First Observations of Asteroid Bennu Reveal Water Was Once Present

Data obtained from the OSIRIS-Rex spacecraft’s two spectrometers, the OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) and the OSIRIS-REx Thermal Emissions Spectrometer (OTES), reveal the presence of molecules that contain oxygen and hydrogen atoms bonded together, known as “hydroxyls,” on the asteroid Bennu. Team members of the mission, which is led by the University of Arizona, suspect that these hydroxyl groups exist globally across the asteroid in water-bearing clay minerals, indicating that at some point, the rocky material interacted with water. While Bennu itself is too small to have ever hosted liquid water, the finding does indicate that liquid water was present at some time on Bennu’s parent body, a much larger asteroid.

This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2, 2018, by the OSIRIS-REx spacecraft from a range of 15 miles. Courtesy of NASA/Goddard/University of Arizona.

Data obtained from the OSIRIS-REx Camera Suite (OCAMS) corroborate ground-based radar observations of Bennu and confirm that the original model, developed in 2013, closely predicted the asteroid’s actual shape. Bennu’s diameter, rotation rate, inclination, and overall shape presented almost exactly as projected. The accuracy of the model means that the mission, spacecraft, and planned observations were appropriately designed for the tasks ahead at Bennu.

The initial assessment of Bennu’s regolith indicates that the asteroid’s surface is a mix of rocky, boulder-filled regions and a few relatively smooth regions that lack boulders. The team will make further observations at closer ranges to more accurately assess where a sample can be taken on Bennu for later return to Earth.

The mission is currently performing a preliminary survey of the asteroid, flying the spacecraft in passes over Bennu’s north pole, equator, and south pole at ranges as close as 4.4 miles to better determine the asteroid’s mass. This survey also provides the first opportunity for the OSIRIS-REx Laser Altimeter (OLA), an instrument contributed by the Canadian Space Agency, to make observations now that the spacecraft is in proximity to Bennu. The spacecraft’s first orbital insertion is scheduled for Dec. 31, 2018, and OSIRIS-REx will remain in orbit until mid-February 2019, when the mission transitions into the next survey phase.

“What used to be science fiction is now a reality,” said UA president Robert C. Robbins. “Our work at Bennu brings us a step closer to the possibility of asteroids providing astronauts on future missions into the solar system with resources like fuel and water.”

From August through early December 2018, the OSIRIS-REx spacecraft has aimed three of its science instruments toward Bennu to begin making the mission’s first observations of the asteroid. The results were presented at the Annual Fall Meeting of the American Geophysical Union in Washington, D.C., Dec. 10-14, 2018.

This preliminary shape model of asteroid Bennu was created from a compilation of images taken by OSIRIS-REx’s PolyCam camera during the spacecraft’s approach toward Bennu during November 2018. This 3D shape model shows features on Bennu as small as 6 meters. More information is available at https://go.nasa.gov/2G9WTDU. Courtesy of NASA/Goddard/University of Arizona.