A History of Planetary Defense

The Silent Sky

For almost all of human history, the sky was a place of serene predictability. The stars wheeled in their fixed patterns, the planets wandered along reliable paths, and the Moon waxed and waned with clockwork precision. Comets were the only exception, appearing suddenly as ephemeral, ghostly visitors. They were widely interpreted not as physical objects, but as omens, portents of plague, war, or the death of kings. The idea that these celestial visitors – or their unseen, rocky cousins – could pose a physical threat to Earth itself wasn’t part of any serious scientific or philosophical discussion.

This perspective began to shift, slowly, with the birth of modern astronomy. When Edmond Halley observed the comet of 1682, he did something novel. Instead of treating it as a one-time apparition, he applied Isaac Newton‘s new laws of gravity. He calculated its orbit and found it strikingly similar to comets seen in 1531 and 1607. He made a bold prediction: it was the same object, and it would return in 1758. When it did, long after his death, Halley’s Comet cemented the idea that comets were physical members of the Solar System, bound by the same laws as Earth.

Even so, the Solar System was still considered a neat, orderly place. The asteroid belt was discovered in the 19th century, but it was seen as a distant collection of rocks orbiting harmlessly between Mars and Jupiter. The notion that some of these rocks might leave the belt and cross Earth’s orbit was a fringe concept. The sky was still, for all intents and purposes, safe.

The first major crack in this peaceful facade didn’t come from an astronomer’s telescope, but from a remote forest in Siberia.

On the morning of June 30, 1908, a massive explosion tore through the sky above the Podkamennaya Tunguska River. The blast was staggering, releasing energy later estimated to be 1,000 times greater than the atomic bomb dropped on Hiroshima. It flattened over 800 square miles of forest, felling some 80 million trees in a radial pattern. Eyewitnesses in trading posts dozens of miles away reported seeing a column of blue light, nearly as bright as the Sun, moving across the sky. They were thrown to the ground by a hot wind, and the sound of the explosion was deafening. Seismic stations across Europe and Asia registered the shockwave. For several nights afterward, the sky over Europe was so bright from atmospheric dust that people could reportedly read newspapers outdoors at midnight.

Because of the region’s remoteness and the political turmoil that would soon engulf Russia, the first scientific expedition, led by Leonid Kulik, didn’t reach the site until 1927. Kulik expected to find an enormous crater, the hallmark of a meteorite impact. He found nothing. There was no hole, no massive fragments of iron or stone. There was only the “great forest,” leveled and scorched at the center.

The Tunguska event remained a mystery for decades, with theories ranging from a volcanic eruption to an exploding pocket of natural gas, and even more exotic ideas involving antimatter or miniature black holes. The correct answer, now widely accepted, was far more disturbing: Earth had been struck by a small asteroid or comet, likely 50 to 60 meters (160-200 feet) across. It hadn’t hit the ground. The object’s immense speed and the pressure of its passage through the atmosphere caused it to disintegrate and explode in a massive air burst several miles above the surface.

Tunguska was the first modern, well-documented proof that Earth is a target in a cosmic shooting gallery. Had that object exploded over a major city like London or Moscow, it would have caused unimaginable death and destruction. Yet, for most of the 20th century, the event was largely treated as a scientific curiosity, a one-in-a-million fluke. The space-faring age began, the Cold War focused scientific attention on rockets and missiles, and the Apollo program took astronauts to the Moon. Through it all, the sky remained, in the public imagination, a benign backdrop.

The Dinosaur Killer

The true paradigm shift, the moment planetary defense began its journey from science fiction to scientific necessity, came not from an impact, but from a thin layer of clay.

In the 1970s, geologist Walter Alvarez was studying rock formations near Gubbio, Italy. He was focused on a specific geological marker: the Cretaceous–Paleogene boundary, or K-Pg boundary. This thin layer of clay, found all over the world, marks a dramatic moment in Earth’s history 66 million years ago. Below this layer, the fossil record is rich with a vast diversity of life, including the dinosaurs. Above it, they are gone. So are more than 75% of all species on the planet. This was the Cretaceous–Paleogene extinction event, and its cause was one of the greatest mysteries in science.

Walter Alvarez was curious about how long it took for this boundary layer to be deposited. He consulted his father, the Nobel Prize-winning physicist Luis Alvarez. They devised a plan. They would measure the concentration of iridium in the clay. Iridium is what’s known as a siderophile, or “iron-loving,” element. When Earth formed, most of its native iridium sank to the core with the iron. It’s exceptionally rare in the Earth’s crust. However, it’s abundant in asteroids and comets. The Alvarezes reasoned that a slow, steady rain of micrometeorites would deposit iridium at a constant rate, allowing them to use it as a clock.

When the results came back from the Lawrence Berkeley National Laboratory, they were stunning. The clay layer didn’t just have a trace amount of iridium; it had 30 times more than the surrounding rock, and in some places, hundreds of times more. The steady rain of micrometeorites couldn’t account for it. This was a sudden, massive spike.

In 1980, the Alvarez team – which also included chemists Frank Asaro and Helen Michel – published what became known as the Alvarez hypothesis. They proposed that the K-Pg extinction was caused by a single, catastrophic event: the impact of an asteroid estimated to be 10 to 15 kilometers (6 to 9 miles) wide.

The hypothesis was met with intense skepticism, even scorn, from many in the geological community, who favored explanations involving massive volcanism or climate change. The Alvarez team had a list of predictions. Such an impact would have thrown up a global dust cloud, blocking sunlight for months or years, killing plants, and collapsing the food chain. The immense energy of the impact would have triggered global firestorms and generated megatsunamis. And, most importantly, it would have left a crater, a “smoking gun,” somewhere on Earth, roughly 150 to 200 kilometers across.

For a decade, the search for the crater was on. Then, in the late 1980s and early 1990s, researchers connected the dots. Geologists working for the Mexican state-owned oil company, Pemex, had detected a massive, semi-circular gravitational anomaly centered on the port town of Chicxulub in the Yucatán Peninsuladecades earlier. It looked like a buried crater, but its significance wasn’t understood. When scientists re-examined samples from the area, they found “shocked quartz” and other markers of a high-velocity impact. The Chicxulub crater, half on land and half under the Gulf of Mexico, was a perfect match in size and, most tellingly, in age. It was dated to precisely 66 million years ago.

The dinosaur-killer was found. The Alvarez hypothesis was confirmed.

The discovery of the Chicxulub impactor changed everything. It wasn’t just a theory; it was a historical fact. The Solar System wasn’t a benign, clockwork mechanism. It was a place where world-ending catastrophes could, and did, happen. The dinosaurs, which had dominated the planet for 160 million years, were wiped out because they didn’t have a space program.

This realization began to filter through the scientific community and, eventually, to policymakers. The key question was no longer “Could it happen?” but “When will it happen again?”

The First Watchers

Even before the Chicxulub crater was confirmed, a few astronomers were already thinking about the danger. The most prominent was Eugene Shoemaker, a geologist who had studied Meteor Crater in Arizona and helped pioneer the field of astrogeology. He had trained the Apollo astronauts, but a medical condition kept him from his dream of walking on the Moon. He dedicated his later career to what he saw as the next great frontier: cataloging the objects that could strike Earth.

In the 1980s, Shoemaker, along with his wife Carolyn Shoemaker and colleague David Levy, began using the Palomar Observatory to hunt for these wandering rocks. They were joined by other small programs, like Spacewatch founded by Tom Gehrels and Robert McMillan at the University of Arizona.

These early searches were painstaking. They used photographic plates or early digital sensors to take multiple images of the same patch of sky, hours apart. They would then manually, or with early computers, look for any “dots” that moved relative to the fixed background stars. These moving dots were the “Near-Earth Objects,” or NEOs.

The terminology began to solidify. An NEO is an asteroid or comet whose orbit brings it within 1.3 astronomical units (AU) of the Sun. One AU is the distance from the Earth to the Sun. A subset of these NEOs were designated Potentially Hazardous Asteroids (PHAs). A PHA is an asteroid that comes within 0.05 AU (about 4.65 million miles) of Earth’s orbit and is large enough – originally defined as over 1 kilometer (0.62 miles) across – to cause a global catastrophe.

In 1990, the United States Congress, spurred by the Alvarez hypothesis and the advocacy of scientists like Shoemaker, directed NASA to study the problem. The result was the 1992 “Spaceguard Survey” report. It outlined the threat in stark terms: a 1-km object would strike Earth with the force of millions of megatons, likely causing a global “impact winter” and ending civilization. The report called for a dedicated, internationally coordinated survey to find 90% of all NEOs larger than 1 kilometer within 25 years.

The term “Spaceguard” stuck, becoming the informal name for all efforts to find these asteroids. But in the early 1990s, it was still a low-budget, low-priority field. Many still viewed the threat as abstract, something to worry about in a thousand or a million years.

Then, in 1994, the Solar System provided a spectacular, terrifying, and perfectly timed wake-up call.

The String of Pearls

In March 1993, the Shoemakers and David Levy were at the Palomar Observatory. They discovered a strange, “squashed” object near Jupiter. It didn’t look like a normal comet. Follow-up observations, including images from the Hubble Space Telescope, revealed something astonishing. It wasn’t a single object; it was a chain of at least 21 fragments, a “string of pearls,” all marching in a line.

Astronomers, including Paul Chodas and Don Yeomans at NASA’s Jet Propulsion Laboratory (JPL), raced to calculate its orbit. The results were mind-boggling. The object, named Comet Shoemaker-Levy 9 (SL9), was in orbit around Jupiter. It had been captured by the giant planet’s gravity on a previous pass, and in July 1992, it had swung so close that Jupiter’s immense tidal forces had ripped it to pieces.

That wasn’t the most shocking part. The calculations showed that the entire train of fragments was on a collision course with Jupiter. The impacts would occur in July 1994.

For the first time in human history, we were about to witness a major planetary impact. Every telescope on Earth, and every spacecraft that could (including Hubble, the Galileo probe on its way to Jupiter, and Voyager 2 in the outer solar system), was pointed at the giant planet.

The show did not disappoint. One by one, from July 16 to July 22, 1994, the fragments of SL9 slammed into Jupiter’s southern hemisphere. Because the impacts happened on the side of Jupiter just out of Earth’s view, scientists had to wait for the planet’s fast rotation to bring the impact sites into view.

The first fragment, “Fragment A,” hit with the force of 200,000 megatons of TNT. When the site rotated into view, astronomers were stunned. It had left a dark, bruised-looking scar in the cloud tops, larger than the planet Earth. The largest fragment, “Fragment G,” hit on July 18. It created a fireball that flashed brighter than the entire planet and left a complex, dark scar over 12,000 km across. The energy released was estimated at 6,000,000 megatons – hundreds of times the combined yield of every nuclear weapon on Earth at the height of the Cold War.

The world watched, live on CNN and the BBC. The abstract threat of a comet impact was suddenly made visceral. Those dark, Earth-sized splotches on Jupiter were a graphic demonstration of what just one of these objects could do. If SL9 had hit Earth instead of Jupiter, it would have been an extinction-level event, full stop.

The impact of Shoemaker-Levy 9 on human consciousness was as powerful as its impact on Jupiter. The debate was over. The threat was real, it was current, and it had to be taken seriously. Funding for planetary defense, while still small, began to flow. The Spaceguard Survey, which had been just a report, became an active, funded mandate.

Charting the Heavens

The late 1990s and 2000s became the golden age of the asteroid survey. The Spaceguard mandate to find 90% of the 1-km NEOs (the “civilization-killers”) was formally adopted by NASA in 1998. This led to the creation of several powerful, semi-automated search programs.

These new surveys were a massive upgrade from the old photographic plates. They used large telescopes with wide fields of view, equipped with sensitive digital CCD cameras. They would image the sky, and powerful software would automatically scan the images for moving objects, flagging candidates for human follow-up.

The major players in this effort included:

• LINEAR (Lincoln Near-Earth Asteroid Research): Run by MIT’s Lincoln Laboratory and funded by the U.S. Air Force and NASA, LINEAR was an early powerhouse, discovering hundreds of thousands of asteroids.
• LONEOS: A survey run by Lowell Observatory in Arizona.
• NEAT: A JPL-run program that used telescopes on Maui and Palomar.
• Catalina Sky Survey (CSS): Based in the Santa Catalina Mountains near Tucson, Arizona, CSS became one of the most productive surveys, finding the majority of new NEOs in the 2000s and 2010s.
• Pan-STARRS (Panoramic Survey Telescope and Rapid Response System): A new-generation system in Hawaii with an enormous 1.8-meter mirror and a 1.4-gigapixel camera, capable of imaging a huge swath of the sky very quickly.

All these discoveries, from all over the world, were sent to a single global clearinghouse: the Minor Planet Center (MPC), operated by the International Astronomical Union (IAU). The MPC would catalog the observations, calculate preliminary orbits, and post new objects for other astronomers to confirm.

Once an orbit was confirmed, the data was passed to specialized centers at JPL (CNEOS) and in Europe (at ESA’s NEOCC) to project the asteroid’s path far into the future, checking for any potential collisions with Earth.

To communicate this risk to the public without causing panic, astronomers developed two scales.

1. The Torino Scale: A simple 0-to-10 scale, like the Richter scale for earthquakes. A “0” means no hazard. A “1” means a routine discovery that’s interesting but almost certain to be downgraded to 0 with more data. A “10” means a certain collision, capable of causing a global catastrophe. To date, no object has ever been rated above a 4.
2. The Palermo Technical Impact Hazard Scale: A more complex, logarithmic scale used by scientists. It compares the calculated risk from a specific object to the “background risk” – the average risk from all objects of that size or larger over time.

By the early 2010s, the original Spaceguard goal was largely met. NASA estimated it had found over 90% of the 1-km objects. The good news: none of them posed any threat for the foreseeable future. The “civilization-killer” problem was essentially solved, for now.

But the job was far from over. Congress, recognizing the success, gave NASA a new, much harder mandate in 2005: Find 90% of NEOs down to 140 meters (about 460 feet) in size by 2020. An object of this size wouldn’t end civilization, but it could wipe out a large city or a small state, causing unprecedented destruction. These “city-killers” are far more numerous – and much, much harder to find – than their 1-km cousins. The 2020 deadline would be missed, as finding these smaller, darker objects proved immensely challenging.

The 2004 Apophis Scare

The process of risk assessment was dramatically illustrated in 2004. On Christmas Eve, the automated systems at JPL’s Sentry and Europe’s NEODyS flagged a newly discovered asteroid, then known only by its provisional designation 2004 MN4. The initial, sparse data suggested a disturbingly high probability of an impact in 2029: 1-in-300, which quickly rose to 1-in-37 as a few more observations came in.

This was the first object to ever reach a “4” on the Torino Scale. The asteroid, soon named 99942 Apophis after the Egyptian god of chaos, was estimated to be about 340 meters wide – a true “city-killer.”

The news made global headlines. For a few days, the world had its first plausible, specific asteroid threat.

But this is where the process of planetary defense worked exactly as it was supposed to. The high-risk calculation was based on very few observations over a short period. As astronomers scrambled to find older, “pre-discovery” images of the asteroid in their archives and new telescopes locked on, the stream of data flowed into the Minor Planet Center. With each new observation, the “error bar” in Apophis’s calculated orbit shrank.

By December 27, just days later, the new data was precise enough to rule out the 2029 impact entirely. The risk level dropped to 0. Apophis would miss, but it would be a spectacular near-miss. On April 13, 2029, Apophis will pass closer to Earth than our own geostationary satellites. It will be easily visible to the naked eye from parts of Europe and Africa, a star-like point of light visibly crawling across the sky.

A secondary risk for 2036 remained for several years, but that too was eventually ruled out, thanks to precise radar astronomy observations from the Arecibo Observatory (before its collapse) and the Goldstone Deep Space Communications Complex. The 2029 close-pass, once a source of fear, is now one of the greatest scientific opportunities of the decade – a chance to study a 340-meter asteroid up close with a fleet of spacecraft.

Apophis was a perfect fire drill for the planetary defense system. It showed the system could find a threat, track it, and, most importantly, refine the data to rule it out.

The View from Space (And a New Warning)

Ground-based telescopes have a fundamental limitation: they can only search at night. They are also hampered by clouds and the blurring effect of the atmosphere. And, most significantly, they have a massive blind spot: they can’t see objects approaching Earth from the direction of the Sun. The Sun’s glare makes them invisible.

To see these hidden objects, and to get a better sense of the entire NEO population, we needed to go to space. The first great asteroid-hunter in space was a repurposed explorer. The Wide-field Infrared Survey Explorer (WISE) was launched in 2009 by NASA to map the entire sky in infrared light.

Infrared is the key to finding asteroids. Many asteroids are “dark,” covered in carbon-rich materials that reflect very little sunlight, making them nearly invisible to optical telescopes. But all asteroids, dark or bright, absorb sunlight and re-radiate it as heat, which glows brightly in infrared wavelengths.

After completing its primary astrophysics mission, WISE was put into hibernation. It was so good at finding asteroids that in 2013, NASA reactivated it with a new name: NEOWISE. Its sole job was to hunt for NEOs.

NEOWISE was a game-changer. It found hundreds of new NEOs, including many dark ones missed by ground surveys. It also provided the first good estimates of size for thousands of others. By combining optical data (how much light it reflects) with infrared data (how much heat it emits), scientists could finally tell the difference between a small, bright, reflective asteroid and a large, dark, non-reflective one – something ground-based optical surveys struggled with. The NEOWISE data helped confirm that the 1-km population was indeed well-cataloged, but that the 140-meter-and-up population was still mostly unknown.

Just as NEOWISE was beginning its new mission, Earth received its second major wake-up call of the modern era.

On the morning of February 15, 2013, an object slammed into the atmosphere over the Russian city of Chelyabinsk. It was a 20-meter (65-foot) asteroid, about the size of a six-story building. It was completely undetected. It had approached from the sunward side of Earth, the blind spot for every ground-based telescope.

The Chelyabinsk meteor exploded in an air burst 18 miles high, unleashing the energy of 30 Hiroshima bombs. The flash was brighter than the Sun, causing painful eyeball burns and sunburns on the frozen morning. A massive shock wave radiated outward, taking two minutes to reach the city. It blew out windows in thousands of buildings, collapsing walls and roofs. Over 1,500 people were injured, not from the impact itself, but from flying shards of glass.

Chelyabinsk was a stark reminder of two things. First, even small objects, far below the “city-killer” 140-meter threshold, are dangerous. Second, we were blind to objects coming from the Sun. It added a new urgency to the field: We needed to find the “city-killers,” but we also needed a way to find the smaller, more numerous “window-breakers.”

From Finding to Fixing

For decades, planetary defense was 99% about one thing: finding the asteroids. Cataloging was the entire job. Because if you have decades or centuries of warning, you have time to develop a solution.

But as the 2010s progressed, the question shifted. What is the solution? If we find one with our name on it, what do we actually do?

Hollywood’s answers, seen in movies like Deep Impact (film) and Armageddon (1998 film), usually involved planting nuclear weapons on the surface. The reality, as studied by scientists and engineers, is more nuanced.

Several mitigation strategies emerged as the most viable.

1. The Kinetic Impactor

This is the “cosmic billiards” approach. It’s a brute-force-but-precise method. You don’t try to destroy the asteroid. Blowing a “rubble pile” asteroid into a thousand pieces just turns one bullet into a shotgun blast, which might be even worse.

Instead, the goal is a simple nudge. You slam a heavy, high-speed spacecraft into the asteroid. The impact itself imparts momentum, pushing the asteroid. But a second, much larger push comes from the ejecta – the plume of rock and dust blasted from the impact site. This debris flies off in the opposite direction, acting like a thruster and multiplying the force of the initial impact.

This method is most effective when you have a long warning time – years or, ideally, decades. A tiny change in velocity (millimeters per second) over a 20-year period translates into a huge change in position, allowing the asteroid to sail harmlessly past Earth. This was, by the mid-2010s, the most mature, technologically ready concept. But it had never been tested.

2. The Gravity Tractor

This is the “slow and steady” approach. It’s elegant and requires no violent collision. You fly a heavy spacecraft to the threatening asteroid and… just park. The spacecraft hovers near the asteroid, using its tiny thrusters to stay in a fixed position just above the surface.

The spacecraft and the asteroid are now gravitationally bound. The spacecraft’s minuscule gravity, which you could overcome with a single breath, pulls on the billion-ton asteroid. And the asteroid’s much larger gravity pulls on the spacecraft.

The spacecraft continuously fires its thrusters, not at the asteroid, but away from it, to keep from falling. This constant, tiny thrust, pulling the spacecraft away, in turn pulls the asteroid with it via their mutual gravitational attraction. It’s a “gravity tow rope.” Over a period of years or decades, this infinitesimal, steady tug can pull an asteroid off its collision course. It’s a slow, precise, and highly controllable method.

3. The Nuclear Option

This is the “panic button.” If a large asteroid (hundreds of meters or more) is discovered only months or a few years before impact, a kinetic impactor or gravity tractor won’t be fast or powerful enough.

The consensus solution is a stand-off nuclear detonation. You don’t drill into the asteroid (a near-impossible task on a spinning, tumbling, low-gravity body). You don’t even hit the surface. You detonate a nuclear device at a precise altitude next to the asteroid.

The blast unleashes a massive, sudden wave of X-rays and neutrons. This intense radiation flash-vaporizes the surface of the asteroid on the side facing the blast. That superheated rock and gas explodes outward, creating a colossal, asymmetric thrust that shoves the asteroid onto a new path. This is by far the most powerful method, the only one capable of moving a large object in a short amount of time.

It is full of geopolitical complications. The 1967 Outer Space Treaty, the foundation of space law, explicitly bans the placement of “weapons of mass destruction” in orbit or on celestial bodies. A planetary defense mission would require an unprecedented international consensus to launch.

The International Response

The Chelyabinsk event galvanized the United Nations (UN) into action. It was clear this was a global problem that required a global, coordinated response. In 2014, the UN endorsed the creation of two key bodies, based on recommendations from the Association of Space Explorers:

• The International Asteroid Warning Network (IAPN): A network of observatories, space agencies, and scientific institutions to share data. If a credible threat is found, IAMPN is responsible for verifying it and issuing a clear, unified warning to the world’s governments.
• The Space Mission Planning Advisory Group (SMPAG): Pronounced “sam-page,” this is the group that does something. It’s a forum for the world’s space-faring nations (NASA, ESA, Roscosmos, JAXA, CNSA, etc.). Their job is to meet, share research, and develop a cooperative plan for a mitigation mission.

At the same time, NASA formalized its own efforts by creating the Planetary Defense Coordination Office(PDCO) in 2016. This office has a clear mandate: “find them, track them, and characterize them.” And if one is found to be a threat, the PDCO is responsible for coordinating the U.S. government’s response, including, if necessary, the development of a mitigation mission.

The pieces were all in place: a global network to find them, an international committee to plan a response, and a suite of well-studied theories on how to do it. But one piece was missing. It was all theory. No one had ever actually tried to move an asteroid.

DART: Humanity’s First Shot

The world’s first planetary defense test mission was born. It was called the Double Asteroid Redirection Test, or DART.

The mission, led by Johns Hopkins Applied Physics Laboratory (APL) for NASA, was a real-world test of the kinetic impactor concept. The team needed a target, but hitting a random asteroid flying solo through space would be problematic. It’s very difficult for Earth-based telescopes to measure the tiny change in an asteroid’s orbit after such a push.

They found the perfect solution: a binary asteroid system. The target was 65803 Didymos, a 780-meter asteroid. But DART’s real target was its tiny moonlet, a 160-meter (530-foot) asteroid named Dimorphos.

Dimorphos circles Didymos in a “nearby,” stable orbit, completing one lap every 11 hours and 55 minutes. This orbital period was the key. It could be measured from Earth with high precision by watching the “winks” in Didymos’s light as Dimorphos passed in front of or behind it (an occultation).

The DART mission was simple: slam the DART spacecraft into Dimorphos. Then, watch from Earth. If the kinetic impactor worked, the orbital period – that 11-hour, 55-minute “clock” – would speed up. The push from the impact would knock Dimorphos into a tighter, faster orbit around Didymos.

The DART spacecraft was essentially a 1,300-pound smart bullet. It was a box about the size of a golf cart, dominated by large solar arrays. Its only scientific instrument was a high-resolution camera called DRACO(Didymos Reconnaissance and Asteroid Camera for Optical navigation). DRACO’s job was twofold: to study the asteroid system on approach, and to act as the “eyes” for the spacecraft’s autonomous navigation system.

That system, called SMART Nav, was the brain. For the last four hours of its life, DART was on its own. It was moving too fast, and the radio signal delay was too long, for any human operator to joystick it. SMART Nav had to identify Didymos, then distinguish the tiny moonlet Dimorphos from its larger parent, and steer the spacecraft directly into its center.

DART launched on November 24, 2021. After a ten-month cruise, on September 26, 2022, it arrived at its target.

The world watched, live, from DART’s own perspective. NASA TV broadcast the DRACO images as they streamed back to Earth, one per second.

At first, Didymos and Dimorphos were just two white pixels. Then, they resolved into distinct objects. The spacecraft’s SMART Nav locked on to the smaller moonlet, Dimorphos. In the final minutes, the images became breathtaking. Dimorphos grew from a pixel to a dot, then to a lumpy, egg-shaped rock. In the final seconds, it filled the entire frame, revealing a surface covered in boulders and rubble. The last image, transmitted just before impact, was a close-up of its rocky surface. Then, the signal went dead.

The DART mission operations center at APL erupted in cheers. The spacecraft had hit its target, a 160-meter rock, 7 million miles from Earth, after a 10-month journey, traveling at 14,000 miles per hour. It was a staggering feat of navigation.

But the real question remained: Did it work? Did it move the asteroid?

A small, Italian-built cubesat called LICIACube, which had piggybacked on DART and been released 15 days earlier, flew past the impact site three minutes later. Its images showed a massive, complex plume of debris and ejecta streaming away from Dimorphos, glowing brightly in the sunlight. The Hubble Space Telescope and James Webb Space Telescope (JWST), along with dozens of ground-based observatories, all pivoted to stare at the event. They saw the Didymos system suddenly brighten by a factor of 10, and then they watched as a vast tail of dust – thousands of miles long – was pushed away from the asteroid by the pressure of sunlight.

Telescopes on Earth began the work of timing the orbit. The minimum requirement for DART to be considered a success was a change of 73 seconds.

Two weeks later, NASA announced the results. DART hadn’t changed the orbit by 73 seconds. It had changed it by 32 minutes.

The original 11-hour, 55-minute orbit was now 11 hours and 23 minutes. It was an overwhelming success, far exceeding expectations. The reason for the massive over-performance was “momentum enhancement,” or beta. The recoil from the massive plume of ejecta had provided a much greater push than the spacecraft’s impact alone. DART had confirmed that Dimorphos was a “rubble pile” asteroid, loosely held together by gravity, and this composition made it easier to move, not harder.

The DART mission was a watershed moment. It moved planetary defense from the realm of theory to practice. Humanity had, for the first time, altered the orbit of a celestial body in a planned, measurable way. We were no longer just passive observers. We had taken our first step to being able to defend our planet.

To get the final piece of the puzzle, the European Space Agency is now preparing the Hera mission. Scheduled to launch in late 2024, Hera will travel to the Didymos system and arrive in 2026. It will perform the “crime scene investigation.” It will map the DART impact crater, precisely measure the mass of Dimorphos for the first time, and study the asteroid’s internal structure and composition. This data will be essential for turning the DART experiment into a repeatable, predictable engineering technique.

Go away

The Future of Planetary Defense

The history of planetary defense is one of slowly dawning awareness, punctuated by sharp, sudden alarms. The Alvarez hypothesis was the theory. Shoemaker-Levy 9 was the demonstration. Chelyabinsk was the warning. And DART was the first active response.

The work is now accelerating. The next great leap in finding them is already being built.

The Vera C. Rubin Observatory, under construction in Chile, is set to revolutionize astronomy. It’s an 8.4-meter telescope with the largest digital camera ever built (3.2 gigapixels). Its mission, the Legacy Survey of Space and Time (LSST), will image the entire available southern sky every few nights.

It’s not just a camera; it’s a motion-detector for the Solar System. It will find millions of new asteroids, including tens of thousands of NEOs in the 140-meter class. It will single-handedly increase the known population of PHAs by a factor of 10. The Vera C. Rubin Observatory will provide the catalog that planetary defense will rely on for decades.

But it still has the Chelyabinsk blind spot: the Sun.

To solve that, NASA is building the NEO Surveyor, a space-based infrared telescope specifically designed to find asteroids. It will be the successor to NEOWISE, but far more powerful. It will be placed at the Sun-Earth L1 Lagrangian point, a stable point in space “upstream” from Earth. From this vantage point, it will be able to look “back” toward Earth’s orbit, finding the objects that ground-based telescopes can’t. NEO Surveyor is the mission designed to find the Chelyabinsk-type objects and to finally complete the congressional mandate of finding the 140-meter “city-killers.”

Other nations are joining the effort. China has announced its own planetary defense mission, a “search-and-deflect” concept that would launch two spacecraft: one to survey a target asteroid, and one to slam into it.

Summary

The story of planetary defense is a story of humanity gradually waking up to its place in the cosmos. We’ve learned that the sky is not a serene and static backdrop, but a dynamic and sometimes violent environment. We’ve learned that the same impacts that sculpted our planet and may have delivered the building blocks of life also brought mass extinction.

For 66 million years, the dinosaurs roamed a planet that they could not understand and could not defend. Their fate was sealed by orbital mechanics, and they never knew what hit them.

Our own story is different. In a geological blink of an eye, a species evolved that could look up, understand the laws of physics, and read the history written in the rocks. We learned of the threat. We built the tools to see it coming. And, in the skies above a binary asteroid 7 million miles away, we proved that we have the power to change our own fate.

The threat remains. There are tens of thousands of “city-killer” asteroids and millions of “Chelyabinsk-sized” objects still undiscovered. The work of finding them is an immense statistical and observational challenge. But the nature of the threat has changed. An asteroid impact is the only major natural disaster that is, in principle, 100% preventable. Unlike an earthquake, a tsunami, or a volcanic eruption, an impact can be predicted decades in advance. And as the DART mission showed, if we have that time, we can do something about it.

The history of planetary defense has just begun, but its first chapter has shown that a combination of global cooperation, scientific ingenuity, and the will to act can protect our world.