Would one error transform a lunar danger into a global catastrophe? That is the query confronting researchers as they consider how to handle asteroid 2024 YR4, a 55–60 meter-sized object now estimated to have a 4% chance of impacting the Moon in December 2032. Though the probability sounds minor, the repercussions of such an impact could have far-reaching ripples well past the lunar surface, endangering spacecraft, astronauts, and valuable orbital hardware.
The risk is the physics of lunar ejecta. Such an impact would catapult regolith dust and rock debris from the surface of the Moon into space at high speeds. Models indicate that this debris will boost micrometeoroid flux in low Earth orbit by as much as 1,000 times over background levels for a period of several days. This spike would likely perforate spacecraft hulls, ruin solar arrays, and undermine spacesuit integrity. Even the International Space Station, were it still in operation by 2032, would be under threat.
The obvious solution appears to be deflecting the asteroid, but there is massive engineering to overcome. The weight of 2024 YR4 is still unknown, estimated as between 51 million kg and more than 711 million kg. This broad estimate is due to a lack of known density figures, which cannot be accurately determined from Earth. The energy needed to change its course even slightly is directly proportional to that mass. A deflection mission based on false assumptions risks sending the asteroid hurtling toward Earth a disastrous miscalculation.
NASA’s 2022 Double Asteroid Redirection Test (DART) proved the kinetic impactor technique by impacting Dimorphos at 24,000 km/h, changing its orbit about Didymos by 30 minutes. But post-mission analysis showed unexpected nuances. As study lead Tony Farnham described, “That additional factor changes the physics we need to consider when planning these types of missions,” in reference to the three times higher momentum of ejected boulders than would be predicted. Such discoveries highlight the challenge of modeling debris behavior paramount to any mission involving high-energy impacts.
With the tight timeline to pre-close approach, reconnaissance missions to hone mass estimates should launch by 2028. Producing and sending such a mission in less than three years would be new ground. Using current spacecraft such as OSIRIS-APEX or Psyche is possible, but it would be at the expense of their originally intended science targets. Janus, a dormant small asteroid mission in storage, might be another possibility, although its potential for high-precision mass determination is uncertain.
If deflection is too dangerous, destruction becomes the backup. A technique is kinetic breakup intentionally colliding a large impactor with the asteroid to break it into small bits less than 10 meters in diameter. The technique has not been tested at scale, but launch windows from April 2030 to April 2032 provide time to design and send such a mission. The catch is preventing fragments themselves from becoming threats to the Moon or Earth.
The more contentious choice is nuclear disruption. Mathematics suggests that a 1 megaton nuclear weapon, exploded at the best “height of burst” over the top of the asteroid, would break apart 2024 YR4 irrespective of its precise mass. This method has never been employed for asteroid deflection, but for space-based nuclear testing like the 1962 Starfish Prime explosion the physics is demonstrated to work. Designing such a mission would involve powerful propulsion systems, perhaps using nuclear thermal or electric propulsion in order to transport the payload within the limited launch window of late 2029 to late 2031.
Such an appeal to nuclear means is as much politics as it is technology. Global agreements for controlling nuclear arms in space, popular opinion, and geopolitical confidence all play into whether or not such a mission would be sanctioned. But time is running out. By 2028, enhanced monitoring will verify if 2024 YR4 is actually on a collision course with the Moon. If that’s the case, planetary defense teams will only have a couple of years to plan, construct, and launch a mission able to prevent a potentially dangerous debris storm within Earth’s orbital community.
