Asteroid Bennu: Its Composition, Mission, and Earth Risk
Analysis of a pristine sample from Asteroid Bennu provides insights into the solar system's past and helps refine our understanding of its future orbit near Earth.
Asteroid Bennu is a near-Earth object that offers a window into the early history of our solar system. Its composition may hold clues about the formation of planets and the ingredients that contributed to the origin of life on Earth. A dedicated space mission was designed to investigate this remnant and retrieve a sample for detailed analysis.
Discovering and Characterizing Bennu
Bennu was first identified in 1999 by the Lincoln Near-Earth Asteroid Research (LINEAR) survey. Initially designated 1999 RQ36, it was later named Bennu, after an Egyptian mythological deity, in a contest won by a student. Classified as a B-type carbonaceous asteroid, it has a distinctive spinning-top shape and a diameter of approximately 525 meters.
This asteroid completes an orbit around the Sun every 1.2 years, and its path brings it close to Earth every six years. This trajectory places it in the Apollo group of near-Earth asteroids, whose orbits cross Earth’s path. Due to its size and proximity, Bennu is designated as a Potentially Hazardous Asteroid (PHA), a classification for asteroids that can make threateningly close approaches to Earth.
The OSIRIS-REx Mission: A Daring Encounter
NASA’s OSIRIS-REx mission launched in September 2016 with the primary objective to collect and return a sample of Bennu’s surface material. Other goals included mapping the asteroid’s surface, documenting its properties, and measuring the Yarkovsky effect.
After arriving at Bennu in December 2018, the spacecraft’s survey revealed a surface far more rugged and boulder-strewn than expected. The asteroid was also unexpectedly active, with the spacecraft observing particles and pebbles being ejected from the surface. This environment required a significant mapping effort to find a safe location for sample collection.
The mission culminated in the Touch-And-Go (TAG) maneuver on October 20, 2020. The spacecraft descended to a selected site, where its robotic arm briefly touched the surface. A puff of nitrogen gas fluidized the loose surface material, called regolith, and captured it within the collector head.
Insights Gleaned from Bennu Pre-Sample Return
Before the samples arrived on Earth, instruments aboard OSIRIS-REx provided valuable information. The spacecraft’s observations from orbit confirmed that Bennu is a carbon-rich body. Spectrometers also detected the widespread presence of hydrated minerals across the asteroid’s surface, indicating that liquid water was once part of Bennu’s parent body.
The data revealed Bennu to be a “rubble-pile” asteroid, a loose collection of rocks and dust held together by gravity with significant empty space in its interior. These remote observations provided context for the samples that would soon be studied in laboratories.
Bennu’s Samples on Earth: Early Findings and Future Promise
The OSIRIS-REx sample-return capsule landed in the Utah desert on September 24, 2023. Initial analysis of the material confirmed the presence of abundant carbon and water-bearing clay minerals. This finding supports the theory that asteroids like Bennu could have delivered water to an early Earth.
The material collected was so plentiful that it was found coating the outside of the collector head as well as inside. Scientists have just begun to study these samples, which represent a direct link to the earliest stages of solar system formation. Future studies will search for specific organic compounds, such as the amino acids that form proteins, which are fundamental to life as we know it.
Bennu’s Long-Term Trajectory and Earth
Bennu’s future path is a subject of intense scientific study. Its orbit is influenced by a phenomenon known as the Yarkovsky effect. This effect occurs when the asteroid absorbs sunlight and re-radiates it as heat, creating a minuscule but constant thrust that slowly alters its trajectory.
Measuring this effect was a goal for OSIRIS-REx, as it is a source of uncertainty in long-term orbital predictions. Data from the mission has allowed scientists to refine Bennu’s orbit with high precision. While a significant impact is not a concern for the next century, there is a calculated probability for an impact in the late 22nd and 23rd centuries, with a close approach predicted for 2182. Organizations like NASA’s Planetary Defense Coordination Office use this data to continuously monitor Bennu and other near-Earth objects.