Asteroid (101955) Bennu is a near-Earth object (NEO) classified as potentially hazardous (PHA) due to its orbit periodically bringing it close to Earth. The NASA OSIRIS-REx mission recently returned samples, providing data to understand its characteristics, trajectory, and origins, believed to hold clues about the early solar system.
Understanding Asteroid Bennu
Asteroid Bennu is a carbonaceous asteroid, approximately 500 meters (1,600 feet) in diameter. Its dark, primitive nature classifies it as a B-type asteroid, a valuable remnant from the early solar system due to minimal geological alteration.
Bennu’s composition is rich in organic and hydrated minerals, including water-containing ones. OSIRIS-REx samples revealed materials from within and beyond our solar system, suggesting its parent body formed from a diverse mixture of cosmic materials.
Bennu belongs to the Apollo group of asteroids, with orbits crossing Earth’s path and passing relatively close every six years. Its orbit is influenced by the Yarkovsky effect, a subtle force from uneven heat re-emission that gradually alters its trajectory.
Bennu is designated a “potentially hazardous asteroid” (PHA) because its orbit brings it within 0.05 astronomical units (7.5 million kilometers or 4.6 million miles) of Earth and it is large enough to cause significant regional damage. Precise data from missions like OSIRIS-REx is important for risk assessment.
Assessing the Likelihood of an Impact
While Bennu is a potentially hazardous asteroid, its impact probability with Earth is very low. OSIRIS-REx data indicates a cumulative probability of approximately 1 in 1,750 (0.057%) for impact between 2178 and 2290, meaning over a 99.94% chance Bennu will miss Earth.
The most significant date for a potential impact is September 24, 2182, with an estimated probability of 1 in 2,700 (0.037%). Before then, Bennu will make a close approach to Earth on September 25, 2135, passing between Earth and the Moon. Earth’s gravity could subtly alter Bennu’s trajectory if it passes through “gravitational keyholes,” though no impact is possible during this flyby.
The OSIRIS-REx mission provided unprecedented precision in tracking Bennu’s movement, allowing scientists to refine trajectory predictions. This improved understanding comes from accurately modeling the Yarkovsky effect, which significantly influences Bennu’s path over long periods.
Space agencies like NASA continuously monitor near-Earth objects using sophisticated systems like Sentry-II. These systems track asteroid orbits and assess potential impact risks over the next century and beyond. Continuous observation and refined models allow for ongoing updates, ensuring a clear picture of any potential threats.
Potential Consequences of a Collision
Should asteroid Bennu hypothetically collide with Earth, consequences would vary significantly by impact location. An object of Bennu’s size would not trigger a global extinction-level event, as the dinosaur-extinction asteroid was considerably larger (10-15 kilometers in diameter compared to Bennu’s 500 meters).
A land impact by Bennu would result in widespread regional devastation. Immediate effects include a significant impact crater (5-10 kilometers wide). The initial blast wave would generate immense pressure, destroying structures up to 15 kilometers away and shattering windows over 100 kilometers away.
The collision would propagate powerful ground shockwaves, equivalent to a magnitude 7.4 earthquake, felt hundreds of kilometers away. Extreme heating could melt the ground and ignite widespread fires. Dust and debris ejected into the atmosphere could lead to a temporary “impact winter” with localized temperature drops and reduced sunlight, affecting ecosystems and agriculture.
If Bennu were to strike the ocean, the initial impact would create a massive surge of water and a high-pressure shockwave. Simulations suggest a deep ocean impact would produce large initial waves that dissipate quickly. The most damaging scenario involves a strike in shallow coastal waters or on the continental shelf, which could generate very large tsunamis impacting broad coastal regions.
An ocean impact would also inject vast quantities of water vapor and sea salt into the atmosphere. This atmospheric disruption could temporarily deplete the ozone layer, increasing harmful ultraviolet radiation and affecting agricultural yields and health. While severe for affected regions, these effects are distinct from the widespread, long-term global climate disruption associated with much larger, extinction-level impacts.
Mitigating Future Asteroid Threats
Global efforts in planetary defense focus on detecting, tracking, and potentially deflecting near-Earth objects. Agencies like NASA, through its Planetary Defense Coordination Office (PDCO), continuously monitor the skies for hazardous asteroids. Future missions, such as NASA’s NEO Surveyor, are designed to enhance discovery and characterization, improving early warning capabilities.
Scientists are developing strategies to alter an asteroid’s trajectory on a collision course. One method is the kinetic impactor, where a spacecraft intentionally crashes into an asteroid to impart a small velocity change. NASA’s DART mission successfully demonstrated this in 2022 by altering Dimorphos’ orbit, validating the concept.
Another proposed method is the gravity tractor, which uses a spacecraft’s subtle gravitational pull to gently tug an asteroid off course over an extended period. This approach offers precise control and avoids physical contact. Additionally, laser ablation is being explored, where high-powered lasers vaporize material from the asteroid’s surface, creating a propulsive force to steer it away.
Addressing asteroid threats is a global undertaking, necessitating international cooperation. Organizations like the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG) facilitate information exchange and coordinate planning among nations. These collaborative efforts aim to build consensus on mitigation strategies and ensure a unified response to safeguard Earth from potential cosmic impacts.