Last updated 4 days ago. Our resources are updated regularly but please keep in mind that links, programs, policies, and contact information do change.
While no known asteroid larger than 140 meters has a significant chance of hitting Earth for at least the next 100 years, the potential consequences of such an event are severe.
This risk has shaped the modern field of planetary defense—all the capabilities needed to detect, warn of, and prevent or mitigate asteroid and comet impacts. Unlike earthquakes or hurricanes, an asteroid impact is potentially the only large-scale natural disaster that can be entirely prevented with enough warning and the right technology.
The United States government, working with international partners, has built a complex strategy involving systematic detection and tracking of potential threats, development of diverse mitigation technologies, and sophisticated interagency response frameworks.
Understanding the Threat from Space
The space around Earth isn’t empty. Millions of rocky and icy bodies from our solar system’s formation 4.6 billion years ago populate nearby regions. While most stay in stable orbits far from Earth, gravitational nudges from planets can send them on paths that cross our own, creating potential hazards.
Scientists use a precise classification system to categorize these objects and prioritize those warranting closest attention.
What Scientists Look For
Near-Earth Objects (NEOs) are any asteroid or comet whose orbit brings it within 1.3 astronomical units (about 121 million miles) of the Sun. This means their path can bring them into Earth’s general orbital neighborhood, roughly within 30 million miles of our planet’s orbit. These objects range from small particles to bodies miles wide.
As of early 2025, astronomers have cataloged over 37,500 NEOs, a number that grows continuously as detection capabilities improve.
Potentially Hazardous Asteroids (PHAs) are a threatening subset within NEOs. An object earns this designation based on two criteria: its orbit must bring it within 4.65 million miles of Earth’s orbit (about 20 times the distance to the Moon), and it must be approximately 140 meters (460 feet) or larger in diameter.
The 140-meter threshold wasn’t chosen arbitrarily. An impact from this size object wouldn’t cause global extinction but could create unprecedented regional devastation, destroying an entire metropolitan area or generating a major tsunami if it struck the ocean. For this reason, PHAs are often called “city killers.”
Of the tens of thousands of known NEOs, about 2,500 have been classified as potentially hazardous, highlighting the scale of the monitoring challenge.
Real-World Impact Evidence
The threat isn’t theoretical—it’s written into Earth’s geological and historical record through two modern events that demonstrate even relatively small cosmic bodies can unleash devastating power.
The Tunguska Event occurred in Siberia on June 30, 1908, when an object estimated between 50 and 60 meters in diameter exploded several miles above ground. The airburst released energy equivalent to 185 Hiroshima bombs, flattening nearly 500 square miles of forest and felling 80 million trees. The explosion was heard 600 miles away, and seismic shockwaves registered as far as England.
More than a century later, on February 15, 2013, a much smaller asteroid about 18 to 20 meters in diameter exploded 14.5 miles above the Russian city of Chelyabinsk. The blast, with the force of about 30 atomic bombs, generated a powerful shockwave that blew out windows across the city and injured more than 1,000 people from flying glass and debris.
The Chelyabinsk event was particularly significant because it demonstrated that objects far smaller than the official 140-meter PHA threshold can cause widespread damage in populated areas, and it occurred with absolutely no warning since the asteroid came from the direction of the sun.
The Global Watch: Finding and Tracking Threats
The foundation of planetary defense strategy is finding potentially hazardous objects before they find us. The sooner a threat is detected, the more time is available to act and the more response options exist.
This global watch transforms faint points of light in the night sky into precisely calculated orbits, allowing rigorous assessment of any potential Earth risk.
The Three-Step Process
Find: The search for NEOs is conducted by a network of ground-based and space-based telescopes. The workhorses are ground-based surveys sponsored by NASA’s Planetary Defense Coordination Office through its Near-Earth Object Observations Program.
Projects like the Catalina Sky Survey in Arizona and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii systematically scan the sky night after night, taking images and using software to identify objects moving against the background of stars. These surveys have discovered the vast majority of all known NEOs.
Space-based assets like the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission use infrared telescopes to detect asteroid heat signatures, making them particularly effective at determining size.
Track: Once a new NEO is discovered, tracking its movement becomes crucial. Astronomers worldwide, including many amateurs supported by organizations like The Planetary Society, perform follow-up observations over days, months, and years. Each new data point helps refine its orbital path around the Sun. Without prompt follow-up, newly discovered NEOs can become “lost,” their orbits so uncertain that astronomers don’t know where to look again.
Characterize: Scientists work to characterize NEO physical properties by analyzing reflected light. This reveals composition (rocky, metallic, or carbon-rich), size and shape, and rotation speed. This information is vital for assessing potential impact damage and planning mitigation missions. For example, kinetic impactor mission effectiveness would depend heavily on whether the target is solid iron or a loosely-held-together “rubble pile.”
Risk Calculation with Sentry
Raw telescope data from around the world flows to the Minor Planet Center, an internationally recognized archive that collects, verifies, and distributes all asteroid and comet observations.
From there, NASA’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory computes high-precision orbits for NEOs and projects their paths into the future to identify close Earth approaches. The centerpiece of this analysis is Sentry, a highly automated system that continuously scans all known NEOs, searching for any possibility of future Earth impact over the next 100 years.
When a newly discovered asteroid enters Sentry, headlines may announce potential impact threats. This is normal part of a process fundamentally about reducing uncertainty. With few observations, calculated orbits are imprecise, creating wide “uncertainty regions” of possible future paths. Some paths may intersect with Earth, and Sentry flags these as “virtual impactors.”
This doesn’t mean impact is likely; it means impact cannot yet be ruled out. As astronomers conduct more observations, orbits are refined, uncertainty regions shrink, and non-threatening trajectories are almost always confirmed. The object is then removed from the Sentry Risk Table.
The Torino Scale is a 0-to-10 scale designed for public communication that combines calculated impact probability with potential kinetic energy to provide a single, color-coded hazard level. A rating of 0 (White) means no hazard, while 1 (Green) indicates routine discovery with extremely unlikely collision chance. No object has ever been rated above 4, and all have eventually been downgraded to 0.
The Palermo Technical Impact Hazard Scale is a more complex logarithmic scale used by astronomers for technical comparison. It compares specific potential impact event risk to average “background risk” of impact from an object of the same size or larger over time until the potential impact.
Planetary Defense Response Options
Once a genuine threat is identified and confirmed, focus shifts from detection to mitigation. Planetary defense involves developing and testing technologies to either deflect an object from its collision course or disrupt it so it no longer poses danger.
There’s no single “best” solution. The choice of mitigation strategy is almost entirely dictated by one critical factor: available warning time.
Kinetic Impactors: The Proven Method
The most mature and well-tested deflection technique is the kinetic impactor. The concept is straightforward: slam a high-mass spacecraft into an asteroid at high velocity to impart a small momentum change, effectively giving it a cosmic nudge. This slight velocity alteration, applied years or decades before predicted impact, is magnified over the asteroid’s long orbital journey, causing it to miss Earth entirely.
This theory was spectacularly tested with NASA’s Double Asteroid Redirection Test (DART) mission. The world’s first full-scale planetary defense technology demonstration targeted Dimorphos, a small 160-meter moonlet orbiting the larger asteroid Didymos. This binary system posed no Earth threat but was ideal for testing because changes in Dimorphos’s orbit around Didymos could be precisely measured by ground-based telescopes.
Launched in November 2021, DART traveled ten months to its target. On September 26, 2022, it successfully collided with Dimorphos at approximately 4.1 miles per second. The results exceeded all expectations. The primary goal was to change Dimorphos’s 11-hour, 55-minute orbital period by at least 73 seconds. Instead, the impact shortened its orbit by a remarkable 33 minutes.
This dramatic success resulted not only from the spacecraft’s momentum but also from powerful recoil effect from tons of rock and dust blasted from the impact site. This debris plume acted like a rocket engine, giving the asteroid much greater push than impact alone. DART unequivocally proved that kinetic impactor technique is viable and effective for asteroid deflection.
Nuclear Devices: The High-Energy Option
For scenarios with much shorter warning times or particularly large, massive NEOs, the gentle nudge of a kinetic impactor may be insufficient. In these high-stakes situations, the most powerful available tool is a nuclear explosive device.
Nuclear planetary defense technology is primarily studied by the Department of Energy’s National Nuclear Security Administration and its three national security laboratories: Los Alamos, Lawrence Livermore, and Sandia. These institutions possess unique nuclear physics expertise and high-performance supercomputing capabilities required to model complex interactions between nuclear detonations and asteroids.
Stand-off Deflection is the preferred approach. A nuclear device is detonated at a calculated distance from the asteroid, not on its surface. Intense X-ray radiation from the explosion instantly vaporizes a layer of the asteroid’s surface material. This vaporized rock expands violently outward, creating massive thrust that pushes the asteroid in the opposite direction, altering its course without shattering it.
Disruption is a last-resort scenario where a nuclear device fragments a large asteroid into many smaller pieces. The goal would be ensuring resulting fragments are small enough to burn up in Earth’s atmosphere or their dispersal pattern is wide enough for most to miss the planet. This is much riskier and less predictable but may be the only choice for very large objects on imminent collision courses.
Gravity Tractors: The Slow and Steady Approach
With decades of warning time, a far more delicate and precise method becomes possible: the gravity tractor. This concept involves sending a massive spacecraft to fly alongside a hazardous asteroid for an extended period—years or even decades. The spacecraft wouldn’t touch the asteroid; instead, it would use engines to maintain fixed position just above the surface. The minuscule but persistent gravitational attraction between spacecraft and asteroid would act as an invisible tow rope, slowly and predictably pulling the asteroid off its impact trajectory.
The primary advantages are precision and control. Because gravity affects the entire asteroid uniformly, the technique doesn’t depend on knowing composition, structure, or spin rate, which are major uncertainties for other methods.
The Enhanced Gravity Tractor (EGT) is a variation where a robotic mission would first land on the asteroid, collect a multi-ton boulder from its surface, and then begin tractoring. The added mass would significantly increase gravitational pull, potentially reducing required deflection time by a factor of 10 to 50.
Future Technologies
Laser Ablation would use powerful lasers to heat spots on asteroid surfaces to thousands of degrees. This intense heat would vaporize rock, creating gas plumes that generate small but continuous thrust like tiny rocket engines. Over long periods, this gentle push could achieve required deflection.
Pulverize-in-Place (PI) is a “terminal defense” strategy for worst-case scenarios with only days or hours of warning. The method proposes using arrays of small, hypervelocity kinetic penetrators to intercept and shatter asteroids just before atmospheric entry. The goal is breaking objects into fragments no larger than about 10 meters in diameter, which would then be dissipated by atmospheric friction and pressure.
Mitigation Strategy Comparison
| Technique | Principle | Technology Readiness | Ideal Warning Time | Key Considerations |
|---|---|---|---|---|
| Kinetic Impactor | Transfers momentum via high-speed collision | Demonstrated in space (DART mission) | Years to decades | Effectiveness depends on target’s mass and composition |
| Nuclear Device | Uses nuclear explosion for deflection or disruption | High-fidelity modeling exists; components mature | Months to years | Most powerful option; significant political implications |
| Gravity Tractor | Uses spacecraft’s gravitational attraction to “tow” asteroid | Conceptual; based on known physics | Decades or more | Very slow but highly precise; doesn’t require contact |
| Laser Ablation | Vaporizes surface material to create thrust | Experimental/Conceptual | Years to decades | Requires significant sustained power source in space |
| Pulverize-in-Place | Shatters asteroid into small fragments before impact | Conceptual; simulations ongoing | Minutes to days | Terminal defense option; relies on atmosphere as shield |
How the U.S. Government Coordinates Defense
Protecting the nation from asteroid impact is a complex challenge requiring seamless integration of scientific research, space technology, emergency management, national security, and international diplomacy. The United States has developed a sophisticated, multi-layered coordination framework extending from the White House to individual agency experts.
Strategic Leadership: The National Space Council
At the apex of U.S. space policy coordination is the National Space Council, reestablished in 2017. This White House policy council is chaired by the Vice President and composed of Cabinet-level secretaries and senior executive branch officials, including the Secretaries of State, Defense, and Commerce, and the NASA Administrator.
The Council’s role in planetary defense isn’t managing day-to-day operations but providing overarching strategic guidance and ensuring policy coherence across the entire U.S. government. It serves as the ultimate forum for resolving high-level interagency issues, securing alignment on national priorities, and ensuring all relevant departments contribute to unified national strategy.
The Action Plan: National Preparedness Strategy
The detailed playbook for U.S. government approach to asteroid hazards is the National Preparedness Strategy and Action Plan for Near-Earth Object Hazards and Planetary Defense. First published in 2018 and updated in 2023, this foundational policy defines national goals and assigns roles to federal agencies.
The strategy outlines six primary goals:
- Enhance NEO detection, tracking, and characterization capabilities
- Improve NEO modeling, prediction, and information integration
- Develop technologies for NEO deflection and disruption missions
- Increase international cooperation on NEO preparedness
- Strengthen and routinely exercise NEO impact emergency procedures
- Improve U.S. governance of planetary defense through enhanced interagency collaboration
Operational Leadership: NASA’s Planetary Defense Coordination Office
While the National Space Council provides strategy and policy, operational leadership rests with NASA’s Planetary Defense Coordination Office (PDCO), established in 2016. With the motto “Hic Servare Diem” (Here to Save the Day), the PDCO acts as central coordinating body for all technical and mission-related planetary defense aspects.
The PDCO’s extensive responsibilities form the core of national capabilities:
Managing Detection: It manages the NEO Observations Program, providing funding to ground-based surveys that discover and track NEOs.
Assessing Risk: It oversees work at JPL’s Center for Near-Earth Object Studies, including the Sentry impact monitoring system performing critical risk calculations.
Issuing Warnings: If a credible threat is identified (typically greater than 1% impact chance), the PDCO provides timely, accurate notification to the White House, Congress, and other federal agencies.
Leading Mitigation Efforts: The PDCO leads development, planning, and execution of planetary defense missions like the successful DART test and upcoming NEO Surveyor telescope.
Interagency Response Roles
In a credible impact threat event, the PDCO wouldn’t act alone but would trigger coordinated response involving multiple federal agencies with clearly defined roles.
Federal Emergency Management Agency (FEMA) is the lead federal agency for domestic disaster preparedness and response. Its role begins where mitigation efforts aren’t feasible or successful. NASA provides FEMA with critical data—predicted impact location, time, and scale of effects—and FEMA coordinates emergency response including public warnings, mass evacuations, and long-term recovery efforts.
Department of Energy/National Nuclear Security Administration provides unique expertise on nuclear mitigation options. The NNSA’s national laboratories use powerful supercomputers to model asteroid composition, nuclear deflection or disruption physics, and potential ground effects of impacts.
Department of Defense provides primarily support and augmentation. The U.S. Space Force and other DoD entities operate vast global networks of powerful radars and telescopes for space domain awareness that can help track and characterize threatening NEOs. In crisis situations, DoD could provide critical logistical support like launch vehicles and global communications networks for mitigation missions.
Department of State handles international diplomacy essential for global threats. State is the lead agency for all international engagement, responsible for notifying foreign governments and international organizations of potential threats. It leads diplomatic efforts to build international consensus for mitigation missions and works to establish legal and policy frameworks for coordinated international response.
U.S. Government Coordination Structure
| Entity | Primary Role | Key Responsibilities |
|---|---|---|
| National Space Council | Top-Level Policy & Strategy | Advise President; synchronize national space policy across sectors |
| NASA (PDCO) | Operational Lead & Coordination | Manage detection/tracking; issue warnings; lead mitigation missions |
| FEMA | Domestic Emergency Response | Plan evacuation and warnings; coordinate post-impact recovery |
| Dept. of Energy (NNSA) | Specialized Technical Support | Provide nuclear mitigation expertise; conduct impact modeling |
| Dept. of Defense | Support & Augmentation | Leverage space tracking assets; provide launch and logistical support |
| Dept. of State | International Diplomacy | Notify foreign governments; lead diplomatic coordination efforts |
International Cooperation Framework
An asteroid on collision course with Earth represents the quintessential global threat. Impact could occur anywhere on the planet, and deflection missions might require international community cooperation and consent. U.S. national strategy is deeply intertwined with international collaboration framework, primarily through two key UN-endorsed organizations.
IAWN: The World’s Asteroid Warning System
The International Asteroid Warning Network (IAWN) was established in 2014 to serve as the planet’s authoritative, science-based “tripwire” for potential impacts. It’s a global collaboration of observatories, space agencies, research institutions, and individual astronomers worldwide.
IAWN’s primary function is acting as internationally recognized clearinghouse for NEO observations and coordinating global observation campaigns. By linking institutions that discover, monitor, and characterize NEOs, IAWN ensures all available data is shared and analyzed collectively.
IAWN establishes criteria and thresholds for credible impact threats—for example, warning of predicted impacts exceeding 1% probability for objects larger than 10 meters. If threats meet these criteria, IAWN provides timely, accurate information to assist governments in analyzing potential consequences and planning responses.
SMPAG: Planning Missions Together
While IAWN focuses on scientific tasks of finding and warning of threats, the Space Mission Planning Advisory Group (SMPAG) handles governmental and technical challenges of responding to them. SMPAG is a forum of representatives from world national space agencies and other governmental bodies capable of contributing to space-based NEO mitigation campaigns.
SMPAG’s mandate is preparing for international response to NEO impact threats. Members work together to lay out frameworks, timelines, and options for initiating and executing space missions to deflect asteroids. This includes promoting international collaboration on deflection technique research and addressing complex technical, operational, and legal questions arising during real missions.
This international architecture represents sophisticated division of labor. IAWN is composed primarily of scientists and astronomers; its role is providing objective, apolitical, credible scientific threat assessment. SMPAG is composed of government space agencies; its role is grappling with political, financial, and logistical realities of mounting complex, high-stakes international space missions.
This separation ensures scientific warning processes are insulated from political response considerations, creating robust global governance model designed to allow humanity to speak with one voice and act as one planet facing cosmic threats.
The Current Situation
Based on current knowledge, no known asteroid larger than 140 meters has a significant chance of hitting Earth for at least the next 100 years. However, this assessment comes with important caveats.
Scientists have likely found and cataloged most of the largest, most dangerous asteroids—those capable of global catastrophe. The ongoing challenge is finding the remaining “city killer” asteroids in the 140-meter to 1-kilometer range. Current estimates suggest we’ve found about 40% of all asteroids in this size category.
The continuous discovery of new NEOs means the risk picture is constantly evolving. Each new discovery must be tracked and analyzed to determine if it poses any future threat. The goal is to push the boundary of “no known threats” further and further into the future, eventually providing centuries of advance warning for any potential impact.
While the immediate future appears safe based on current knowledge, the planetary defense community emphasizes that their work is fundamentally about being prepared. The goal isn’t just to detect threats but to develop and maintain the capabilities needed to respond effectively when a genuine threat is eventually discovered.
The combination of improved detection capabilities, proven deflection technologies like DART, and coordinated international response frameworks means humanity is better prepared than ever before to face an asteroid threat.
Our articles make government information more accessible. Please consult a qualified professional for financial, legal, or health advice specific to your circumstances.