Asteroid impact avoidance
Based on Wikipedia: Asteroid impact avoidance
Sixty-six million years ago, a rock about ten kilometers wide—roughly the size of San Francisco—slammed into what is now Mexico's Yucatan Peninsula. The explosion released more energy than a billion nuclear bombs. Forests ignited across entire continents. A blanket of dust and debris choked the atmosphere, blocking sunlight for years. When the skies finally cleared, the dinosaurs were gone.
It will happen again. Not might. Will.
The only question is whether we'll be ready.
The Uncomfortable Math
Here's the thing about asteroid impacts that keeps planetary scientists awake at night: on a long enough timeline, a catastrophic collision is essentially guaranteed. The solar system is littered with rocky debris left over from its formation, and Earth is a big target orbiting through this cosmic shooting gallery.
Most of these objects miss us entirely. Many burn up harmlessly in our atmosphere—we call those shooting stars. But every so often, something large enough to cause real damage finds its way to our doorstep.
In 2013, a relatively small asteroid—about twenty meters across, the length of a bowling lane—exploded over the Russian city of Chelyabinsk. It didn't even hit the ground. The airburst alone shattered windows across the city and injured over 1,500 people. That same day, astronomers were tracking a completely different asteroid that passed within 28,000 kilometers of Earth. They never saw the Chelyabinsk object coming.
The Chelyabinsk impactor was a pebble compared to what's out there. An object fifty meters across—still small by cosmic standards—created Arizona's Barringer Crater, a hole nearly 1.2 kilometers wide that you can visit today. And the dinosaur-killer? That was in a different league entirely.
Finding What's Out There
You cannot dodge what you cannot see. This simple truth has driven decades of increasingly sophisticated efforts to catalog every significant rock that might cross Earth's path.
The systematic hunt began in earnest in the 1990s, when NASA established what became known as the Spaceguard program. The goal was ambitious: find ninety percent of all near-Earth objects larger than one kilometer in diameter. Why one kilometer? Because that's roughly the threshold for global catastrophe. An object that size wouldn't just devastate a region—it could trigger what scientists grimly call an extinction-level event.
Today, a network of telescopes scans the sky every night. The Catalina Sky Survey in Arizona. The Lincoln Near-Earth Asteroid Research program—LINEAR for short—in New Mexico. Pan-STARRS in Hawaii. Spacewatch at Kitt Peak. Each uses different techniques, but they share a common purpose: spotting anything that might be heading our way.
The Minor Planet Center in Cambridge, Massachusetts, has been cataloging asteroid and comet orbits since 1947. Think of it as the world's database of potentially dangerous rocks. Every newly discovered object gets logged, its orbit calculated, its future trajectory projected decades or even centuries into the future.
We've gotten quite good at this. As of the 2020s, astronomers have found the vast majority of kilometer-sized near-Earth objects. The existential threats—the dinosaur-killers—are mostly accounted for. None currently pose a threat.
But here's the catch: smaller objects remain largely uncatalogued. In 2005, Congress directed NASA to find ninety percent of near-Earth objects 140 meters or larger by 2020. That goal has not been met. Objects this size wouldn't end civilization, but they could obliterate a major city or, if they struck the ocean, generate devastating tsunamis.
And the Chelyabinsk impactor? At twenty meters, it was far below even this threshold. We have barely begun to inventory objects that small.
The Comet Problem
Asteroids are relatively cooperative targets. They orbit the sun in well-behaved ellipses, returning to the inner solar system on predictable schedules. Spot one, track it for a while, and you can calculate where it will be for centuries to come.
Comets are different. Long-period comets spend most of their existence in the frigid outer reaches of the solar system, far beyond Neptune, in a region called the Oort Cloud. Some have orbital periods measured in thousands or millions of years. When one of these ancient wanderers falls inward toward the sun, it arrives with almost no warning.
Worse, comets move fast. A long-period comet might hit Earth at fifty or sixty kilometers per second—several times faster than a typical asteroid. Because kinetic energy scales with the square of velocity, this means a comet packs far more destructive punch, size for size.
If a large long-period comet were discovered on a collision course with Earth, we might have only months of warning. Current deflection technologies would be useless. This scenario, thankfully, is statistically unlikely in any given century. But "unlikely" and "impossible" are very different words.
How to Move a Mountain
Suppose you've found an asteroid. Suppose the orbital calculations confirm the worst: in fifteen years, it will hit Earth. What do you do?
The first thing to understand is that you're not trying to destroy the asteroid. Hollywood loves dramatic explosions, but shattering an incoming object is usually a terrible idea. You might turn one large impactor into a shotgun blast of smaller ones, spreading the damage across a wider area. Or the fragments might simply reassemble under their own gravity—research published in 2019 suggests asteroids are far harder to permanently break apart than previously thought.
No, the goal is deflection: nudging the asteroid's orbit just enough that it misses Earth entirely. This sounds impossible—how do you move a mountain through space?—but the physics are actually in your favor.
Here's the key insight: you don't need to stop the asteroid or dramatically alter its course. You just need to change its timing. An asteroid and Earth are both moving through space on their respective orbits. A collision happens only if they arrive at the same point at the same time. Make the asteroid arrive a few minutes early or late, and it passes harmlessly through the space Earth occupied moments ago or will occupy moments hence.
The earlier you act, the less force you need. Astronomers have calculated that an asteroid on a direct collision course can be deflected with a velocity change of just 3.5 centimeters per second per year of warning time. With a decade of lead time, you need to change the asteroid's velocity by only 3.5 millimeters per second—slower than a snail crawls.
This is why detection matters so much. With sufficient warning, even modest technologies become effective. Without warning, even our most powerful rockets are useless.
The Kinetic Impactor: Ramming Speed
The simplest deflection method is also the most intuitive: hit the asteroid with something heavy, going fast.
This is exactly what NASA's Double Asteroid Redirection Test—DART for short—accomplished in September 2022. The mission targeted Dimorphos, a small moonlet orbiting a larger asteroid called Didymos. Neither posed any threat to Earth; they were chosen specifically because the binary system allowed scientists to precisely measure any change in Dimorphos's orbit.
DART was roughly the size of a refrigerator and weighed about 570 kilograms. It hit Dimorphos at roughly 6.6 kilometers per second—about 24,000 kilometers per hour. The impact shortened the moonlet's orbital period around Didymos by 32 minutes, far exceeding expectations.
This was the first time humanity had intentionally changed the trajectory of a celestial object.
The results were encouraging for planetary defense. DART imparted more momentum than its own mass and velocity alone would suggest. This is because the impact kicked up a massive plume of debris—ejected material that flew off into space, providing additional push like a rocket exhaust. The more rubble an asteroid ejects when struck, the more effective a kinetic impactor becomes.
But there's a catch: kinetic impactors require years of warning. You need time to design, build, and launch the spacecraft. You need time for it to reach the asteroid. And you need to hit the asteroid early enough that the tiny velocity change accumulates into a significant orbital shift by the time the collision would have occurred.
For a threatening asteroid discovered only months or a year before impact, kinetic impactors simply won't work.
The Nuclear Option
When you need to deliver enormous energy to a target in space, nuclear explosives are hard to beat. A single nuclear warhead can release millions of times more energy than the same mass of conventional explosives. For very large asteroids, or situations with very short warning times, nuclear devices may be the only viable option.
The physics here are counterintuitive. You don't actually want to blow up the asteroid—remember, fragmentation often makes things worse. Instead, you detonate the weapon near the surface. The intense burst of radiation vaporizes a thin layer of rock on the near side, and this superheated material jets outward like a rocket exhaust, pushing the asteroid in the opposite direction.
This approach, called a nuclear standoff explosion, can deliver far more momentum change than a kinetic impactor. It's the heavy artillery of planetary defense, reserved for worst-case scenarios: very large objects, very short warning times, or both.
Of course, nuclear explosives in space raise obvious political and legal complications. The Outer Space Treaty of 1967 prohibits placing nuclear weapons in orbit or on celestial bodies, though it arguably doesn't forbid a one-time defensive use. Any actual mission would require unprecedented international cooperation.
The Slow Push: Gravity Tractors and Ion Beams
At the opposite extreme from nuclear explosions are methods that work slowly but continuously over months or years.
A gravity tractor is exactly what it sounds like: a spacecraft that hovers near an asteroid and uses nothing more than gravitational attraction to gradually pull it off course. The spacecraft constantly fires its thrusters to maintain position, and the tiny gravitational tug between spacecraft and asteroid accumulates over time.
This sounds almost absurdly gentle, and it is. A gravity tractor would need years or decades to significantly alter an asteroid's orbit. But it has one crucial advantage: it works on any asteroid regardless of composition, spin rate, or surface properties. You don't need to land on it, drill into it, or even know what it's made of. Gravity is universal.
Ion beam deflection uses a different approach. A spacecraft positions itself near the asteroid and fires a stream of ions—electrically charged particles—at its surface. The ions transfer their momentum to the asteroid, slowly accelerating it. This is essentially the same principle as a kinetic impactor, but delivered continuously in tiny increments rather than all at once.
These gentle methods require extraordinary lead times. But for objects discovered decades before a potential impact, they offer precise control and the ability to make fine adjustments as new orbital data comes in.
Painting the Sky
Perhaps the most elegant deflection concept exploits an obscure physical effect named after a Russian engineer who never lived to see space travel.
The Yarkovsky effect occurs because asteroids absorb sunlight on their day side, warm up, and then radiate that heat away as infrared radiation. Because the asteroid rotates, the warmest point is typically on the afternoon side—the part that's been exposed to sunlight longest. This warm region radiates more energy, and that radiation carries momentum. The result is a tiny but continuous thrust, like an invisible hand gently pushing the asteroid.
Over millions of years, this effect measurably alters asteroid orbits. The question is whether we can harness it.
One approach is simply to change the asteroid's reflectivity. Paint it white (or cover it in reflective material), and it absorbs less sunlight and thus radiates less heat. Paint it black (or cover it in dark material), and the opposite occurs. Either way, you change the Yarkovsky force and alter the trajectory.
This is still theoretical. No mission has attempted it. But it represents the kind of creative thinking planetary defense demands—finding ways to move mountains with minimal force.
The Human Factor
In May 2021, NASA and international partners ran a tabletop exercise simulating an asteroid on a collision course with Earth. The scenario: a 35-meter object discovered six months before impact, with initial calculations showing a possible hit somewhere along a swath stretching from Europe to North Africa.
The exercise was designed to fail. Six months was simply not enough time to mount any deflection mission. Participants had to confront an uncomfortable truth: with current technology and infrastructure, short-warning impacts are unsurvivable in the sense that we cannot prevent them. The best we can do is evacuate and prepare.
The simulation drove home several lessons. First, early detection is everything. A threatening asteroid discovered five years before impact presents very different options than one discovered five months out. Second, international coordination is essential. An asteroid doesn't care about national borders, and neither can our response. Third, we need hardware ready to go before we need it—designing a mission from scratch after a threat is discovered wastes precious time.
The political dimensions are thorny. A deflection mission doesn't just make an asteroid miss Earth entirely—it shifts the impact point across the planet's surface until the object passes safely by. During that process, the predicted impact zone might move from one country to another, potentially crossing through the territories of nations that had nothing to do with the mission. How do you get permission to temporarily increase the risk to uninvolved parties, even if the end result benefits everyone?
In 2013, the United Nations established the International Asteroid Warning Network to share detection data and the Space Missions Planning Advisory Group to coordinate potential deflection efforts. These organizations exist precisely to work through such questions before a crisis forces ad hoc decisions.
Where We Stand
The 2016 NASA scientist who warned that Earth was unprepared for an asteroid impact wasn't exaggerating. The same year, the B612 Foundation—a nonprofit dedicated to planetary defense—stated bluntly: "It's one hundred percent certain we'll be hit by a devastating asteroid, but we're not one hundred percent sure when."
Stephen Hawking, in his final book published in 2018, called asteroid collision the biggest threat facing our planet.
These assessments might seem alarmist, but they reflect a simple reality. We've made enormous progress in detection. We've demonstrated that deflection is physically possible. But we haven't actually built the infrastructure for a rapid response. If a dangerous asteroid were discovered tomorrow, we would have to design and build a mission from scratch—a process that takes years.
China plans to launch a deflection test mission to asteroid 2015 XF261 in 2027, with an expected impact in 2029. Europe's Hera mission, launched in 2024, will arrive at the DART impact site to study the results in detail. These missions continue building our knowledge and capabilities.
But capability is not the same as readiness. The United States released a "National Near-Earth Object Preparedness Strategy Action Plan" in 2018, acknowledging that the country remained unprepared. Progress since then has been incremental.
The Stakes
Congressman George E. Brown Jr. of California spent years championing planetary defense in an era when many considered it science fiction. He once said that if we ever successfully deflect an asteroid that would have caused mass extinction, "it will be one of the most important accomplishments in all of human history."
He wasn't wrong. Consider what we're really talking about.
Asteroid defense is the only category of natural disaster that we can prevent entirely—if we choose to invest in the capability. Earthquakes, hurricanes, volcanic eruptions: we can predict them (to varying degrees), prepare for them, respond to them. But we cannot stop them.
Asteroids are different. The physics of deflection is well understood. The technology exists or can be developed. The only barriers are attention, funding, and political will.
This makes planetary defense a uniquely existential test for our species. Can we muster the foresight to prepare for a threat that may not materialize for centuries? Can we cooperate across national boundaries on a truly global challenge? Can we think on timescales longer than election cycles and quarterly reports?
The dinosaurs had no choice. They couldn't track the incoming object, couldn't calculate its trajectory, couldn't design a mission to deflect it. They simply waited, oblivious, for the end.
We have a choice. Whether we make the right one remains to be seen.