Like boot prints on the moon, the University of Arizona-led OSIRIS-REx spacecraft left its mark on asteroid Bennu. Now, new images – taken during the spacecraft's final flyover on April 7 — reveal the aftermath of the historic encounter with the asteroid.

The spacecraft flew within 2.3 miles of the asteroid – the closest it has been since the touch-and-go, or TAG, sample collection event on Oct. 20. During TAG, the spacecraft's sampling head sunk 1.6 feet into the asteroid's surface and simultaneously fired a pressurized charge of nitrogen gas, churning up surface material and driving it into the collection chamber. The spacecraft's thrusters also launched rocks and dust during the maneuver to reverse course and back away from the asteroid.

Comparison of the two images reveal surface disturbance. At the sample site, there appears to be a depression, with several large boulders evident at the bottom, suggesting that they were exposed by sampling. There is a noticeable increase in the amount of highly reflective material near the TAG point against the mostly dark background of the surface, and many rocks were moved around.

Where thrusters fired against the surface, scientists found more substantial movement. Multiple small boulders were mobilized by the plumes into a shape similar to a campfire ring – similar to rings of boulders seen around small craters pocking the surface.

Jason Dworkin, the mission's project scientist at Goddard Space Flight Center, noticed that one boulder measuring 4 feet across on the edge of the sampling site seemed to appear only in the post-TAG image.

"The rock probably weighs around a ton, with a mass somewhere between a cow and a car," he said.

Dante Lauretta, UArizona planetary sciences professor and mission principal investigator, believes that this boulder likely was present in the pre-TAG image, but much nearer the sampling location, and the forces from the TAG event launched it 40 feet.

To compare the before and after images, the team had to meticulously plan this final flyover.

"Bennu is rough and rocky, so if you look at it from a different angle or capture it at a time when the sun is not directly overhead, that dramatically changes what the surface looks like," said Dathon Golish, a member of the OSIRIS-REx image processing working group, headquartered at UArizona. "These images were deliberately taken close to noon, with the sun shining straight down, when there's not as many shadows."

"These observations were not in the original mission plan, so we were excited to go back and document what we did," Golish said. "The team really pulled together for this one last hurrah."

The spacecraft will remain in Bennu's vicinity until departure on May 10, when the mission will begin its two-year cruise back to Earth. As it approaches Earth, the spacecraft will release the Sample Return Capsule that contains the sample from Bennu. The capsule will then travel through Earth's atmosphere and land under parachutes at the Utah Test and Training Range on Sept. 24, 2023.

Once recovered, the capsule will be transported to the curation facility at the agency's Johnson Space Center in Houston, where the sample will be removed for distribution to laboratories worldwide, enabling scientists to study the formation of the solar system and Earth as a habitable planet.

The OSIRIS-REx mission is the first NASA mission to visit a near-Earth asteroid, survey the surface, and collect a sample to deliver to Earth.

The University of Arizona leads the science team and the mission's science observation planning and data processing. NASA's Goddard Space Flight Center, located in Greenbelt, Maryland, provides overall mission management, systems engineering, and the safety and mission assurance for OSIRIS-REx. Lockheed Martin Space in Denver built the spacecraft and provides flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA's New Frontiers Program, which is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama, for the agency's Science Mission Directorate in Washington.