If asteroid Apophis struck Earth, it would release roughly 375 megatons of energy, equivalent to about 7,500 times the yield of the atomic bomb dropped on Hiroshima. That would make it the most powerful impact event in recorded human history by a wide margin, capable of devastating a region the size of a small country or generating catastrophic ocean waves if it hit water. Apophis is roughly 370 meters across, placing it in an awkward middle range: too small to trigger the kind of mass extinction the dinosaur-killing impactor caused, but far too large for the atmosphere to slow down or break apart before it reached the surface. The good news is that current tracking has ruled out an impact for at least the next century, but the hypothetical remains one of the most studied disaster scenarios in planetary science.
How Much Energy Apophis Carries
Apophis belongs to a class of rocky asteroids called Sq-type, meaning its composition resembles a type of stony meteorite known as LL ordinary chondrite. Spectral observations suggest it is made primarily of silicate minerals, with a total porosity (the fraction of empty space inside the body) estimated at around 40 percent. Based on those physical properties, researchers have estimated its mass at about 20 billion kilograms, which, combined with its expected impact velocity, yields an energy release of approximately 375 megatons of TNT equivalent.1Icarus. Spectral properties and composition of potentially hazardous Asteroid (99942) Apophis To put that in perspective, the largest nuclear weapon ever detonated, the Soviet Tsar Bomba, had a yield of about 50 megatons. Apophis would deliver more than seven times that energy in a single instant.
This places Apophis solidly in the category that planetary scientists call “regionally devastating.” It would not kick up enough debris to threaten global civilization the way a 10-kilometer impactor would, but it would be far beyond anything humans have experienced. The 2013 Chelyabinsk meteor, which shattered windows across a Russian city, was only about 20 meters across and released the energy of roughly 500 kilotons. Apophis is nearly 20 times wider and carries orders of magnitude more energy.
The Blast Wave and Ground Effects
The single most destructive effect of an Apophis-sized impact would be the atmospheric shock wave. Numerical modeling of asteroid entries into Earth’s atmosphere shows that the damage caused by shock waves covers an area roughly ten times larger than any other hazardous effect, including the crater itself or the thermal flash.2Meteoritics & Planetary Science. Atmospheric shock waves after impacts of cosmic bodies up to 1000 m in diameter The same research found a counterintuitive detail: oblique impacts, where the asteroid comes in at an angle rather than straight down, produce a larger damage zone on the surface than vertical ones. Since most impacts arrive at an angle close to 45 degrees, the real-world scenario would likely produce a more widespread blast than the worst-case vertical strike might suggest.
The overpressure from the shock wave would flatten buildings, shatter glass, and knock down trees across hundreds of kilometers from ground zero. Beyond the blast zone, the thermal pulse from the impact plume would ignite fires. Modeling of thermal radiation from impact plumes indicates that for a 300-meter asteroid striking at typical speed and a 45-degree angle, fires could ignite at distances of roughly 250 kilometers from the impact site.3Meteoritics & Planetary Science. Thermal radiation from impact plumes That is a circle of potential fire roughly the width of a large metropolitan area and its surrounding suburbs. A direct hit on or near a major city would be catastrophic; a strike in a remote desert or ocean would still produce effects felt across a wide region.
The crater itself would be significant but not enormous by geological standards. Based on scaling from known impact craters, an Apophis-sized strike on land would carve out a crater perhaps three to five kilometers across, depending on the angle, speed, and composition of the surface rock. Ejected material would blanket the surrounding area, burying terrain near the rim under meters of debris while scattering finer material across hundreds of kilometers.
What If It Hits the Ocean
About 70 percent of Earth’s surface is water, so statistically an Apophis impact is more likely to hit ocean than land. An ocean strike would trade the crater and immediate blast damage for a different threat: tsunamis. Numerical models of asteroid-generated tsunamis show that the initial water displacement creates enormous waves near the impact site, but how dangerous those waves remain at great distances depends heavily on the ocean’s depth and the shape of nearby coastlines.4Geophysical Journal International. Numerical modelling of generation, propagation and run-up of tsunamis caused by oceanic impacts: model strategy and technical solutions
Unlike earthquake-generated tsunamis, which involve long-wavelength waves that travel efficiently across entire ocean basins, impact-generated waves tend to be shorter in wavelength and lose energy more quickly as they spread. The wave height decays roughly in proportion to the inverse of the distance from the impact. This means a strike in the middle of the Pacific would produce devastating waves near the impact site and along the nearest coastlines but would weaken substantially before reaching the far side of the ocean. A strike in a more confined body of water, or close to a populated coastline, would be far worse. If Apophis hit the Atlantic a few hundred kilometers off the U.S. East Coast, for instance, coastal cities could face waves many meters high with very little warning time.
Even an open-ocean impact would have secondary effects. The energy dumped into the water would vaporize enormous quantities of seawater, injecting steam and salt aerosols into the upper atmosphere. This vapor plume would contribute to the atmospheric disruption discussed below, though the scale would be smaller than what a land impact’s dust cloud would produce.
Would It Change the Climate
This is where the science gets genuinely uncertain for an asteroid of Apophis’s size. The question is whether an impact at 375 megatons would loft enough fine dust and aerosols into the stratosphere to block sunlight and cool the planet, or whether most of the debris would settle quickly without producing lasting climate effects.
A 2025 study simulating the collision of a medium-sized asteroid (similar in scale to the near-Earth asteroid Bennu, which is somewhat larger than Apophis at about 500 meters) found dramatic results when large quantities of dust reached the stratosphere. Injecting up to 400 million tons of dust into the stratosphere in their simulations produced a global temperature drop of about 4°C and a 15 percent decrease in global precipitation, along with serious disruptions to photosynthesis on both land and sea.5PubMed Central. Climatic and ecological responses to Bennu-type asteroid collisions A temperature drop of that magnitude, even if temporary, would shorten growing seasons worldwide, threaten crops already in the ground, and stress ecosystems from coral reefs to boreal forests.
But an earlier and widely cited review of impact hazards reached a more cautious conclusion for this energy range. That analysis found that in the nominal case, an impact of this scale does not inject enough fine dust (specifically, particles smaller than a micrometer, which are the ones that stay aloft longest) into the stratosphere to produce major global cooling. The caveat was significant, though: if a larger fraction of pulverized rock than expected reached the stratosphere, dust-driven cooling would become an important factor.6Reviews of Geophysics. Environmental perturbations caused by the impacts of asteroids and comets
The disagreement comes down to assumptions about how efficiently impact energy converts rock and soil into fine particles and how high those particles get lofted. Apophis, being somewhat smaller than Bennu, would likely produce less stratospheric dust than the worst-case scenarios modeled for the larger asteroid. But whether the result is a brief, regional haze or a months-long global dimming depends on variables that remain poorly constrained, including the surface composition at the impact site (ocean sediment versus granite versus desert sand), the angle of impact, and the altitude at which the fireball stabilizes. The honest answer is that a Apophis-scale impact sits right at the boundary where climate effects could range from locally severe to globally significant, and no one can say with confidence which side of that line the real outcome would land on.
Regional Versus Global Catastrophe
To be clear about what Apophis would not do: it would not end civilization, trigger mass extinction, or render the planet uninhabitable. The Chicxulub impactor that killed the non-avian dinosaurs was roughly 10 to 12 kilometers across and released energy measured in the tens of billions of megatons. Apophis, at 375 megatons, is about a hundred million times less energetic. There would be no years-long “impact winter,” no collapse of the global food chain, and no threat to the survival of the human species as a whole.
What it would do is devastate whatever region it hit and stress global systems for months to years afterward. The immediate destruction zone, combining the crater, blast wave, thermal pulse, and fires, could cover tens of thousands of square kilometers. If the impact hit a populated area, casualties could reach into the millions. Even a remote strike would disrupt global supply chains, trigger financial instability, and produce a refugee crisis if it occurred near an inhabited region. An ocean strike near a major population center could cause mass casualties from tsunamis alone, with coastal flooding extending inland for kilometers depending on local topography.
The secondary effects, including disrupted agriculture from temporary cooling or altered rainfall, damaged infrastructure from seismic shaking, and atmospheric contamination from vaporized rock and soil, could affect communities thousands of kilometers from the impact site. How long those effects lasted would depend heavily on the specifics of the impact location and geometry.
Could We See It Coming in Time
One of the few reassuring aspects of the Apophis scenario is that we already know exactly where it is. Apophis was discovered in 2004 and has been tracked extensively ever since. Its orbit is now known with enough precision to rule out any impact for at least the next century. But the broader question, whether we could detect a similar-sized asteroid on a collision course with enough lead time to do something about it, is worth examining.
Current ground-based survey systems like ATLAS (Asteroid Terrestrial-impact Last Alert System) are designed to spot incoming objects, but their reach for Apophis-sized bodies is limited. ATLAS can detect a 140-meter asteroid roughly three weeks before impact and a 50-meter one about a week before arrival.7Publications of the Astronomical Society of the Pacific. An Early Warning System for Asteroid Impact Three weeks is enough time to evacuate a city, but it is nowhere near enough to mount a deflection mission. For an object the size of Apophis, ATLAS and similar systems would likely provide more lead time because it would be brighter and easier to spot at greater distances, but a “new” Apophis discovered on a short-notice collision course would still be a nightmare scenario for planetary defense planners.
The upcoming NEO Surveyor space telescope, designed specifically to hunt for potentially hazardous asteroids from an orbit around the Sun, aims to close this gap. Modeling of the mission’s expected performance suggests that a five-year survey would identify about 87 percent of potential impacts at or above the 100-megaton energy level, with a ten-year survey raising that to about 94 percent.8The Planetary Science Journal. Asteroid Impact Hazard Warning from the Near-Earth Object Surveyor Mission Since Apophis is well above that energy threshold, a fully operational NEO Surveyor would almost certainly have caught it years or decades before a potential impact, giving humanity time to respond.
Deflection Options If We Had Warning
With enough lead time, deflecting an Apophis-sized asteroid is not science fiction. NASA’s DART mission in 2022 provided the first real-world proof that a spacecraft slamming into an asteroid at high speed can meaningfully change its orbit. The mission hit the small asteroid Dimorphos and reduced its orbital speed by about 2.7 millimeters per second. Crucially, the escaping debris blasted off the surface by the collision transferred substantially more momentum to the asteroid than the spacecraft itself carried. Researchers calculated that the momentum enhancement factor ranged between roughly 2.2 and 4.9, meaning the debris did two to nearly five times as much pushing as the spacecraft alone.9PubMed Central. Momentum transfer from the DART mission kinetic impact on asteroid Dimorphos
That result was encouraging, though Dimorphos is only about 160 meters across and far less massive than Apophis. Deflecting Apophis with a kinetic impactor would require a much larger spacecraft, multiple impactors, or a much longer lead time so that a small velocity change could accumulate into a large enough orbital shift. Pre-impact modeling for the DART mission had predicted momentum enhancement factors broadly in the range of 1 to 5 depending on the target material, and the actual result fell within that window, which gives some confidence that the physics is understood well enough to plan a real deflection.10The Planetary Science Journal. Effects of Impact and Target Parameters on the Results of a Kinetic Impactor: Predictions for the Double Asteroid Redirection Test (DART) Mission
For shorter warning times or larger asteroids, a nuclear device detonated near the surface is the most powerful tool in the theoretical toolkit. Rather than trying to blow the asteroid apart (which could create a dangerous debris cloud), a standoff nuclear detonation would vaporize a thin layer of the asteroid’s surface, and the expanding vapor would act like a rocket exhaust, pushing the body off course. Ongoing simulation work has refined models for how X-ray energy from a nuclear detonation deposits into asteroid material, with recent codes matching detailed radiation-hydrodynamics calculations to within about 10 percent for the resulting blowoff momentum.11The Planetary Science Journal. X-Ray Energy Deposition Model for Simulating Asteroid Response to a Nuclear Planetary Defense Mitigation Mission Additional simulations using smoothed-particle hydrodynamics have confirmed that different modeling approaches agree closely on the expected momentum transfer, with deviations of less than one percent in benchmark scenarios.12The Planetary Science Journal. Nuclear Mitigation of Hypothetical Asteroid Threats in Smoothed Particle Hydrodynamics No nuclear deflection has ever been tested on an actual asteroid, but the modeling consensus is strong enough that this approach is considered a realistic backup for scenarios where kinetic impactors alone would not provide sufficient push.
A gentler option is the gravity tractor, a concept in which a spacecraft flies alongside the asteroid for months or years, using its own tiny gravitational pull to slowly tug the asteroid off course. The beauty of this approach is that it does not care about the asteroid’s structure, surface properties, or spin rate, and it cannot accidentally fragment the target.13PubMed. Gravitational tractor for towing asteroids The drawback is that it is extremely slow and would only work with decades of advance warning for an object as massive as Apophis.
The 2029 Flyby and the Keyhole Problem
On April 13, 2029, Apophis will pass closer to Earth than the ring of geostationary communications satellites, coming within about 31,000 kilometers of the surface. This will be close enough to see with the naked eye from parts of Europe, Africa, and western Asia. The flyby itself poses zero risk of impact, but it introduces a subtle concern that planetary scientists take seriously: gravitational keyholes.
A keyhole is a narrow corridor in space near Earth. If an asteroid passes through a keyhole during a close flyby, Earth’s gravity bends its orbit just enough to set it up for an impact on a future pass, often years or decades later. For Apophis, the 2029 flyby is well characterized enough that scientists know it will not pass through any of the dangerous keyholes. But researchers have studied how sensitive the 2029 trajectory is to tiny perturbations. They found that a velocity change as small as 0.0003 meters per second, applied before the flyby, could shift Apophis’s position on the target plane by about 200 kilometers, enough to nudge it into the nearest keyholes. A somewhat larger perturbation of about 0.003 meters per second could shift it 1,500 kilometers, reaching a more distant cluster of keyholes associated with a potential 2036 return.14PubMed Central. On the sensitivity of Apophis’ 2029 Earth approach to small asteroid impacts
What could cause such a tiny velocity change? The researchers considered collisions with small space rocks, the kind of random event that happens occasionally in the asteroid belt. They concluded that any particular collision is equally likely to push Apophis away from a keyhole as toward one, so there is no systematic bias toward danger. Still, the finding underscores why continued tracking matters. Even after the 2029 flyby, astronomers will need to refine Apophis’s orbit to confirm that it did not pick up any unexpected nudges from the encounter.
Who Pays If a Deflection Goes Wrong
One of the stranger corners of the Apophis question involves international law. If a country or coalition launched a deflection mission and something went wrong, say the nuclear device fragmented the asteroid into multiple pieces that struck a different country, or a kinetic impactor nudged the impact point from an ocean into a populated area, who would be liable?
Existing space law offers partial and ambiguous answers. The 1972 Liability Convention establishes that launching states bear absolute liability for damage caused by their space objects on Earth’s surface. But whether a planetary defense mission gone awry would fall under this framework is genuinely unclear. The convention was written to cover accidents like a satellite crashing, not deliberate missions intended to protect the planet that inadvertently cause harm. Legal scholars have noted that the convention is victim-oriented, allowing any affected state to sue any state involved in the launch, but its application to a planetary defense scenario remains contested.15Acta Astronautica. Enabling planetary defense: Science, law, ethics
There is also no international body with the authority to order a deflection mission, approve its methods, or decide which countries should bear the costs and risks. The United Nations Committee on the Peaceful Uses of Outer Space has discussed planetary defense, and groups like the International Asteroid Warning Network exist to coordinate information sharing, but none of these have binding authority over who launches what. In a real emergency, the countries with the technical capability to mount a mission (primarily the United States, and potentially China, the European Space Agency member states, and Russia) would likely act first and negotiate the legal implications later. The governance gap is real, and no one has closed it.
How Risk Communication Shapes the Response
Even if Apophis never hits Earth, the way scientists and media communicate about impact risks has real consequences. When Apophis was first discovered in 2004, initial calculations gave it about a 2.7 percent chance of hitting Earth in 2029, which was briefly the highest impact probability ever assigned to a known asteroid. Headlines were alarming. Further observations quickly lowered the probability to near zero, but the episode highlighted how easily impact risk can be misunderstood by the public. Researchers in the field have argued that responsible communication about asteroid hazards requires careful attention to framing, because the statistical risk is real but falls so far outside everyday experience that it is easy for people to either dismiss it entirely or to catastrophize.15Acta Astronautica. Enabling planetary defense: Science, law, ethics A public that panics is as dangerous as a public that ignores the threat, and the line between informed concern and irrational fear is thin when you are talking about rocks from space.
Asteroid impacts are unique among natural disasters in one respect: they are, in principle, entirely preventable with current or near-future technology, as long as we find the threat early enough. That combination of extreme rarity, extreme consequence, and genuine preventability makes the communication challenge unlike anything else in disaster preparedness. The Apophis scenario, even as a hypothetical, serves as one of the most useful test cases the field has ever had.