What’s the fastest way to stop a city-killer asteroid? Some scientists say: detonate it. Asteroid 2024 YR4, which was initially detected in December 2024, started life as a public space menace. Initial trajectory models put it on a possible collision trajectory with Earth in December 2032, with the highest-ever recorded 3.1% probability of impact and the highest ever for a big near-Earth object. Measuring almost 300 feet long, roughly the height of a 10-story building, YR4 contains enough kinetic energy to destroy a city, the equivalent of hundreds of bombs the size of Hiroshima. But closer observations from the James Webb Space Telescope and telescopes on the ground have since eliminated an Earth collision. The moon, however, now has increasing chances of impact, rising from 1.7% in February 2025 to 4.3% by May.
While a lunar impact would not change the moon’s orbit, its effects would resonate throughout the technological underpinnings of Earth. The collision of that size could send high-speed debris into cislunar space, endangering satellites in geostationary orbit and astronauts on the International Space Station. It has even led scientists, including those at NASA, to suggest a contentious countermeasure: launching nuclear explosive devices to intercept and destroy the asteroid before it hits the moon.
The idea is based on decades of nuclear physics research for space use. As Lawrence Livermore National Laboratory physicist Mary Burkey put it, “We would then detonate the device and either deflect the asteroid, keeping it intact but providing a controlled push away from Earth, or we could disrupt the asteroid, breaking it up into small, fast-moving fragments that would also miss the planet.” Nuclear devices have the greatest energy density per unit mass of any technology developed by humans and are therefore especially well suited for imparting the huge impulse needed in short-warning situations.
In practice, the ideal technique would be a standoff blast detonating the device near but still a short distance from the surface of the asteroid. This boils volatile materials off, causing an immediate outgassing that transfers momentum and changes direction. High-fidelity simulations with LLNL’s Kull radiation-hydrodynamics code simulate X-ray energy deposition into asteroid-similar materials, including porosity, composition, and reradiation effects. Such modeling is essential to predict whether the blast would deflect YR4 without fracturing it into hazardous fragments.
The urgency is compounded by orbital mechanics. YR4’s path is now better constrained thanks to Webb’s infrared imaging, which improved positional accuracy by nearly 20%. But the asteroid’s mass remains unknown, making kinetic impact strategies like NASA’s 2022 Double Asteroid Redirection Test harder to calibrate. DART successfully reduced the orbit of asteroid Dimorphos around Didymos by 33 minutes, well over the mission’s minimum threshold. That achievement depended on Dimorphos’ fragile “rubble pile” composition, which enhanced momentum transfer through ejecta recoil. Without mass information about YR4, the same kinetic strategy could fail or even send the asteroid towards Earth.
Time is of the essence. The opening nuclear mission window is suggested as occurring between 2029 and late 2031 for the ability to launch, transit, and detonate before the anticipated December 2032 encounter. However, the placement of nuclear bombs in space faces insurmountable legal hurdles. The 1963 Limited Test Ban Treaty and the 1967 Outer Space Treaty ban nuclear explosions in outer space outside Earth’s atmosphere and the positioning of nuclear weapons in orbit or on celestial bodies. Even “peaceful” nuclear tests for planetary protection are still categorically excluded. United Nations Security Council approval under Chapter VII of the UN Charter, overriding treaty commitments temporarily for a very specific mission, is one route around these prohibitions.
The stakes are higher than in engineering and law. Planetary defense specialists caution that massive asteroid collisions are a question of “when, not if.” Although YR4’s risk from the moon is insignificant compared to the risk of a collision with Earth, the situation presents a golden moment to perfect high-energy deflection methods before a genuinely existence-cursing object is in the pipeline. As NASA Administrator Bill Nelson phrased it following DART’s achievement, “NASA has proven we are serious as a defender of the planet. This is a watershed moment for planetary defense and all of humanity.”
For YR4, the question is whether the world’s most powerful technology will be unleashed not to save Earth, but to defend its silent partner and in doing so, safeguard the fragile shell of satellites and spacecraft that surround our world.
