Asteroid 2024 YR4, a roughly 180 foot wide rock initially labeled a “city killer,” has focused attention on how quickly a single object could upend life on Earth. Some scientists now argue that a carefully planned deflection could steer such a threat toward the moon instead of our planet, turning a potential catastrophe into a controlled impact. I look at five missions that together form the backbone of that planetary defense playbook.
1. NASA’s DART Mission: Kinetic Impactor Test
NASA’s Double Asteroid Redirection Test, or DART, proved that a spacecraft can physically shove a hazardous object off course. In 2022 the mission slammed into the 160-metre-wide asteroid Dimorphos, a result described in detail through city killer defense analyses, and measurably changed its orbit. Earlier coverage of how NASA launched its “asteroid killer” highlighted that The DART spacecraft was designed as a one-way kinetic impactor, not a science probe.
That proof of concept is central to any plan to redirect a “city killer” like 2024 YR4. If tracking showed a future Earth impact, engineers could adapt the DART playbook, timing a collision so that the asteroid’s new trajectory intersected the moon instead. The stakes are obvious: a lunar impact would scar the surface, but a similar strike on Earth could devastate a metropolitan region and trigger global economic shock.
2. ESA’s Hera Mission: Follow-Up Rendezvous and Characterization
Europe has launched Hera to study the aftermath of DART and refine models of how rubble pile asteroids respond to impacts. Reporting on Europe’s first dedicated planetary defense spacecraft notes that it is meant to help protect Earth from future city-killer strikes by gathering close-up data on Dimorphos and its companion. Hera will map the crater, measure the mass, and track subtle changes in the system’s orbit.
Those measurements are exactly what mission designers would need before attempting to steer an object like asteroid 2024 YR4 toward the moon. By showing how much momentum transfer a given impact actually delivers, Hera will tighten the error bars on any future deflection campaign. For governments weighing billion-dollar interventions, that level of predictive confidence could determine whether they authorize a shot at a controlled lunar impact.
3. NASA’s NEO Surveyor: Enhanced Detection Telescope
NASA’s NEO Surveyor is an infrared space telescope built to find hazardous asteroids long before they appear in backyard telescopes. Its designers emphasize that early discovery is the single most important factor in planetary defense, because it buys time to plan and launch missions. With enough warning, a relatively small nudge can shift an orbit so that a rock like the “city killer” 2024 YR4 misses Earth entirely.
For scenarios where a miss is not guaranteed, that same lead time could enable a more nuanced strategy, such as redirecting the object toward the moon. Without a surveyor constantly scanning the sky, planners might spot a threat too late to do anything but brace for impact. I see NEO Surveyor as the quiet foundation beneath every flashier kinetic or nuclear option, because no mission can target an asteroid that has not yet been found and tracked.
4. JAXA’s Hayabusa2 Extended Mission: Sample Return Expertise
JAXA’s Hayabusa2, famous for returning samples from asteroid Ryugu, is now on an extended mission that showcases precision navigation around small bodies. Its success in touching down, collecting material, and departing safely demonstrates a deep understanding of low-gravity operations. That experience is directly relevant to any attempt to fine-tune the trajectory of a hazardous object, especially one with complex surface geology.
For a potential threat like 2024 YR4, which some scientists suggest could be steered toward the moon instead of Earth, Hayabusa2-style proximity work could precede a deflection. A spacecraft might first survey the asteroid, deploy small landers, or even install thrusters. I view that granular knowledge as crucial, because misjudging an asteroid’s structure could turn a controlled nudge into fragmentation that multiplies the danger.
5. NASA’s OSIRIS-APEX: Nuclear Option Exploration
OSIRIS-APEX, the repurposed OSIRIS-REx spacecraft, is heading to study the near-Earth asteroid Apophis after completing its sample return. While its primary goal is scientific, the mission will test how close operations and surface interactions can subtly alter an asteroid’s motion. In parallel, planetary defense studies have examined more extreme tools, including nuclear devices, as a last resort for objects on a collision course.
Analyses of DART and other concepts argue that nuclear explosives would be reserved for the largest or most urgent threats, potentially including a “city killer” like the 180 foot wide object described in 180 foot estimates. If tracking showed that 2024 YR4 was locked on Earth, a carefully designed nuclear standoff burst could, in theory, redirect it toward the moon instead. I see OSIRIS-APEX as a bridge between pure science and that sobering contingency planning.
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