Planetary defense is the collective effort to find, track, and deflect asteroids or comets that could collide with Earth. It spans telescope surveys that catalog thousands of space rocks, test missions that prove we can nudge them off course, and international networks designed to sound the alarm if a threat materializes. NASA formalized this work in 2016 by creating the Planetary Defense Coordination Office (PDCO), but the broader effort involves space agencies, laboratories, and observatories around the world.
What Counts as a Threat
Space is full of rocky and icy objects, but only a small fraction pose any risk. The ones that matter are called near-Earth objects (NEOs), meaning their orbits bring them relatively close to our planet. An NEO graduates to “potentially hazardous” status when two conditions are met: it is larger than 140 meters (about 460 feet) across, and its projected path brings it within roughly 8 million kilometers (5 million miles) of Earth’s orbit.
That 140-meter threshold isn’t arbitrary. An object that size striking land could devastate an entire region. Smaller rocks can still cause damage, as the 20-meter asteroid that exploded over Chelyabinsk, Russia in 2013 proved, but the 140-meter mark is where consequences scale up dramatically. Finding and tracking these objects is the first pillar of planetary defense, because you can’t deflect what you haven’t spotted.
How Scientists Communicate Risk
When astronomers discover a new asteroid that passes near Earth, they assess its risk using the Torino Impact Hazard Scale, a 0-to-10 rating system designed to give the public and governments a clear sense of the danger. A rating of 0 means no hazard: either the chance of collision is essentially zero or the object is too small to survive the atmosphere. A rating of 1 is routine, flagging a flyby that poses no unusual concern. Most newly discovered objects start at 1 and quickly drop to 0 as astronomers refine their orbital calculations with additional observations.
Ratings of 3 or 4 indicate a 1% or greater chance of a collision that could cause localized or regional destruction. These warrant public attention if the encounter is less than a decade away. Ratings of 5 through 7 represent increasingly serious threats, from regional devastation to potential global catastrophe, where government contingency planning becomes appropriate. An 8, 9, or 10 means a collision is certain, with the number reflecting the scale of expected damage. No known object currently rates above 0.
The DART Mission Proved Deflection Works
In September 2022, NASA’s Double Asteroid Redirection Test (DART) spacecraft deliberately slammed into Dimorphos, a small asteroid moonlet orbiting a larger asteroid called Didymos. The goal was straightforward: hit it and see if the impact changes its orbit enough to matter.
It worked better than expected. Dimorphos’s orbital period shortened by 33 minutes. Scientists had predicted roughly 7 minutes of change if the spacecraft simply transferred its momentum in a head-on collision. The much larger result came from a bonus effect: debris blasted off the asteroid’s surface flew away with enough force to give Dimorphos an additional push, almost like a jet of material acting as a thruster. This “momentum enhancement” is a critical finding because it means kinetic impactors are significantly more effective than a simple billiard-ball calculation would suggest.
The kinetic impactor approach works best against asteroids less than a few hundred meters across, and it requires at least a decade of warning time. The deflection itself is tiny in the moment, but because it alters the asteroid’s orbit permanently, that small nudge compounds over years. By the time the asteroid reaches the point where it would have intersected Earth’s path, it’s in a completely different place.
Other Ways to Move an Asteroid
Kinetic impactors aren’t the only option. A gravity tractor is a spacecraft that parks itself near an asteroid and uses nothing but its own gravitational pull to slowly tug the object off course. It requires no physical contact, which makes it appealing for asteroids with uncertain surface conditions. The tradeoff is time: simulations suggest that with a 10-year warning, a gravity tractor could defend against a 50-meter asteroid, while a 20-year warning would be needed for a 100-meter object. For anything larger, the technique becomes impractical on its own.
For the biggest and most dangerous scenarios, nuclear devices remain the highest-energy option available. A nuclear standoff detonation (exploding the device near the asteroid rather than on its surface) would vaporize a layer of material, creating a rocket-like push that deflects the asteroid while keeping it intact. If the warning time is extremely short or the asteroid is very large, a more aggressive approach could break it into small, fast-moving fragments that all miss Earth. Lawrence Livermore National Laboratory continues to develop detailed simulations of how the energy from a nuclear detonation interacts with an asteroid’s surface and interior, building on insights from DART. Nuclear deflection has the highest energy-to-mass ratio of any technology humans possess, which makes it the leading candidate for scenarios where nothing else would deliver enough force.
Finding What We Haven’t Found Yet
Ground-based telescopes have already cataloged most near-Earth asteroids larger than a kilometer. The bigger challenge is the 140-meter class, where a significant fraction remains undetected. The U.S. Congress mandated that NASA find more than 90% of all NEOs larger than 140 meters, and a dedicated space telescope called NEO Surveyor is being built to close that gap.
NEO Surveyor is scheduled to launch no earlier than September 2027. Unlike optical telescopes that rely on reflected sunlight, it uses two infrared imaging channels to detect the heat that asteroids radiate. This is a major advantage: dark-colored asteroids that are nearly invisible to optical surveys glow brightly in infrared. The telescope will also produce more accurate size measurements, since an asteroid’s brightness in visible light depends on how reflective its surface is, while its heat signature correlates directly with its physical size. Over a five-year survey, NEO Surveyor is designed to find at least two-thirds of potentially hazardous objects 140 meters and larger, with the goal of reaching the 90% congressional mandate when combined with existing ground-based efforts.
International Coordination
An asteroid doesn’t respect national borders, so planetary defense requires global cooperation. Two key bodies handle this. The International Asteroid Warning Network (IAWN) links observatories worldwide to share tracking data, verify discoveries, and coordinate public communication when an object warrants attention. It establishes agreed-upon thresholds for when governments should be notified of an emerging threat and maintains communication plans tailored to different audiences, from scientists to heads of state to the general public.
The Space Mission Planning Advisory Group (SMPAG), also organized through the United Nations Office for Outer Space Affairs, focuses on the response side. If IAWN identifies a credible threat, SMPAG coordinates the mission planning: which agencies could launch a deflection mission, what technologies are available, and how to share the cost and responsibility. Together, these two bodies form the international framework that would activate if a dangerous asteroid were ever confirmed on a collision course.
Planetary defense is unusual among global threats in one important way: it is entirely preventable with enough warning. The physics of deflection are proven, the detection infrastructure is expanding, and the international agreements for coordination are already in place. The remaining challenge is almost entirely one of time, finding hazardous objects early enough to act.