10 “City‑Killer” Asteroids That Scientists Are Actively Tracking

Every year, scientists track thousands of asteroids that pass near Earth. Some of these are big enough to destroy an entire city if they ever hit—earning them the nickname “city-killers.” While none of the asteroids on this list are expected to strike Earth anytime soon, their size and close paths make them important to watch.

From long-known threats to newly discovered objects, here are 10 city-killer asteroids that scientists are keeping a close eye on right now.

Related: Top 10 Disturbing Secrets About Space Agencies

10 2024 YR4 — A Potential Lunar & Earth Threat

Asteroid 2024 YR4 has been making headlines not because it’s targeting Earth directly—but because it might strike the Moon. Discovered in late 2024, the asteroid is estimated to be between 174 and 220 feet (53 and 67 meters) wide, roughly the size of a Boeing 747. That’s large enough to destroy a city if it hit Earth—similar to the 2013 Chelyabinsk meteor, which injured over 1,500 people despite exploding in midair. Initially, scientists were concerned that the asteroid had a 3.1% chance of impacting Earth in 2032. Fortunately, that probability has since dropped to nearly zero.

But what makes YR4 fascinating—and still dangerous—is that it now carries a 4.3% chance of hitting the Moon on December 22, 2032. If it does, it could create a crater over a kilometer wide, visible through many backyard telescopes and even from lunar orbiters. Scientists hope that if an impact does occur, it could provide valuable scientific data. NASA and the European Space Agency (ESA) are already considering how to use the event to study impact mechanics, lunar geology, and ejecta dispersal patterns in real time.

The asteroid’s trajectory is still being refined, and it remains under active surveillance using radar, optical telescopes, and potentially even space-based assets, such as the James Webb Space Telescope. The fact that it was identified as a potential threat so soon after discovery underscores how many objects like it remain undetected until they’re uncomfortably close. While Earth is safe for now, YR4 is a sobering reminder that cosmic hazards often come with very little warning—and that even our Moon isn’t immune.^[1]

9 99942 Apophis — The Return of the Aten Threat

Asteroid 99942 Apophis was once the poster child for planetary danger. Discovered in 2004, this massive Aten-class asteroid—estimated at 1,082 and 1,476 feet (330 to 450 meters) wide—initially had a 2.7% chance of striking Earth in 2029, sending scientists, news outlets, and doomsday theorists into a frenzy. It was the first object to briefly reach a Level 4 rating on the Torino Scale, a rare and alarming metric indicating a significant threat of regional destruction. That might sound mild, but Apophis is big enough to devastate an entire country.

Thankfully, additional observations ruled out a 2029 collision. Instead, Apophis is expected to swing just 19,644 miles (31,600 km) from Earth—closer than some satellites. That close approach will provide scientists with an unprecedented opportunity to study the asteroid’s structure, spin, and potential for future deflection. Already, NASA and other space agencies have been planning flyby missions, including ESA’s proposed RAMSES spacecraft, which may launch to observe Apophis during or after its close encounter.

Despite being “cleared” for the 2029 approach, Apophis remains under intense surveillance. Some astronomers worry that gravitational interactions with Earth could slightly alter its orbit, setting up a dangerous resonance for later in the 21st century. Although the current data suggests no impact through at least the next 100 years, the asteroid’s size, orbital behavior, and eerie namesake (Apophis is the Egyptian god of chaos) make it a perennial favorite in both planetary defense plans and Hollywood screenplays. Its passage will be visible with the naked eye—and a cosmic close call we’ll all be watching.^[2]

8 101955 Bennu — Long-Term Hazard

Asteroid 101955 Bennu is among the best-studied space rocks in the solar system, thanks to NASA’s OSIRIS-REx mission, which collected and returned a sample from its surface in 2023. Bennu is roughly 490 meters wide—large enough to destroy a metropolitan area—and has a 1-in-2,700 chance of hitting Earth in the year 2182, according to updated NASA models. That may sound remote, but it gives Bennu one of the highest known impact probabilities for a large object.

Bennu orbits the Sun every 1.2 years and makes close approaches to Earth roughly every six years. Scientists are particularly interested in how solar radiation affects Bennu’s path through a phenomenon called the Yarkovsky effect—a tiny but cumulative push caused by heat emissions from its surface. This unpredictable force makes long-term trajectory modeling difficult, which is part of why Bennu’s exact path in the late 2100s remains under close analysis. The good news is that we have over a century to prepare, and thanks to the OSIRIS-REx data, more tools than ever to predict and prevent impact.

Even if Bennu never strikes Earth, it has already reshaped our understanding of asteroid dynamics, surface composition, and what a real-world planetary defense mission might look like. OSIRIS-REx revealed that Bennu’s surface behaves like loose rubble rather than solid rock, which could complicate deflection attempts if the asteroid ever moves into a more threatening trajectory. For now, it remains a case study in both cosmic risk and interplanetary preparedness.^[3]

7 (144898) 2004 VD17 — A 2102 Concern

Asteroid 2004 VD17 once held the title of the most dangerous known asteroid for a brief period in the early 2000s. With a diameter of around 580 meters, it is more than large enough to cause widespread destruction. Early projections placed a 1-in-1,320 chance of impact in the year 2102, which caused it to be temporarily assigned a Torino Scale rating of 2, a rare status typically reserved for objects that require serious tracking and impact assessments.

As more data came in, astronomers revised the probability downwards, eventually eliminating the 2102 impact scenario. However, that hasn’t taken the object off watchlists. Because of its highly elliptical orbit, 2004 VD17 frequently crosses Earth’s path. Any future perturbations—from gravitational tugs by planets or solar radiation pressure—could still nudge it into a collision course over the next few centuries. It’s a classic example of how long-term monitoring is essential, even for asteroids that temporarily fall off the “risk radar.”

What makes VD17 stand out is its combination of size, speed, and prior risk level. While it’s no longer considered an emergency-level threat, its history as a top concern proves how quickly scientific projections can change. As asteroid detection systems like NASA’s NEO Surveyor and ESA’s Flyeye telescope come online, objects like VD17 will be reevaluated in greater detail, potentially resurfacing as future hazards—or reassuring blips in our increasingly crowded cosmic neighborhood.^[4]

6 (12538) 1998 OH — Approaching in 2042 & 2132

Asteroid (12538) 1998 OH is an Apollo-class near-Earth object estimated to be between 328 and 656 feet (100 and 200 meters) in diameter—large enough to obliterate a city and cause regional devastation if it ever made landfall. First observed in 1998, this asteroid has drawn attention due to two upcoming close approaches to Earth, expected in 2042 and 2132, with distances narrowing to just 0.03 astronomical units (about 2.8 million miles or 4.5 million kilometers).

Though that sounds far, it places the asteroid within the category of potentially hazardous objects (PHOs)—those that approach Earth within 0.05 AU and are large enough to cause serious damage. Its elliptical orbit crosses Earth’s path every few years, and while current data doesn’t predict an impact, the asteroid’s trajectory remains sensitive to gravitational nudges and solar radiation effects. These subtle forces can alter orbits over time, especially over multiple decades or centuries.

Because of its size and orbital behavior, 1998 OH is included in long-term planetary defense simulations, helping scientists refine impact modeling, response planning, and early warning systems. As of now, there are no mission plans to intercept or study it up close, but radar observations during the 2042 flyby could yield more accurate predictions for its 2132 path. If it were to ever veer closer to Earth, it would become an urgent priority for mitigation efforts.^[5]

5 (4953) 1990 MU — Future Encounters Approaching

Discovered in 1990, asteroid 4953 MU is another large and concerning Apollo-class asteroid that orbits the Sun every 1.5 years. It’s estimated to be between 984 and 1,968 feet (300 and 600 meters) wide, making it significantly larger than the 1908 Tunguska object, which flattened 772 square miles (2,000 square km) of Siberian forest. Classified as potentially hazardous, 1990 MU has several future encounters with Earth scheduled—most notably in 2027 and 2058, where it will pass within a few million kilometers.

Although no immediate danger is forecasted, 1990 MU is being closely monitored due to its high orbital eccentricity and tendency to pass through the Earth’s neighborhood. Like many other large PHOs, its orbit could be affected by gravitational interactions with planets or thermal forces like the Yarkovsky effect, which change its trajectory very subtly but significantly over time. Because of its long history of observation, it’s a prime candidate for refining our understanding of orbital drift.

Some researchers have suggested 1990 MU could become a target for a future reconnaissance mission, especially given its size and frequent proximity. As more near-Earth objects are cataloged each year, 1990 MU remains one of the better-known threats on the books—not because it’s heading our way soon, but because it’s large enough to remind us of how devastating even a “near miss” would be if trajectories ever shifted. It’s a quiet, massive rock—just a little too close for comfort.^[6]

4 2007 FT3 — A Lost Threat

Asteroid 2007 FT3 is one of those rare cases where a once-worrisome threat essentially disappeared—not because it no longer exists, but because astronomers lost track of it. Discovered in March 2007, 2007 FT3 was initially given a relatively high impact probability for several future dates, including October 3, 2024, and October 2, 2030. Estimated at approximately 1,082 feet (330 m) in diameter, its mass and potential speed made it a formidable threat, placing it briefly on NASA’s Sentry Risk Table as a potential “city-killer.”

However, the problem with FT3 is that it was only observed for a very short period before it slipped out of view—just 1.2 days, in fact. That minuscule observation arc left a massive amount of uncertainty in its predicted orbit. While models could suggest dozens of potential close approaches or even impacts, the data was simply too vague to calculate anything with confidence. The result was a sort of statistical Schrödinger’s asteroid—maybe dangerous, maybe not even close.

NASA eventually downgraded the impact probabilities for all forecasted dates and removed it from high-risk classifications. But 2007 FT3 remains a cautionary tale: even today, many potentially hazardous asteroids are discovered, briefly tracked, and then lost due to short viewing windows or poor sky visibility. Without constant vigilance—and perhaps a dedicated space-based asteroid observatory—we risk repeating this scenario with a rock we won’t have time to stop.^[7]

3 2023 TL4 — Emerging Risk

Discovered in late 2023, asteroid 2023 TL4 immediately caught attention due to its size (estimated ~1,082 feet or 330 mm wide) and its long-term risk potential. Within months of discovery, scientists identified a 0.00055% chance of impact in the year 2119, placing it on the lower end of risk probabilities—but still notable due to its destructive potential. A rock of this size would unleash energy many times greater than the Hiroshima bomb, possibly leveling a city or causing widespread secondary effects like fires, shockwaves, or tsunamis.

Like many newly discovered objects, 2023 TL4 was initially burdened by uncertainty. Small changes in its orbital path—caused by gravitational tugs or solar radiation—could significantly alter where it ends up a century from now. It’s now part of a growing catalog of next-generation PHOs (potentially hazardous objects) that are being tracked with improved models. These asteroids are crucial to study not because they pose an immediate threat but because they give scientists decades to test, refine, and improve long-term planetary defense strategies.

More observations are needed to determine whether 2023 TL4 will eventually be removed from watchlists, but its discovery highlights the importance of global early-warning systems. It’s also a testament to the power of modern sky surveys: telescopes like Pan-STARRS and ATLAS are spotting these kinds of mid-sized city-busters earlier than ever before. TL4 reminds us that humanity is getting better at identifying danger—but still has a long way to go in terms of prevention.^[8]

2 Asteroids Co-Orbiting Venus — Hidden City Killers

In 2024, a group of astronomers revealed that Venus may be hiding a group of potentially hazardous asteroids—some of which could be large enough to wipe out a city if their orbits ever shift. These co-orbital asteroids, which orbit the Sun alongside Venus, are particularly difficult to detect due to their proximity to the Sun’s glare. Earth-based telescopes have a hard time observing this region of the sky, meaning that some of these threats could be flying under the radar—literally and figuratively.

What makes this class of asteroids especially concerning is their potential orbital instability. While they currently travel in relatively predictable paths near Venus, any gravitational interaction—say, a nudge from Venus, Mercury, or even solar heating—could send them into Earth-crossing orbits over the course of decades. According to a study published in Astronomy & Astrophysics, at least 20 Venus co-orbitals have been identified, including some classified as potentially hazardous to Earth. One of these, 2020 AV2, is locked into a rare orbit that never crosses Earth’s path—for now.

This discovery has sparked calls for space-based detection systems, such as the upcoming NEO Surveyor mission, which will orbit the Sun in a way that allows it to observe objects near and inside Earth’s orbit. Because city-killers like these remain effectively invisible from Earth until they’re much too close, detecting them early is essential for any meaningful defense. The Venus co-orbital group is a cosmic blind spot—and one of the best arguments for turning our eyes toward the Sun, not just the stars.^[9]

1 2025 Close-Flyers — A Window on Destruction

In May 2025, NASA tracked five near-Earth asteroids making close flybys in a single day—none of them large enough to end civilization, but several measuring up to 250 feet (76 meters) wide, or roughly the size of the Statue of Liberty. One of the objects, 2025 JO1, passed within just 1.1 million miles (1.7 million km) of Earth—a cosmic hair’s breadth in astronomical terms. While none of these rocks were on collision courses, they illustrated just how frequently space throws fast, chunky surprises our way.

Events like this are sometimes downplayed by the media, but among planetary defense experts, they’re viewed as critical drills. Each close approach provides valuable data for testing tracking software, refining orbital models, and rehearsing emergency alert systems. Had even one of the 2025 flybys been on a direct course, it could have flattened a city or caused a tsunami if it struck the ocean. These events help researchers assess the effectiveness of early warning tools and highlight gaps in asteroid detection coverage—especially for fast-moving or sun-approaching objects.

Perhaps most unsettling is how routine these events are becoming. Dozens of city-killer-sized asteroids pass near Earth each year—many discovered only days or weeks in advance. The 2025 cluster is a sobering example of how little warning time we might have if a true impact threat were inbound. It’s also a call to action: improve sky surveys, develop deflection technologies, and treat near-misses not as curiosities, but as previews of what could one day be a global catastrophe.^[10]