5 Critical Facts About 'City Killer' Asteroids Following The 2024 YR4 Near-Miss

As of December 10, 2025, the threat of a devastating asteroid impact remains a top priority for space agencies worldwide, a reality underscored by the recent close encounter with the "city killer" asteroid 2024 YR4. This event, which dominated headlines in early 2025, served as a crucial real-world test for global planetary defense systems, highlighting both the speed of modern detection and the unpredictable nature of Near-Earth Objects (NEOs). The term "city killer" is not sensationalism; it refers to space rocks large enough to wipe out a major metropolitan area, causing death and destruction on a scale not seen since World War II. While the Earth-impact probability for 2024 YR4 was ultimately reduced to zero, the incident has fueled a new urgency in tracking, modeling, and preparing for the next inevitable close call.

The Anatomy of a 'City Killer': Size, Scale, and Shockwaves

The phrase "city killer" is a scientific designation for an asteroid whose impact could cause catastrophic damage over a wide, populated area, typically defined by its size and kinetic energy.

What Defines a 'City Killer' Asteroid?

The size threshold for an object to be considered a city killer is generally smaller than most people think.

• Smallest City Killer: Asteroids as small as 40 to 80 meters (130 to 260 feet) in diameter are considered capable of devastating a city. The 2013 Chelyabinsk meteor, which was only about 20 meters, released an energy equivalent of 30 Hiroshima bombs and caused over 1,600 injuries from its shockwave.
• The PHA Threshold: The most critical category is the Potentially Hazardous Asteroid (PHA), which is defined by NASA's Planetary Defense Coordination Office (PDCO) as an NEO larger than 140 meters (460 feet) that passes within 4.7 million miles of Earth. An impact from a 140-meter object would cause regional, potentially continental-scale devastation.
• Impact Effects: A city-killer impact would generate an enormous blast wave, causing widespread destruction from supersonic winds and massive seismic shaking. A secondary, but equally deadly, threat is the resulting firestorm, which could engulf a wide area around the impact zone.

The 2024 YR4 Near-Miss: A Real-World Test Case

The discovery and subsequent tracking of Asteroid 2024 YR4 in late 2024 and early 2025 provided a dramatic, high-stakes example of modern planetary defense in action.

Discovered by the ATLAS survey on December 27, 2024, the Apollo-class NEO quickly became the focus of intense international scrutiny.

The Probability Rollercoaster

Initial orbital calculations for 2024 YR4 caused alarm when its impact probability for a 2032 collision with Earth rose sharply, peaking at over 3%. The asteroid’s size was estimated to be between 130 and 300 feet, squarely placing it in the "city killer" category.

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Scientists and agencies like the European Space Agency (ESA) and NASA PDCO worked around the clock to refine the trajectory.

Crucially, the James Webb Space Telescope (JWST) was utilized to make precise measurements of the asteroid's size and composition, providing data that helped to dramatically reduce the uncertainty in its orbit. By February 2025, new data confirmed that the Earth-impact probability had dropped to zero, deeming the planet safe from this particular threat.

The Remaining Lunar Threat

While Earth is safe, the story of 2024 YR4 is not over. The refined trajectory models now indicate a small, but notable, 4% chance that the asteroid could strike the Moon in December 2032.

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This lunar impact possibility has led to theoretical discussions among scientists about potential mitigation strategies, including the extreme, last-resort option of using a nuclear device to deflect or destroy the asteroid before a lunar collision. This hypothetical scenario underscores the complexity of planetary defense planning for all celestial bodies in the Earth-Moon system.

Humanity's Defense Grid: The State of Planetary Defense in 2025

The close call with 2024 YR4 highlights the incredible progress made in planetary defense, which is now considered a fundamental pillar of global space policy. The current strategy focuses on three main areas: detection, tracking, and deflection.

Detection and Tracking: The Eyes in the Sky

The ability to detect and track NEOs is rapidly improving. Key entities driving this effort include:

• NASA’s PDCO: The primary US body responsible for coordinating observations and issuing threat warnings. They release monthly updates on near-Earth asteroids.
• New Telescopes: NASA is on track to launch its next-generation asteroid-hunting telescope, a critical tool for cataloging the remaining undiscovered PHAs, with a projected launch in 2027.
• ESA's NEOMIR: The European Space Agency’s Near-Earth Object Mission in the Infrared (NEOMIR) is another crucial development, designed to revolutionize the early detection of asteroids like 2024 YR4 by observing them in infrared light, which is less affected by solar glare.
• Specific NEOs: Beyond 2024 YR4, other objects like 2025 LK and 2025 ME90 are currently being monitored by the Minor Planet Center, demonstrating the continuous, vigilant nature of the threat assessment.

Deflection: The DART and Hera Legacy

The most significant leap in defense technology remains the success of the Double Asteroid Redirection Test (DART) mission.

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In 2022, the DART spacecraft successfully impacted the small moonlet asteroid Dimorphos, which orbits the larger asteroid Didymos. This hypervelocity collision was the first demonstration of the "kinetic impactor" technique, proving that humanity can alter the trajectory of a celestial body.

The long-term analysis of DART’s impact continues in 2025, with scientists studying the ejected dust particles and the precise orbital change.

The next step is the ESA's Hera Mission, which is scheduled to launch and arrive at the Didymos-Dimorphos system to perform a detailed post-impact survey. Hera will measure the resulting crater, analyze the internal structure of Dimorphos, and provide the data needed to turn the DART test into a fully validated, reliable deflection strategy for future threats.

The Future: A Constant, Vigilant Watch

While the immediate threat from 2024 YR4 has passed, the incident serves as a stark reminder that the solar system is a dynamic, unpredictable place. The continuous discovery of new Potentially Hazardous Asteroids (PHAs) and Near-Earth Objects (NEOs) demands sustained investment in global planetary defense infrastructure. The combined efforts of NASA PDCO, the ESA NEOCC, the James Webb Space Telescope (JWST), and upcoming missions like Hera and NEOMIR are creating a robust defense grid. The ultimate goal remains to find, track, and—if necessary—divert every potential "city killer" asteroid long before it becomes a disaster.