Asteroid Bennu has roughly a 1-in-2,700 chance of striking Earth on a single day in 2182

Among the millions of rocks tumbling through the inner solar system, one small carbon-rich asteroid gets more attention from planetary defense scientists than almost any other. Bennu, a roughly 500-meter-wide near-Earth object, has a well-documented, if extremely small, chance of hitting Earth more than a century and a half from now. That number did not come from guesswork. It came from years of tracking the asteroid up close, including a NASA spacecraft that touched its surface and brought a piece of it home.

Impact-probability figures like this exist because space agencies now run standing survey programs that catalogue and track thousands of near-Earth objects year after year, refining each one’s orbit as fresh observations come in. Most of those objects never warrant a second look once their paths are pinned down with confidence, but Bennu’s combination of size, orbit, and unusually precise tracking data has kept it near the top of the watch list for more than a decade. Understanding how that single number was derived, and what it does and does not imply about the centuries ahead, requires walking through the mission that made the measurement possible in the first place.

Where the 1-in-2,700 figure comes from

NASA’s OSIRIS-REx mission spent nearly two years orbiting and studying Bennu before collecting a sample from its surface in 2020 and returning it to Earth in 2023. That close-range tracking let scientists refine Bennu’s orbit with far greater precision than ground-based telescopes alone could achieve, because the spacecraft’s own position could be measured so exactly relative to the asteroid. Feeding that data into orbital models, researchers calculated that Bennu’s single highest-risk encounter falls on September 24, 2182, with an impact probability of about 1 in 2,700, according to the NASA news release detailing the analysis.

What the returned sample revealed about Bennu’s makeup

Beyond refining Bennu’s orbit, the OSIRIS-REx mission’s sample return answered questions no telescope could. The spacecraft’s capsule parachuted into the Utah desert in September 2023, delivering roughly 121.6 grams of black, carbon-rich rock and dust, the largest asteroid sample any mission has brought back to Earth. Laboratory analysis of that material found clay minerals holding water locked inside their crystal structure, an abundance of carbon and nitrogen compounds, and amino acids, the building blocks proteins are made from, along with phosphate minerals not previously identified in remote observations of the asteroid. Those findings support a long-standing idea among planetary scientists that carbon-rich asteroids like Bennu could have helped deliver some of the water and organic chemistry that let life get started on the early Earth, long before humans existed to study the rocks that may have carried it. Bennu itself is thought to be a fragment of a much larger parent body that broke apart in the asteroid belt between Mars and Jupiter roughly one to two billion years ago; its rubble-pile structure, a loose aggregation of boulders and dust rather than solid stone, has held together loosely ever since, drifting over time into the Earth-crossing orbit that now brings it back for repeated close approaches.

What “1 in 1,750 through 2300” actually means

Beyond that single 2182 date, NASA also calculated a cumulative probability across every close approach Bennu makes through the year 2300, arriving at roughly 1 in 1,750. That combined figure is higher than any individual date’s odds because it adds up dozens of separate low-probability encounters over more than two centuries. Even so, both numbers describe an overwhelmingly likely safe pass: a 1-in-2,700 chance means better than 99.9 percent odds that Bennu misses Earth on that specific day, a distinction researchers at the mission’s lead science institution have stressed in explaining why the figure is notable to scientists without being alarming to the public, as detailed in NASA’s Jet Propulsion Laboratory writeup of the orbital study.

Why a keyhole in 2135 matters more than the odds suggest

The mechanism behind Bennu’s risk is not a direct collision course but something orbital dynamicists call a gravitational keyhole. In September 2135, Bennu will pass close enough to Earth that the planet’s gravity could bend its path just enough to send it through a narrow region of space, the keyhole, that would set up an impact decades later. Whether Bennu passes through that specific keyhole depends on tiny uncertainties in its position that are still being narrowed down, which is exactly why the sample-return data and continued tracking matter: each refinement either closes off possible impact scenarios or narrows the small number that remain open.

Ranking Bennu among the solar system’s watched objects

Bennu and another asteroid, 1950 DA, are generally regarded as the two objects with the highest known cumulative impact probabilities among all catalogued near-Earth asteroids, a status confirmed by the University of Arizona, which leads the OSIRIS-REx science team. That ranking says more about how well these two objects have been measured than about how uniquely dangerous they are; thousands of other asteroids simply have less precise orbital data, which keeps their calculated uncertainty, and therefore their nominal risk, higher. As tracking improves across the broader near-Earth object population, researchers expect similar refinements to either rule out or better quantify risks for other objects, a process the University of Arizona describes in its summary of what the Bennu sample-return data taught scientists about hazardous asteroids generally.

What would actually happen with more than a century of notice

Unlike the sudden discovery of a previously unknown object, Bennu’s case is a study in advance warning. Planetary defense agencies now have more than 150 years of lead time before the riskiest date arrives, and NASA has already demonstrated one deflection technique with the 2022 DART mission, which altered the orbit of a small moonlet by deliberately crashing a spacecraft into it. If future tracking data ever pushed Bennu’s odds meaningfully higher, that lead time would give engineers room to test and refine deflection methods long before any mission would need to fly. For now, Bennu remains what it has been since its risk was first calculated: a well-understood, low-probability, extremely long-range scenario that illustrates how precisely modern orbital mechanics can forecast the distant future of a piece of rock a third of a mile across.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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