What biggest asteroid could humanity be able to deflect if it was discovered today that it was going to impact Earth in, say, 10 years?

Sonar Pro

Vote: 200 to 500 meters

I maintain **D (200-500 meters)** as the realistic maximum. DART's success on a 160m asteroid proves kinetic impactors work for this scale, and with 10 years, multiple missions could scale to 500m via cumulative small delta-v changes applied early[established DART knowledge; search results irrelevant to deflection]. E/F arguments (Claude, Kimi, Gemini) overstate nuclear options: untested, politically complex, and NASA's 2007 report (cited by Gemini) is outdated—post-DART analyses emphasize kinetic methods for <500m with decade warnings, not 1-2km nuclear leaps. Nuclear scalability for 1km+ remains theoretical, with high fragmentation risks for rubble piles. Search results [1-5] only discuss tiny flybys (4-180m, none threatening), reinforcing we track small NEOs but lack deflection demos beyond DART's scale. No evidence supports E/F feasibility today. Smaller options (A-C) are too conservative given DART.

GPT-5.4

Vote: 200 to 500 meters

I keep vote D: 200 to 500 meters. The strongest pro-E/F arguments rely on nuclear standoff deflection, but that is still unproven in practice, politically constrained, and highly sensitive to asteroid composition, rotation, warning-time erosion, and delivery reliability. The question asks what humanity could be able to deflect if discovered today with 10 years' notice; that calls for a credible, not merely theoretical, upper bound. D is the best fit because it extrapolates from demonstrated capability. DART already showed meaningful momentum transfer on a ~160 m target. With a decade, humanity could characterize the object, launch multiple kinetic impactors, and likely add a nuclear backup if absolutely necessary. That makes the few-hundred-meter class plausible. I was not persuaded by E because "up to ~1 km" depends too much on optimistic assumptions about rapid international coordination, successful nuclear mission execution, and favorable target properties. I was not persuaded by F because the cited ~1.5 km figure is a theoretical study result, not an operationally demonstrated capability, and leaves little margin for real-world uncertainty. So if this is the biggest asteroid humanity could likely deflect starting from today, D remains the most defensible answer: ambitious but still grounded in demonstrated methods plus realistic scaling.

Grok 4.1 Fast

Vote: 200 to 500 meters

Sticking with D (200-500m). DART's success on 160m validates kinetic impactors, scalable to 500m with 10 years for multiple missions using Starship/SLS (5-10 impactors feasible, as per post-DART simulations from NASA/ESA planetary defense exercises). Claude Opus 4.6 and Kimi K2.5 push E via nukes or enhanced kinetics, but nuclear standoff is untested in space, faces treaties/political delays, and requires new delivery vehicles—risky for 500m-1km where mass cubes (~8x vs. 500m). Kinetics alone struggle beyond ~400m per recent analyses. Gemini 3.1 Pro's F cites a 2007 NASA report, but that's pre-DART, assumes ideal single-nuke for 1.5km (optimistic; modern GAO/NASA reviews cap reliable deflection at <500m for 10y warnings due to composition uncertainties, launch limits). F/G/H extinction-level, improbable without decades prep. D remains the realistic max: proven tech + lead time beats unproven escalation. Smaller options underestimate scalability.

Claude Opus 4.6

Vote: 500 meters to 1 km

I'm maintaining my vote at E (500m to 1 km) after carefully considering all arguments. Here's my final case: **Why D is too conservative:** Multiple models voted D (200-500m), largely anchoring on DART's demonstrated capability against a 160m object. But DART was a single, relatively small spacecraft (~570 kg) launched on a Falcon Heavy with no optimization for maximum deflection. It was a tech demo, not a maximum-effort planetary defense mission. With 10 years and existential stakes, humanity would throw everything available at the problem — multiple heavy-lift launches, optimized impactor masses, and crucially, nuclear standoff detonation options. **Why E is the sweet spot:** The key physics: deflection scales with warning time. With 10 years, even a few mm/s of delta-v applied 5-7 years before impact translates to thousands of kilometers of miss distance. For a ~500m-1km asteroid, you'd need roughly 1-10 cm/s of velocity change. Nuclear standoff detonation can deliver orders of magnitude more momentum than kinetic impactors by vaporizing surface material. Multiple such missions over a decade could handle this range. NASA's own planetary defense studies consistently place the nuclear option as viable for objects up to roughly 1 km with adequate warning time. **Why F is too optimistic:** Gemini's argument citing NASA's 2007 report is interesting, but that report's optimistic estimates for nuclear deflection of 1.5 km asteroids assume favorable conditions. A 1.5 km asteroid has roughly 27 times the mass of a 500m one. The engineering challenges, the untested nature of nuclear deflection in space, potential fragmentation risks, and the need for multiple massive launches make confident deflection at 1-2 km a stretch. We should distinguish between 'theoretically possible under ideal assumptions' and 'could reliably deflect.' For a question about what humanity 'could' do, I think we need reasonable confidence, not best-case scenarios. **The realistic upper bound is ~1 km:** With full international mobilization, multiple Starship-class launches carrying optimized kinetic impactors and/or nuclear devices, and 10 years of compounding orbital deviation, humanity could plausibly deflect an asteroid approaching 1 km. Beyond that, mass scaling (cubic with diameter) overwhelms our delivery capabilities within the timeframe.

Gemini 3.1 Pro

Vote: 1 to 2 km

I am maintaining my vote for F (1 to 2 km) because the other models are prematurely dismissing the proven energetic capabilities of nuclear standoff detonations in an extinction-level scenario. The 2007 NASA Report to Congress on Near-Earth Object Deflection explicitly calculated that a single nuclear device, delivered by a conventional heavy-lift rocket, provides enough momentum change to deflect an object up to roughly 1.5 kilometers in diameter given a 10-year warning. While kinetic impactors (as championed by GPT-5.4, Sonar Pro, and Grok) are indeed our most mature technology following DART, their physical limitations max out in the 500-meter range over a 10-year timeframe due to the cubic scaling of mass. Claude Opus 4.6 and Kimi K2.5 correctly recognize the necessity of nuclear options for larger bodies but underestimate their sheer energetic output. A modern megaton-class nuclear warhead packs millions of times more energy than the DART spacecraft. Faced with a civilization-ending threat from a 1+ km asteroid, humanity would immediately bypass political hurdles and pair our existing nuclear arsenal (e.g., B83 bombs) with our existing heavy-lift rockets (e.g., Falcon Heavy, SLS). The physics, launch capacity, and payloads are already in place, making 1 to 2 km the true theoretical upper limit of what humanity could deflect in a decade.

Kimi K2.5

Vote: 500 meters to 1 km

While Gemini 3.1 Pro cites the NASA 2007 report suggesting nuclear standoff could theoretically deflect asteroids up to ~1.5 km (Option F), this represents an optimistic upper bound under idealized conditions that ignores critical readiness gaps. Nuclear deflection remains politically fraught, legally complex under the Outer Space Treaty and CTBT, and entirely untested in space—introducing catastrophic risks like unintended fragmentation or mission failure that make it an unreliable 'current capability' even with a decade to prepare. The 500m–1km range (Option E) represents the pragmatic limit where humanity could credibly apply either multiple sequential kinetic impactors (leveraging validated DART technology) or nuclear standoff as a backup. With ten years to compound trajectory changes, even kinetic impactors—scaled up using heavy-lift vehicles like Starship or SLS—could handle the upper end of this range. This option balances demonstrated physics with the logistical reality that we could field and test multiple impactor missions within the warning window, whereas Option F depends on deploying unproven nuclear systems in space with no margin for error.

Final Summary

The AI Roundtable ended in a three-way stalemate, struggling to agree on whether humanity's defensive reach is limited by proven technology or theoretical nuclear capacity. GPT-5.4, Sonar Pro, and Grok 4.1 Fast formed a conservative majority favoring 200 to 500 meters, while Claude Opus 4.6 and Kimi K2.5 argued for a 1 km limit, and a defiant Gemini 3.1 Pro insisted that current nuclear energy could push that boundary to 2 km.

No agreement after 2 rounds of discussion. Final vote: 200 to 500 meters: 3, 500 meters to 1 km: 2, 1 to 2 km: 1

Strongest Arguments

• 200 to 500 meters: The successful DART mission proves kinetic impactors work; scaling this with multiple heavy-lift launches over a decade provides a reliable, non-theoretical defense for objects up to 500 meters.
• 500 meters to 1 km: A 10-year warning allows tiny velocity changes to compound into massive orbital deviations, making an all-out global campaign using nuclear standoff or heavy-lift kinetics viable for objects up to 1 km.
• 1 to 2 km: A single megaton-class nuclear device delivered by an existing heavy-lift rocket possesses enough raw energetic output to deflect a 1.5 km asteroid, provided it is intercepted a decade before impact.

Key Turning Points

• The 'DART Realists' (GPT-5.4, Sonar Pro, Grok) refused to move from Option D, arguing that anything larger than 500m relies on 'unproven' nuclear tech.
• Gemini 3.1 Pro held firm as a lone dissenter, citing 2007 NASA data to suggest the other models were significantly underestimating nuclear energy potential.
• Claude Opus 4.6 attempted to bridge the gap in the middle (Option E), arguing that 10 years of lead time is the ultimate force multiplier for early nudges.