Among the thousands of asteroids that pass near Earth’s orbit, one modest-sized rock has drawn more sustained scientific attention than almost any other, not because it is especially large, but because a spacecraft has actually visited it and returned samples to refine exactly how dangerous it might one day be. That asteroid, known as Bennu, carries a small but real chance of striking Earth roughly a century and a half from now, a risk precise enough that scientists can name the single most likely date.
A Small Asteroid With an Outsized Watch List Ranking
Bennu measures roughly 500 meters across, small compared to the mountain-sized asteroids capable of a truly civilization-ending impact, but large enough that a direct hit would cause regional devastation on the scale of thousands of nuclear weapons. First discovered in 1999, Bennu was later classified as a potentially hazardous asteroid because its orbit brings it close enough to Earth’s own path to warrant long-term tracking. Despite its modest size, Bennu currently holds one of the highest rankings of any known asteroid on the scales scientists use to compare impact risk, largely because researchers have been able to measure its orbit and physical properties with unusual precision compared with most other cataloged near-Earth objects.
How NASA Landed on the Numbers
That precision came from NASA’s OSIRIS-REx mission, which traveled to Bennu, orbited it for nearly two years, and in 2020 briefly touched down to collect a sample of rock and dust before returning it to Earth in 2023. According to Wikipedia’s overview of asteroid Bennu, the mission’s close-up tracking allowed scientists to measure a subtle effect called Yarkovsky drift, in which sunlight absorbed and re-radiated by the asteroid’s surface nudges its orbit over time, a factor too small to detect from Earth-based telescopes alone but significant enough to reshape long-term impact forecasts. Feeding that refined orbital data into NASA’s Sentry monitoring system, researchers calculated a cumulative impact probability of roughly one in 1,750 through the year 2300, a small number but far more precise than earlier estimates made before the spacecraft’s visit.
One Afternoon in September, 2182
Within that long list of possible future close approaches, one date stands out as carrying the highest individual risk: September 24, 2182. Scientists have calculated the odds of an impact on that specific date at roughly one in 2,700, making it the single most probable scenario within the broader window of possible encounters that stretches from 2178 through 2290. That level of specificity is unusual in planetary defense; most potentially hazardous asteroids carry far vaguer, less precisely dated risk estimates, and Bennu’s case stands out largely because of how thoroughly OSIRIS-REx characterized its size, shape, mass, and orbital path during its extended survey of the asteroid.
What an Impact Would Actually Look Like
If Bennu were to strike Earth, researchers estimate the impact energy would be roughly equivalent to 1,200 megatons of TNT, an amount capable of leveling a large metropolitan area or devastating a wide swath of countryside, though nowhere near the scale of the extinction-level impacts associated with objects several kilometers across, such as the asteroid linked to the dinosaurs’ extinction. A strike anywhere on land would flatten structures and vegetation for a considerable distance, while an ocean impact would risk a regional tsunami, though the object is not large enough to trigger the kind of global climate disruption that a much bigger asteroid could cause. Scientists classify this category of impact as regionally, rather than globally, catastrophic.
Why the Odds Still Favor Earth
Despite the attention Bennu receives, the numbers still overwhelmingly favor no impact ever occurring at all; a one-in-1,750 cumulative chance means Earth is far more likely than not to avoid a Bennu strike across every possible encounter scientists have modeled through 2300. Researchers continue refining the calculation as more tracking data accumulates and as scientists analyze the sample OSIRIS-REx brought home, work that could push the odds even lower, as has happened repeatedly with other asteroids once initially alarming estimates were replaced by more precise tracking. Asteroid Apophis, once briefly flagged with an unusually high impact probability for a 2029 flyby, later had its risk reduced essentially to zero once additional observations pinned down its orbit, a pattern scientists expect could eventually apply to Bennu as well.
Planetary Defense Beyond Just Watching
Bennu’s well-mapped orbit has also made it a useful test case for planetary defense planning more broadly, since agencies use asteroids with well-understood trajectories to rehearse how they would respond if a genuinely dangerous object were ever discovered on a collision course. NASA has already demonstrated one potential response, deliberately crashing a spacecraft into a small asteroid moon in 2022 to test whether a kinetic impact could measurably alter an asteroid’s path, a technique that could eventually be applied to an object like Bennu if its odds ever worsened. For now, Bennu remains less an imminent threat than a case study in how far planetary defense science has advanced, precise enough to name a date more than 150 years away while still leaving little reason for anyone alive today to worry.
This article was produced with the assistance of AI and reviewed by Morning Overview editors.
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