Near-Earth Objects: Mapping and Deflecting the Rocks That Could End Civilization
There are hundreds of thousands of asteroids with orbits that bring them near Earth. A significant fraction are large enough to cause regional or global catastrophe. In 2022, NASA proved it could change an asteroid's orbit. The question now is whether we would have enough warning.
Sixty-six million years ago, an asteroid or comet roughly 10 km across struck the present-day Yucatan Peninsula at roughly 20 km/s, releasing energy equivalent to a billion nuclear bombs and triggering the fifth mass extinction. It ended the reign of non-avian dinosaurs and wiped out roughly 75% of all species on Earth. That event was not unique in Earth's history — the planet bears the scars of hundreds of large impacts — and it will not be the last. What is unique about our moment is that for the first time in the planet's 4.5-billion-year history, a species inhabiting it has the technology to detect incoming impactors well in advance, to characterize their trajectories with extraordinary precision, and — as of September 2022 — to demonstrably change an asteroid's orbit.
What happened
The survey of near-Earth objects (NEOs) began in earnest in the early 1990s following the Spaceguard recommendation of a 1992 NASA workshop. The goal: catalog all NEOs larger than 1 km within a decade. That goal was largely achieved by 2010. The current Spaceguard survey, implemented through programs like LINEAR, NEAT, Spacewatch, Pan-STARRS, and the Catalina Sky Survey, has now catalogued over 35,000 NEOs including roughly 2,350 classified as potentially hazardous asteroids (PHAs — those larger than roughly 140 meters that come within 7.5 million km of Earth's orbit).
The 1 km threshold is not arbitrary: an impactor of that size could cause regional to continental devastation and potentially a global climate impact (an "impact winter" from lofted dust). The current assessment is that over 95% of NEOs larger than 1 km have been catalogued and none pose a serious threat in the next century. The 140-meter threshold represents roughly the minimum size to destroy a large city — and the catalogue at that size is only about 40% complete. Smaller objects are largely uncatalogued.
NASA's DART (Double Asteroid Redirection Test) mission launched in November 2021 and impacted Dimorphos — the moonlet of the binary asteroid Didymos — at 6.1 km/s on September 26, 2022. Before impact, Dimorphos orbited Didymos every 11 hours 55 minutes. After DART's impact, follow-up observations confirmed the orbital period had changed to 11 hours 23 minutes — a change 33 times larger than the minimum required to declare the mission a success. DART proved that a kinetic impactor can meaningfully deflect an asteroid.
The European Space Agency's Hera mission, launched in October 2024, is currently en route to Didymos for a detailed post-impact survey of the DART impact site, arriving in 2026. It will characterize the crater, the ejecta plume's contribution to the orbital change, and the internal structure of Dimorphos — data that will refine planetary defense models for future real deflection missions.
Why it matters
Planetary defense is unique among existential risks: unlike pandemic preparedness, climate change, or nuclear war, the asteroid threat is one that we demonstrably have the technology to eliminate, given sufficient warning time. A 10 km impactor — the kind that ended the dinosaurs — is so rare (roughly once per 100 million years) that it is not a near-term concern. The relevant risk is from impactors in the 140-meter to 1 km range, which strike on timescales of thousands to tens of thousands of years and could devastate regions or countries.
The critical variable is warning time. A deflection mission using a kinetic impactor needs to intercept the asteroid years to decades before impact, when a small change in velocity propagates into a large change in orbital position. A 10-year warning allows a relatively modest kinetic impactor to deflect even a kilometer-scale asteroid. A 6-month warning might require a nuclear standoff detonation — technically and politically more complex. A 2-month warning might leave no viable options.
This is why completing the NEO survey — particularly finding the 60% of 140-meter asteroids still uncatalogued — is urgently important. NASA's Near-Earth Object Surveyor (NEO Surveyor) mission, an infrared space telescope launching in 2027, is specifically designed to find NEOs that ground-based surveys miss because they are best detected in infrared light (warm rocks glowing) rather than visible reflected light.
- DART demonstrated that kinetic impactors are an effective and technically straightforward deflection method for asteroids given sufficient warning time.
- The current NEO catalogue is comprehensive for large (1+ km) impactors, eliminating the surprise scenario for the most catastrophic events over the next century.
- Planetary defense is the only existential threat where prevention is fully within technological reach — a well-funded and sustained survey and deflection program eliminates the risk for all but the rarest events.
- The survey for 140-meter and smaller NEOs is incomplete — roughly 60% of the 140m+ population is uncatalogued, meaning a city-destroying impactor could arrive with limited warning.
- Long-period comets (with orbital periods of thousands of years) are essentially undetectable until they enter the inner solar system, potentially giving only years of warning — too little time for most deflection strategies.
- International coordination for an actual deflection mission — agreeing on a response strategy, which nation launches what spacecraft, liability for the deflected asteroid's changed trajectory — has not been tested and could be politically fraught.
How to think about it
Planetary defense is actuarial. The probability of a civilization-ending 10 km impactor in the next thousand years is very low (roughly 0.001%). The probability of a city-destroying 140-meter impact is much higher (roughly 0.1% per century). But the consequence asymmetry justifies treating even low-probability events as priorities: a 0.1% per century chance of an event that kills millions or more of people warrants significant investment in prevention.
The cost of a complete NEO survey and a standing planetary defense capability — a space telescope, a kinetic impactor fleet ready to launch — is in the billions of dollars over decades, comparable to the cost of running the Large Hadron Collider. It would reduce a genuine existential category of risk essentially to zero for any impactor detectable with sufficient warning. On a cost-per-life-saved basis, it is one of the most efficient investments a civilization can make.
The DART success is worth pausing to appreciate. In September 2022, humans changed the orbit of an asteroid for the first time in history. That represents a transition from a species that was helpless against one category of existential risk to one that is not. The same technology, scaled appropriately, could eventually protect Earth from any future impact that gives us enough warning.
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