A city-block-sized asteroid named 2024 YR4 is hurtling through space, and it has our Moon in its sights.
Asteroid 2024 YR4 caused quite the stir last year when astronomers calculated a 3% chance of hitting Earth. Since then, the trajectory models have been refined, and it’s no longer believed it has a chance of hitting Earth. But it does have a 4% chance of hitting the Moon in December 2032.
This would be far from the first sizeable asteroid to hit the moon. But it could be problematic for us. The impact could kick up a massive cloud of lunar rock and dust. Some of this debris would reach low-Earth orbit, creating a major hazard for vital satellites and even asteroids working in space.
So, astronomers have an idea. What if we blow it up?
A Gentle Nudge is Off the Table
The idea is described in a yet-to-be-peer-reviewed study spearheaded by Brent Barbee from the Department of Aerospace Engineering at the University of Maryland and a flight Dynamics Engineer at NASA’s Goddard Space Flight Center. The 40-page study essentially reads as a blueprint for dealing with asteroids that can become a problem. Although this particular asteroid is headed for the moon, the same approach could be deployed for an asteroid headed for the Earth, making it an important test.
When an asteroid threatens your planet (or in this case, your planet’s largest satellite) you have two basic choices: push it or pulverize it.
The first option, technically called deflection, is the more delicate approach. The goal is to give the asteroid a relatively small shove, changing its velocity just enough to alter its orbit so it misses its target. Given the enormous distances in space, a small, strategic nudge would be good enough to change its trajectory. NASA’s successful DART mission in 2022 was a proof-of-concept for this, slamming a spacecraft into an asteroid to change its path.
For 2024 YR4, however, deflection is a dangerously risky game. The problem is that we don’t know what the asteroid is made of. Observations from the James Webb Space Telescope put the asteroid at somewhere around 60 meters (197 feet) in diameter. We know it’s a stony asteroid (technically called an S-type asteroid). But its mass is a big question mark. It could be a dense, solid boulder, but it could also be a porous pile of rubble. The paper authors describe a wide range of plausible structures, with the densest possibility being over 13 times more massive than the least dense.
This uncertainty is a deal-breaker for deflection. If your nudge ends up breaking the asteroid into pieces, this could be unpredictable. In this case, you could even end up sending one of the pieces to Earth.
The Other Options
If you can’t gently nudge it, the next option is robust disruption. This involves hitting the asteroid with so much energy that even in its highest mass scenario, it’s blasted into fragments no larger than 10 meters. This energy would scatter the fragments so widely they wouldn’t pose any real threat.
The researchers found multiple launch windows where a mission could attempt this approach, through a heavier and faster version of the DART spacecraft. We’d have time to do this by 2032.
The last option would be the “Hollywood” boom: a nuclear explosive device (NED). It sounds sci-fi, but the authors explain it’s well within our current technological means. The plan doesn’t involve drilling into the asteroid but rather using a “standoff detonation” at a calculated distance from the asteroid’s surface. The radiation would vaporize a thin layer of the surface rock and that vaporized material would shatter the asteroid.
The authors calculate that a 1-megaton device would provide enough power to robustly disrupt even the highest-mass version of the asteroid while allowing for a safe detonation distance of about 85 meters. This explosion would be around 60 times more powerful than the one detonated at Hiroshima.
Eyes on the Prize
The key variable here is information. The more we know about this asteroid, the better decisions researchers can make. At the moment, we don’t know everything that would be useful.
At the same time, it’s remarkable that we’ve learned so much about it, and it’s yet another reason why investing in space missions is so important. Without James Webb, we’d be missing even more information about this asteroid.
This entire exercise is like an important fire drill for humanity. The 4% probability of a lunar impact is high enough to be taken seriously but low enough that it may not happen. Scientists face a difficult decision. Do they recommend starting an expensive spacecraft development program now, based on uncertain odds? Or do they wait? An observation planned with the James Webb Space Telescope in early 2026 could refine the orbit and change the odds dramatically, and by June 2028, ground-based telescopes will be able to see the asteroid again, likely confirming or ruling out the impact for good
But waiting until 2028 would eliminate some of the most effective mission options.
The clock is ticking. The best opportunity to launch a purpose-built reconnaissance spacecraft is in late 2028, enabling it to reach the asteroid by mid-2029. That leaves only about three years for development — a challenging but potentially feasible timeline.
We’re not dinosaurs gazing helplessly at the sky. We have the ability to look up, see the danger, and for the first time in the history of life on Earth, do something about it.
