Would humanity’s initial nuclear mission to distant space be directed not at defending the Earth, but the moon? A building-sized asteroid known as 2024 YR4, which was found in late 2024, now has a 4% probability of hitting the lunar surface in December of 2032. The Earth itself is safe NASA has eliminated any risk of impact on our planet but the moon’s possible collision suggests a new frontier in planetary defense: safeguarding extraterrestrial infrastructure and orbital assets from indirect damage.
The asteroid is thought to be about 60 meters in diameter, though its mass is impossible to pin down, varying from 51 million to 711 million kilograms. That uncertainty is a central engineering problem. Any deflection mission will need to deliver a specific change in velocity in order to avoid undesirable consequences, including the nightmare possibility of sending YR4 to Earth. As Dr. Paul Wiegert of Western University said, “We now have things worth protecting that are a bit further away from Earth, so our vision is hopefully expanding a little bit to encompass that.”
The implications are not limited to the surface of the moon. A high-energy impact could create a crater about a kilometer wide, spewing up to 100 million kilograms of lunar regolith into space. Models indicate micrometeoroid flux in low-Earth orbit would increase by a factor of 1,000 above background levels for several days, posing a threat to satellites and space stations. Even particles as minuscule as a sugar cube, traveling at bullet speed, could penetrate solar arrays or sensitive instruments. Dan Oltrogge of COMSPOC cautioned, “Space touches almost every aspect of our lives today… a loss of access to and effective use of space presents a serious risk to humanity.”
NASA’s 2022 Double Asteroid Redirection Test (DART) proved that kinetic impact can deflect an asteroid’s path. DART’s half-ton craft hit Dimorphos at 13,645 mph, decreasing its orbit around Didymos by 32 minutes three times the projected amount due to momentum transfer from ejecta. But the same mission highlighted the dangers of fragmentation. UCLA astronomer David Jewitt compared the resulting swarm of boulders to “a cloud of shrapnel expanding from a hand grenade,” each of which had the potential to impart energy equivalent to the Hiroshima bomb if on Earth.
For YR4, researchers hold that kinetic deflection is impractical in the absence of accurate mass information. Reconnaissance missions would be able to refine estimates, yet the first chance to study the asteroid again will be in 2028, with little room for spacecraft design and launch. Re-purposing missions like OSIRIS-APEX or Psyche can be done, but at the cost of abandoning their principal science mission.
The other option is destruction. One idea is a “kinetic disruption mission” employing impactors that will shatter the asteroid into sub-10-meter-sized fragments. Another, more radical, is nuclear interference. The present plan involves two 100-kiloton nuclear devices five to eight times the power of those exploded over Hiroshima and Nagasaki exploded on or near YR4. Estimates indicate that even a 1-megaton weapon, exploded at an ideal “height of burst,” would interfere with the asteroid regardless of density. The window for launch for such a mission would be between late 2029 and late 2031.
Designing a nuclear space mission requires expertise in both the physics of explosions and orbital mechanics. The timing and distance of the detonation from the surface of the asteroid would be paramount to ensure optimal momentum transfer while keeping the potential for multiple dangerous fragments to a minimum. There is no precedence in historical nuclear tests, however, save the 1962 Starfish Prime test, showing that nuclear weapons are capable of operation in space but not for deflection of asteroids.
The political as well as technical choice to seek nuclear disruption is a strong possibility. Treaties regarding the use of nuclear weapons in space would need to be negotiated internationally, and the justification of the mission relies on risk analysis that can be further developed when YR4 returns in 2028. Telescopes like NASA’s NEO Surveyor and ESA’s NEOMIR could reduce blind spots by then, providing planetary defense crews with more time to prepare for anticipated dangers.
Until now, YR4 is a case study in the expanding realm of planetary defense a case that takes humanity’s shield beyond Earth’s orbit, out to the moon and tenuous web of satellites that support modern society.
