Of all the near-Earth objects tracked by NASA, one asteroid holds the highest known probability of colliding with our planet. Asteroid Bennu, a roughly 500-meter-wide rubble pile, has a single date that concentrates the greatest risk: Sept. 24, 2182, carrying a 1-in-2,700 chance of impact. That figure, refined using years of close-range spacecraft data, also gives Bennu a cumulative impact probability through 2300 of approximately 1 in 1,750, making it the most statistically significant impact threat on record.
How OSIRIS-REx data sharpened Bennu’s 2182 risk window
The odds attached to Bennu are not guesswork or back-of-the-envelope estimates. They come from precise tracking data collected by NASA’s OSIRIS-REx spacecraft, which orbited the asteroid from late 2018 through 2021 and even briefly touched its surface to collect a sample. That proximity gave scientists an unprecedented dataset for calculating where Bennu will be decades and centuries from now. A peer-reviewed analysis in the journal Icarus used those observations to build a refined orbit model, identifying September 2182 as the most significant individual impact solution with a probability of 3.7 times 10 to the negative fourth power.
Before OSIRIS-REx, ground-based telescopes and radar had already flagged Bennu as a potential threat, but the uncertainty bands around its future path were wide enough that scientists could not pin down which close approaches posed real danger. The spacecraft’s tracking data dramatically narrowed those error bars. According to NASA’s overview, the single-date impact probability for Sept. 24, 2182 stands at approximately 0.037%, while the cumulative probability through 2300 is roughly 0.057%.
A critical event along the way is Bennu’s close flyby of Earth in 2135. During that encounter, the asteroid will pass close enough for Earth’s gravity to bend its trajectory. If Bennu threads a narrow corridor in space known as a gravitational keyhole during the 2135 pass, its orbit would shift just enough to line up a collision seven years later. The keyhole concept explains why a single flyby more than a century from now can determine whether the 2182 date becomes a genuine threat or a statistical footnote. NASA’s Jet Propulsion Laboratory has described the 2135 approach as the hinge point for all subsequent impact scenarios through 2300.
Why the Yarkovsky effect keeps the forecast uncertain
Even with spacecraft-quality tracking data, predicting where a small asteroid will be in 156 years involves forces that are difficult to measure. The most consequential of these is the Yarkovsky effect, a subtle push caused by the asteroid absorbing sunlight and re-radiating that energy as heat. Because Bennu rotates unevenly and has an irregular surface, the thermal radiation creates a tiny but persistent thrust that shifts its orbit over time.
OSIRIS-REx allowed scientists to characterize Bennu’s shape, spin, and surface roughness in much greater detail than was possible from Earth. Those properties feed directly into models of the Yarkovsky effect, which must estimate how much of the incoming solar energy is stored, how quickly it is released, and in which directions the heat is emitted. Small errors in those parameters compound over decades, and the resulting uncertainty in Bennu’s position can grow to thousands of kilometers by the late 2100s.
JPL researchers have emphasized that non-gravitational forces like this thermal recoil are now a dominant source of uncertainty in Bennu’s long-term trajectory. Gravity from the Sun, planets, and large asteroids can be modeled with high precision, but the Yarkovsky effect depends on surface properties that may evolve over time. Micrometeorite impacts, shifts in Bennu’s rubble-pile structure, or changes in its rotation state could all slightly alter the way it absorbs and emits heat, introducing additional drift that is hard to predict in advance.
The hypothesis that additional ground-based radar observations before 2030 could push the 2182 impact probability below 1 in 10,000 and trigger Bennu’s removal from the Sentry risk list is plausible in principle but not confirmed by any published analysis. NASA’s Sentry system, which continuously monitors potential impact threats, uses ensembles of virtual asteroids to compute probabilities. Each new radar measurement tightens the orbital solution and can eliminate potential impact corridors. But the Yarkovsky effect introduces a source of uncertainty that radar alone cannot fully resolve, because it depends on surface properties, thermal inertia, and rotational dynamics that change over decades. Without a dedicated study modeling how specific future observations would reduce the probability, the threshold for removal from the risk list is speculative.
What scientists still cannot resolve about Bennu’s trajectory
Several questions remain open. The OSIRIS-REx data provided the best orbital fix to date, but the Icarus study’s authors worked with observations collected through mid-2021. No publicly available update since then has revised the 1-in-2,700 figure for the 2182 date or the 1-in-1,750 cumulative probability through 2300. Whether Bennu still holds the top position on the Palermo Technical Impact Hazard Scale, which ranks objects by how much their impact probability exceeds the background risk for objects of similar size, has not been independently confirmed by NASA’s Center for Near Earth Object Studies since the original data release.
The 2135 flyby will be the next major inflection point. Ground-based and space-based telescopes will track Bennu as it approaches, and the precision of those measurements will determine whether the 2182 keyhole can be definitively ruled in or out. If the asteroid’s path is observed to miss all relevant keyholes, the 2182 impact scenario would effectively be eliminated, and Bennu’s risk rating would drop accordingly. If, however, the post-2135 orbit remains compatible with one or more keyholes, the statistical threat could persist or even increase as uncertainties shrink around a smaller set of possible futures.
Another unresolved issue is how much Bennu’s physical state might change over the coming century. As a loosely bound rubble pile, the asteroid is susceptible to internal rearrangements driven by spin, tidal forces during planetary flybys, or minor impacts. Any shift in its mass distribution could slightly alter its rotation or wobble, which in turn would modify the direction and magnitude of the Yarkovsky thrust. Current models treat Bennu’s properties as effectively constant, but real-world deviations from that assumption could introduce subtle discrepancies between predictions and reality over long timescales.
Scientists are also watching for opportunities to refine Bennu’s orbit using future spacecraft or radar campaigns. The OSIRIS-REx sample return has already improved knowledge of Bennu’s composition and surface mechanics, which feed into thermal models. Additional radar apparitions in the coming decades could further tighten its position and velocity, especially if they coincide with relatively close passes by Earth. Yet without a new mission dedicated to re-visiting the asteroid, the level of detail achieved during OSIRIS-REx’s prime operations is unlikely to be matched again soon.
For now, Bennu stands as a benchmark case for planetary defense planning. Its relatively small but well-quantified impact probability illustrates both the progress and the limits of current orbit-determination techniques. On one hand, scientists can already identify specific calendar dates more than a century away that carry elevated risk. On the other, they must still contend with non-gravitational forces and long-term uncertainties that keep those forecasts probabilistic rather than definitive.
As new data arrive and models improve, Bennu’s risk assessment will continue to evolve. Whether the asteroid ultimately proves to be a once-in-many-millennia hazard or a well-studied near miss, its story is shaping how agencies think about detecting, tracking, and, if necessary, deflecting future threats. The numbers attached to Sept. 24, 2182 may change, but the broader lesson is already clear: understanding small forces acting over long times is essential to predicting the paths of potentially hazardous asteroids.
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*This article was researched with the help of AI, with human editors creating the final content.
