Asteroid Bennu has a small but real chance of striking Earth in the year 2182

A city-block-sized asteroid named Bennu periodically draws attention because of a distant, low-probability encounter with Earth more than a century and a half from now. NASA has tracked the object for more than two decades, and a dedicated sample-return mission sharpened the underlying orbital math considerably. The resulting odds are small enough that nobody is bracing for a collision, but they are precise enough that Bennu remains one of the most thoroughly modeled objects in the solar system.

Why September 24, 2182 Is the Date That Matters

In August 2021, NASA researchers published a study in the journal Icarus that used precision-tracking data collected by the agency’s OSIRIS-REx spacecraft to recalculate Bennu’s long-term path through the solar system. Led by Davide Farnocchia of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, the team determined that Bennu’s total probability of striking Earth sometime between now and the year 2300 is about 1 in 1,750, or 0.057%, according to a NASA news release summarizing the findings.

Within that broader window, a single date carries the highest individual risk. NASA’s analysis identified September 24, 2182, as the most significant potential impact date, with a probability of about 1 in 2,700, or roughly 0.037%. Those numbers come directly from the peer-reviewed paper, titled “Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data,” and they represent a substantial narrowing of uncertainty compared with ground-based telescope tracking alone.

A Near-Earth Object Discovered by a Military Radar Survey

Bennu was first spotted in September 1999 by the Lincoln Near-Earth Asteroid Research survey, a joint project of MIT’s Lincoln Laboratory, the U.S. Air Force, and NASA that scanned the sky for objects passing close to Earth. Its orbit brings it across Earth’s path roughly every six years, which is what earned it a spot on NASA’s list of potentially hazardous asteroids long before OSIRIS-REx ever left the launch pad. That repeated close contact is also precisely why the object became an attractive sample-return target: a short trip compared with more distant asteroids, paired with a well-documented, if still uncertain, long-term orbit.

NASA’s Planetary Defense Coordination Office, which oversees the broader search for hazardous asteroids and comets, treats Bennu as a case study in how much uncertainty modern tracking can remove. Kelly Fast, program manager for the office’s Near-Earth Object Observations Program, said in the 2021 release that the OSIRIS-REx results helped refine not just Bennu’s own path but the models scientists use to predict the orbits of other asteroids discovered by ongoing sky surveys.

A 2135 Flyby Sets Up the Later Risk

Before Bennu’s more distant odds become relevant, the asteroid has a closer and better-understood appointment with Earth. In 2135, Bennu will pass near the planet in a flyby that NASA has said poses no danger on its own. That close approach will still nudge the asteroid’s orbit through the pull of Earth’s gravity in ways that are difficult to predict with total precision more than a century in advance.

Scientists describe certain narrow regions of space near Earth as gravitational keyholes. If Bennu happens to pass through one of these zones during its 2135 approach, the resulting gravitational tug could set the asteroid on a trajectory that intersects Earth’s orbit generations later, including on the 2182 date NASA has flagged as the riskiest. Determining exactly how likely that keyhole passage is became the central task of the 2021 study.

The Yarkovsky Effect and Other Forces Scientists Had to Weigh

Pinning down whether Bennu will thread one of those keyholes required accounting for forces far subtler than gravity alone. Chief among them is the Yarkovsky effect, a phenomenon in which sunlight heats one side of a slowly spinning asteroid, which then radiates that stored heat away as infrared energy once it rotates into shadow, producing a faint but continuous thrust. JPL scientist Steve Chesley, a co-investigator on the study, compared the resulting push on Bennu to the constant weight of three grapes resting on the asteroid, a force too small to measure over days but significant when it accumulates across centuries.

The research team also modeled the gravitational influence of the sun, the planets, their moons, and more than 300 other asteroids, along with drag from interplanetary dust and pressure from the solar wind. Investigators even checked whether OSIRIS-REx’s own touch-and-go sample-collection maneuver on October 20, 2020, might have altered Bennu’s path, ultimately confirming that the brief contact had a negligible effect on the asteroid’s future course.

How Bennu Compares to Other Hazardous Asteroids

Bennu measures roughly 500 meters, or about a third of a mile, across, based on more than two years of close-up mapping by OSIRIS-REx before the spacecraft departed the asteroid in May 2021. Despite the long odds against any impact, NASA has described Bennu as one of the two most hazardous known asteroids in the solar system, alongside a separate object called 1950 DA, largely because the combination of its size, orbit, and now well-characterized trajectory makes its risk unusually easy to quantify compared with less-studied objects.

The same mission that sharpened these calculations also delivered a physical piece of Bennu to Earth. OSIRIS-REx released its sample capsule on September 24, 2023, exactly 41 years before the date NASA identified as carrying the highest individual impact probability, giving laboratories material to study the thermal and physical properties that drive the Yarkovsky effect shaping Bennu’s long-term path. Continued observation, including work coordinated through NASA’s Center for Near-Earth Object Studies, is expected to keep refining these odds long before the 22nd century arrives.

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

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