An asteroid large enough to damage a city, devastate a region or affect the entire planet could strike Earth. Such impacts are part of Earth’s geological history, but the larger and more destructive the object, the less frequently these events occur.
What has changed is humanity’s ability to see some of them coming.
NASA, the European Space Agency and observatories around the world continuously search for and track near-Earth objects, refining their trajectories as new observations arrive. NASA has also demonstrated that a spacecraft can deliberately alter an asteroid’s motion, turning planetary defence from a theoretical idea into a tested technology.
The challenge is not simply knowing that asteroids exist. It is finding potentially hazardous objects early enough, determining whether their predicted paths genuinely intersect Earth and, if necessary, deciding whether an intervention is possible.
How Asteroid Threats Are Found
A near-Earth object, or NEO, is an asteroid or comet whose orbit brings it into Earth’s celestial neighbourhood. NASA defines NEOs as objects that pass within about 30 million miles, or 48 million kilometres, of Earth’s orbit.
A potentially hazardous asteroid is more specific: it is generally at least about 140 metres across and has an orbit that can bring it within roughly 7.5 million kilometres of Earth’s orbit. The designation does not mean the asteroid is expected to hit Earth. It identifies objects deserving particular attention.
After an asteroid is discovered, repeated observations allow astronomers to determine its orbit. NASA’s Center for Near Earth Object Studies at the Jet Propulsion Laboratory operates the Sentry impact-monitoring system, which continually examines the current asteroid catalogue for possible future collisions.
An early impact probability can sometimes rise before falling. That is not necessarily evidence that an asteroid has suddenly become more dangerous. A newly discovered object’s orbit initially contains substantial uncertainty. As astronomers collect additional observations, the range of possible trajectories becomes narrower. Earth can temporarily remain inside that shrinking uncertainty region, increasing the calculated probability, before later observations exclude an impact.
This is why a headline reporting a 1%, 2% or even higher probability should not be interpreted as a prediction that an impact will occur.
The Torino Impact Hazard Scale helps communicate that distinction. Adopted by the International Astronomical Union in 1999, it combines an object’s impact probability and potential consequences into a rating from 0 to 10. Level 0 represents effectively no hazard; levels 8 through 10 represent certain collisions of increasing severity. Most newly identified potential impacts remain at or return to the lowest levels as their orbits are refined.
Detection is also improving. NASA’s NEO Surveyor, an infrared space telescope currently targeted for launch no earlier than September 2027, is being built specifically to discover and characterize potentially hazardous asteroids and comets. Its infrared instruments should help identify dark objects that visible-light surveys can have difficulty detecting.
Asteroid Deflection Has Already Been Tested
Finding a threatening object years or decades before impact could create another possibility: changing its orbit rather than waiting for it to arrive.
NASA tested that principle with the Double Asteroid Redirection Test, or DART.
On September 26, 2022, DART deliberately struck Dimorphos, the small moon of asteroid Didymos, at roughly 22,500 kilometres per hour. Neither asteroid threatened Earth. The binary system instead provided scientists with a controlled experiment in which a change in Dimorphos’ orbit could be measured from Earth.
It worked.
Later analysis determined that Dimorphos’ orbital period around Didymos ultimately became about 33 minutes shorter. Material blasted from the asteroid contributed additional momentum, strengthening the effect beyond the spacecraft’s impact alone. Research published in 2026 also found that the collision produced a measurable change of about 0.15 seconds in the binary asteroid system’s roughly 770-day orbit around the Sun.
DART therefore demonstrated the kinetic impactor concept: deliberately hitting an asteroid to alter its trajectory.
For planetary defense, the goal would not normally be to destroy an incoming asteroid. With sufficient warning, even a small velocity change could accumulate over years until the object arrives at Earth’s orbit too early or too late to collide with the planet.
Other proposed techniques include using a spacecraft’s gravity to slowly tug an asteroid onto a different trajectory, sometimes called a gravity tractor. Nuclear devices have also been studied as a potential option for difficult scenarios, particularly when warning time is short or an object is too large for other methods. Which approach could work would depend on the asteroid’s size, composition, structure, orbit and the amount of warning available.
ESA’s Hera spacecraft is now travelling to the Didymos system and is scheduled to rendezvous in late 2026. It will closely examine Dimorphos and the aftermath of DART, helping researchers determine how reliably kinetic impacts could be applied to other asteroids with different physical properties.
How Large Would an Asteroid Have to Be?
Size matters enormously, although diameter alone does not determine the consequences. Impact velocity, composition, density, angle and whether the object explodes in the atmosphere or reaches the surface also matter.
NASA estimates that an object around 10 metres across would typically produce a bright fireball and powerful sonic boom. Objects around 50 metres could cause local devastation. At approximately 140 metres, an impact could produce destruction across a metropolitan area or wider region depending on where it occurred.
At roughly one kilometre, the consequences become potentially global. NASA estimates that an object of this scale could produce a crater around 10 kilometres wide and cause global devastation, potentially threatening civilization. An asteroid around 10 kilometres across, the general scale associated with the Chicxulub impact 66 million years ago, could produce mass-extinction-level consequences.
Those categories should not be treated as precise thresholds. A National Academies assessment similarly notes that objects around 25–30 metres can produce damaging airbursts, roughly 140-metre objects can cause regional or national-scale destruction, and kilometre-scale impacts can produce global effects.
Fortunately, destructive impacts become dramatically rarer as asteroid size increases. NASA estimates that roughly 50-metre objects strike Earth on average about once every 1,000 years, 140-metre objects about once every 20,000 years and kilometre-scale objects roughly once every 700,000 years. These are statistical averages, not schedules for when the next impact will happen.
Planetary defense therefore rests on a straightforward principle: warning time is protection.
The earlier astronomers discover an asteroid, the longer they have to refine its orbit and determine whether Earth is actually in danger. If an impact is eventually confirmed, years of warning could make a relatively small change in the asteroid’s motion sufficient to prevent a collision.
DART showed that humans can alter an asteroid’s trajectory. The remaining challenge is ensuring that a dangerous object is discovered early enough to put that capability to use.
About the Author Brad Socha is the founder of The Universal Record, focused on sourced, factual global reporting. Coverage includes international news, geopolitics, technology, and major developments.
