Asteroid (101955) Bennu is a carbonaceous Near-Earth Object (NEO) classified as potentially hazardous due to its proximity to Earth’s orbit. This primitive, dark asteroid is a relic from the solar system’s earliest days, offering a unique opportunity to study the materials that formed our planets. Bennu was the target of NASA’s OSIRIS-REx mission, which successfully collected and returned a sample of its surface material to Earth in September 2023. Analyzing this pristine material provides unprecedented data on the asteroid’s composition and structure, which is essential for accurately forecasting its future trajectory near our planet.
Bennu’s Physical Characteristics and Orbital Path
Bennu is a relatively small body, measuring approximately 490 to 510 meters across. It is a C-type, or carbonaceous, asteroid, meaning it is rich in carbon compounds and volatile materials, giving it an extremely dark surface. OSIRIS-REx observations revealed that Bennu is not solid rock but a “rubble pile”—a collection of rock fragments and dust held together loosely by gravity. This internal structure is important because it affects how the asteroid might break up or react to deflection attempts.
Bennu is categorized as a Potentially Hazardous Object (PHO) because its 1.2-year orbit brings it close to Earth’s path. Its trajectory is subject to subtle, non-gravitational forces. The most influential of these is the Yarkovsky effect, where sunlight warms one side of the rotating asteroid, and the re-radiation of that heat creates a small, continuous thrust. This subtle push has caused Bennu’s orbit to drift toward the Sun, complicating long-term orbital predictions.
Assessing the True Probability of Collision
Highly precise calculations, informed by OSIRIS-REx data, place the cumulative probability of Bennu impacting Earth between 2178 and 2290 at approximately 1 in 1,750 (0.057%). Although this is a low chance, it remains the highest impact probability of any known asteroid. The single date of greatest concern is September 24, 2182, with an estimated impact probability of 1 in 2,700 (0.037%).
The risk depends heavily on a close flyby in 2135, when Earth’s gravity will strongly influence Bennu’s path. During this approach, the asteroid must avoid passing through specific, tiny regions of space known as “gravitational keyholes.” Passing through a keyhole would cause Earth’s gravity to precisely alter Bennu’s trajectory, setting it on a collision course for a subsequent orbit late in the 22nd century.
The Torino Impact Hazard Scale, which ranges from 0 (no risk) to 10 (certain global catastrophe), is used for potential impacts less than 100 years away. Since Bennu’s highest risk is over a century away, it does not carry an active rating on this scale. However, its probability is monitored on the Sentry Risk Table and quantified using the more complex Palermo Technical Impact Hazard Scale, which accounts for the long-term nature of the threat.
Scale of Destruction from a Hypothetical Impact
If Bennu were to strike Earth, the energy released would be immense, estimated to be the equivalent of approximately 1,200 megatons of TNT. This energy is far greater than the largest nuclear weapons ever tested and would cause regional, but not global, devastation. The outcome would depend heavily on the impact location, specifically whether it struck land or ocean.
A land impact would generate a massive shockwave, forming an initial crater up to 6.4 kilometers (4 miles) wide, with destruction extending for hundreds of kilometers. Immediate effects would include devastating seismic activity, widespread wildfires from the heat pulse, and an air blast capable of collapsing buildings tens of miles from the site. The sheer amount of pulverized rock and dust ejected into the atmosphere would also have severe longer-term consequences.
An oceanic impact would pose an equally catastrophic threat by generating enormous tsunamis. The resulting waves would be capable of devastating coastlines thousands of miles away from the strike zone. Regardless of the location, the injection of hundreds of millions of tons of dust and aerosols into the stratosphere would cause a global “impact winter.” This cloud would block sunlight, potentially causing global average temperatures to drop by several degrees and significantly reducing plant photosynthesis for three to four years. While this would lead to severe climate and agricultural disruption, Bennu is not large enough to trigger an extinction-level event. The destruction would be localized, but the long-term environmental effects would be felt globally.
Planetary Defense and Mitigation Strategies
The potential for an impact from an object like Bennu underscores the importance of NASA’s Planetary Defense Coordination Office (PDCO). The PDCO is responsible for finding, tracking, and characterizing Near-Earth Objects to provide early warning of any potential threat to our planet. Data gathered by missions like OSIRIS-REx directly refines the orbital models used for prediction.
Should an impact trajectory be confirmed, various deflection strategies exist, though all require decades of lead time. The most proven method is the kinetic impactor technique, which involves smashing a spacecraft into the object at high velocity to slightly alter its orbital path. This method was successfully demonstrated by NASA’s Double Asteroid Redirection Test (DART) mission. Another concept is the gravity tractor, where a spacecraft flies in close formation, using its small gravitational pull to gently tug the object off course over many years. These deflection options represent proactive planetary defense, and the continuous monitoring of Bennu provides the necessary lead time to plan and execute such a mission if the risk were to increase substantially.