Roughly 66 million years ago, Earth was struck by a city-size asteroid. The impact released the energy equivalent to the detonation of 72 trillion tons (65 metric tons) of TNT, carving a 100-mile-wide (180 kilometers) scar in what is now Mexico's Yucatán Peninsula.
The most infamous effect of this asteroid, named the Chicxulub impactor, was the death of the nonavian dinosaurs along with around three-quarters of Earth's species in an event called the Cretaceous-Paleogene extinction.
In an effort to prevent similarly catastrophic collisions, NASA's Center for Near Earth Object Studies (CNEOS) operates the Sentry impact-monitoring system, which continuously performs long-term analyses of possible future orbits of potentially hazardous asteroids. Here are the five asteroids that pose the greatest risk to Earth.
Related: 'Planet killer' asteroids are hiding in the sun's glare. Can we stop them in time?
1. Bennu
Size: 0.30 mile (0.49 kilometer)
Mass: 74 million tons (67 million metric tons)
Discovered in September 1999 and officially designated "101955 Bennu (1999 RQ36)," the near-Earth asteroid Bennu currently poses the greatest risk of impacting our planet — but fortunately, not for some time. NASA scientists calculate that when Bennu makes a close approach to Earth on Sept. 24, 2182, there is a 0.037% — or 1 in 2,700 — chance that the asteroid will strike our planet.
In preparation for this, scientists are learning as much as they can about this carbon-rich asteroid, which is believed to have broken away from a larger asteroid between 2 billion and 700 million years ago. On Sept. 24, 2023, NASA's OSIRIS-REx spacecraft returned a sample of Bennu to Earth, and the space rock has been analyzed by teams of researchers across the globe. Early results suggest that Bennu contains the building blocks of life, including the simple amino acid glycine, as well as many water-bearing minerals. This suggests Bennu's parent body witnessed many water-related episodes before eventually fracturing, Live Science previously reported.
If Bennu impacted Earth, it would release the energy equivalent to the detonation of 1.4 billion tons of TNT, causing regional destruction but lacking the potential to cause global devastation. If it were to impact a densely populated area, Bennu could cause millions of deaths.
Related: Could scientists stop a 'planet killer' asteroid from hitting Earth?
2. 29075 (1950 DA)
Size: 0.81 mile (1.3 kilometers)
Mass: 78 million tons (71 metric tons)
The second-riskiest object is the asteroid 29075 (1950 DA), which was lost after its initial discovery in February 1950 and rediscovered 50 years later. 1950 DA is believed to be a loose-rubble-pile asteroid with high iron-nickel content. Currently, 1950 DA has a 0.0029% — or 1 in 34,500 — chance of impacting Earth on March 16, 2880.
If 1950 DA were to hit Earth, it would release the energy equivalent to 75 billion tons of TNT — enough to trigger a global catastrophe that could potentially wipe out humanity.
3. 2023 TL4
Size: 0.20 mile (0.33 kilometer)
Mass: 47 million tons (43 million metric tons)
2023 TL4, discovered in 2023, shows that a newfound space object can immediately become one of the most potentially hazardous asteroids. From observations collected between Oct. 8 and Oct. 19, 2023, astronomers calculated that 2023 TL4 has a 0.00055% — or 1 in 181,000 — chance of striking Earth on Oct. 10, 2119. Should such an impact occur, 2023 TL4 would release the energy equivalent to the detonation of 7.5 billion tons of TNT.
4. 2007 FT3
Size: 0.21 mile (0.34 kilometer)
Mass: 54 million tons (49 million metric tons)
2007 FT3 is defined as a "lost asteroid" because astronomers haven't seen it since 2007. This object's orbit is currently not well constrained, but NASA predicts that the asteroid has a 0.0000096% — or 1 in 10 million — chance of striking our planet on March 3, 2030. The asteroid has a slightly lower probability of 0.0000087% — or 1 in 11.5 million — chance of striking Earth on Oct. 5, 2024.
Should such an impact happen in 2024 or 2030, FT3 would release the energy equivalent to the detonation of 2.6 billion tons of TNT — enough to cause massive regional damage, but not to trigger a global catastrophe.
5. 1979 XB
Size: 0.41 mile (0.66 kilometer)
Mass: 390 million tons (354 metric tons)
Another lost asteroid, 1979 XB, hasn't been seen for around 40 years and thus also has a poorly understood orbit. From what CNEOS scientists do understand, the asteroid, which was first observed on Dec. 11, 1979, has a 0.000055% — or 1 in 1.8 million — chance of striking Earth on Dec. 14, 2113. Such a collision would release the same energy as the detonation of 30 billion tons of TNT.
(Dis) Honorable mention: 99942 Apophis
Size: 0.21 mile (0.34 kilometer)
Mass: 27 million tons (24 metric tons)
It's hard to talk about risky asteroids without mentioning 99942 Apophis.This asteroid — with a suitably apocalyptic name referencing Apep the Uncreator, an enemy of the Ancient Egyptian sun god Ra — sat atop the risky-asteroid table for almost two decades.
Discovered in June 2004, Apophis was quickly identified as one of the most hazardous asteroids that could impact Earth. But that changed in 2021, when a radar observation campaign better constrained the asteroid's orbit. This led astronomers to conclude that the 1,100-foot-wide (340 m) Apophis poses no risk to Earth for at least 100 years. It now sits in the "removed objects" section of the Sentry Risk table.
That means skywatchers will be able to watch Apophis during its close approach in 2029 as it passes within 20,000 miles (32,000 km) — closer than some satellites — with binoculars and telescopes, without the fear that its returns in 2036 and 2068 will threaten the planet.
So what is the risk?
The idea of an asteroid strike is frightening, but all of the space rocks on this list are listed as "zero," or "white," on the Torino Impact Hazard scale. Adopted by the International Astronomical Union in 1999, the Torino scale gives asteroids a number ranging from 0 to 10, indicating both the risk of impact and the consequences of such a strike.
Level 0, the white zone, indicates zero risk of impact or at least a risk so low it might as well be zero. This level applies not only to asteroids that will miss Earth but also to small objects from space that will burn up in the atmosphere and thus pose no threat.
Levels 8 to 10, on the other hand, are in the red zone, representing asteroids that will certainly collide with Earth, with effects ranging from localized destruction (Level 8) and unprecedented regional devastation (Level 9) to global climatic catastrophe (Level 10) that may threaten the future of civilization as we know it.
At the moment, there are no objects on the Sentry Risk table about level 0. Bennu and 1950 DA don't have Torino ratings because their predicted impacts are more than 100 years into the future. As NASA states, "There is currently no known significant threat of impact for the next hundred years or more."
Of course, there still could be potentially hazardous objects out there waiting to be found — indeed, thousands of potential "city-killers" and even some "planet-killers" may be hiding in the sun's glare — which is why CNEOS is ever vigilant in its search for near-Earth asteroids.
Editor's note: This article was updated on June 18, 2024, to include new information about asteroid Bennu revealed during the OSIRIS-REx mission.
