasing dust particles that trail behind it. They originate in the Kuiper Belt and the Oort Cloud, on the outermost fringes of the solar system. Asteroids tend to be rocky or metallic, and true to their name are usually found within what’s known as the asteroid belt, between Mars and Jupiter. While films like The Empire Strikes Back make it seem as if an asteroid belt is so dense with rock that the odds of successfully navigating it are—to quote C-3PO—approximately 3,720 to 1, our belt is actually so sparse, as the planetary scientist Carrie Nugent points out in her delightful book Asteroid Hunters, that the actual odds are closer to 1:1.25 If you took all the asteroids in the asteroid belt and squashed them together, you’d have an object less than 4 percent of the mass of our moon.26
Every once in a while, gravity perturbations from Jupiter or Mars can kick asteroids out of the main belt, like a big child nudging a smaller one off a merry-go-round. Those loose asteroids might spin out of the solar system altogether or be swallowed up by the sun, but some can end up in shallower orbits that bring them close enough to our planet to be classified as an NEO. An NEO’s orbit must bring it within about 30 million miles of the Earth’s orbit, around 125 times the distance between the Earth and the moon. A potentially hazardous object, or PHO—the ones to watch out for—is any NEO big enough to potentially survive the plunge through our atmosphere, meaning larger than 100 or so feet wide, and with an orbital path that can bring it within about 5 million miles of the Earth.27
That we rarely notice asteroid impacts is in part a sign of just how empty space is—by one estimate, only 0.0000000000000000000042 percent of the universe actually contains matter.28 But in truth we’re under near-constant bombardment, though mostly by the cosmic equivalent of BBs. Each day the Earth is battered by about 100 tons of dust and sand-sized particles;29 keep a close enough watch on the night sky and you’ll see the larger pieces flaring briefly as they burn up in the atmosphere as what we call, inaccurately, shooting stars. The size and number of asteroids in the solar system follows a proportional relationship called a power law, so that smaller asteroids are far more numerous than large ones. That’s why it has been easier for NASA to locate asteroids above 1 kilometer than it has been to locate those above 140 meters—the big ones are bigger, so they were easier to spot, and there are fewer of them to find.
Though the English astronomer Edmond Halley—he of the eponymous comet—theorized as early as the seventeenth century that impacts could create craters on the Earth’s surface,30 the first NEO wasn’t discovered until 1898.31 For years astronomers found asteroids—which showed up as smudges on the pictures they took from telescopes, just as any moving object is blurred in a photo—more annoying than intriguing. They were searching for the interesting stuff—planets, moons, and stars, all of which are far rarer than what appeared to be the dregs of the solar system. In her book, Nugent writes that the astronomer Edmund Weiss found asteroids so irritatingly common that he began referring to them as “those vermin in the sky.”32
One scientist’s vermin can be another’s life work, however, and by the 1960s interest in hunting asteroids surged. The Palomar Observatory, where the Shoemakers and David Levy discovered their comet; the Leiden Observatory in the Netherlands, led by another husband-and-wife team, Cornelis and Ingrid van Houten; the LONEOS survey in the mountains near Flagstaff, Arizona—all were staffed by astronomers dedicated to finding NEOs.33 But no search program has been more success
ful than the Catalina Sky Survey at the University of Arizona, which as of 2018 had found nearly half of the roughly 18,000 NEOs that have been discovered to that point.34 As Eric Christensen, Catalina’s primary investigator, told me when I visited him at the University of Arizona: “Finding near-Earth objects is our only mission, our only goal, and we’re free to optimize everything we can towards it.”
If a civilization-threatening asteroid or comet does zero in on Earth, there’s a better than even chance that the NEO hunters of Catalina will be the ones to spot it. They are the first line of defense this planet has against existential threats from space—which is why I decided to travel to Arizona to see their work firsthand.
Greg Leonard was right—I should have brought a jacket. The Mount Lemmon Observatory, where the Sky Survey does its work, is perched high in the Santa Catalina Mountains north of Tucson, Arizona. Tucson is a desert town and warm even in March, but the observatory is more than 9,000 feet above sea level. By the time I’d navigated the switchback mountain roads, the sun had vanished and temperatures were plunging, so much so that I was shivering as I walked the last few steps to the telescope. I was warned to point my car away from the observatory when I parked—light is the enemy of asteroid hunting, and even a brief flash of headlights could be enough to spoil an observation. The sky was mostly clear as the evening began, save for a few spindly clouds framed against the materializing stars. I was lucky—had the cloud cover been fuller, I would have spent my time on Mount Lemmon watching Leonard, the observer on duty that night, filing old data.
