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Bennu ejects particles from its surface Jan. 19, 2019. The image was created by combining two images taken aboard OSIRIS-REx.

After nearly five years in space, NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer – OSIRIS-REx – spacecraft is on its way back to Earth with an abundance of rocks and dust from the near-Earth asteroid Bennu.

On May 10 at 3:23 p.m., the spacecraft fired its main engines full throttle for seven minutes – its most significant maneuver since it arrived at Bennu in 2018. This burn thrust the spacecraft away from the asteroid at 600 miles per hour, setting it on a 2 ½-year cruise toward Earth.

“It was very exciting to see such a perfect execution of the final spacecraft operations at Bennu,” Solveig Irvine, OSIRIS-REx mission manager at Marshall Space Flight Center, said. “While it is the end of our time with the asteroid, it is also the beginning of the final journey that will allow us to unlock many of the questions we have held throughout this amazing adventure.”

ORISIS-REx will continue to head toward Earth and will fire its engines to fly by Earth safely, putting it on a trajectory to circle the sun inside of Venus’ orbit. After orbiting the Sun twice, OSIRIS-REx is due to reach Earth on Sept. 24, 2023. Upon return, the capsule containing pieces of Bennu will separate from the rest of the spacecraft and enter the atmosphere. After releasing the sample capsule, OSIRIS-REx will have completed its primary mission. The capsule will parachute to the Utah Test and Training Range in Utah’s west desert, where scientists will be waiting to retrieve it.

“OSIRIS-Rex’s many accomplishments demonstrated the daring and innovate way in which exploration unfolds in real time,” Thomas Zurbuchen, associate administrator for science at NASA headquarters, said. “The team rose to the challenge, and now we have a primordial piece of our solar system headed back to Earth where many generations of researchers can unlock its secrets.”

To realize the mission’s multiyear plan, a dozen navigation engineers made calculations and wrote computer code to instruct the spacecraft when and how to push itself away from Bennu. After departing from Bennu, getting the sample to Earth safely is the team’s next critical goal. This includes planning future maneuvers to keep the spacecraft on course throughout its journey.

“Our whole mindset has been, ‘Where are we in space relative to Bennu?’” Mike Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center, said. “Now our mindset has shifted to ‘Where is the spacecraft in relation to Earth?’”

The navigation cameras that helped orient the spacecraft in relation to Bennu were turned off April 9, after snapping their last images of the asteroid. With Bennu in the rearview mirror, engineers are using NASA’s Deep Space Network of global spacecraft communications facilities to steer OSIRIS-REx by sending it radio signals. By measuring the frequency of the waves returned from the spacecraft transponder, engineers can tell how fast OSIRIS-REx is moving. Engineers measure how long it takes for radio signals to get from the spacecraft back to Earth in order to determine its location.

The May 10 departure date was precisely timed based on the alignment of Bennu with Earth. The goal of the return maneuver is to get the spacecraft within about 6,000 miles of Earth in September 2023. Although OSIRIS-REx still has plenty of fuel remaining, the team is trying to preserve as much as possible for a potential extended mission to another asteroid after returning the sample capsule to Earth. The team will investigate the feasibility of such a mission later this year.

The spacecraft’s course will be determined mainly by the Sun’s gravity, but engineers will need to occasionally make small course adjustments via engine burns.

The team will perform course adjustments a few weeks prior to Earth reentry in order to precisely target the location and angle for the sample capsule’s release into Earth’s atmosphere. Coming in too low could cause the capsule to bounce out of the atmosphere like a pebble skipping off a lake; too high and the capsule could burn up due to friction and heat from the atmosphere. If OSIRIS-REx fails to release the capsule, the team has a backup plan to divert it away from Earth and try again in 2025.

OSIRIS-REx exceeded many expectations. Most recently, in the midst of a global pandemic, the team flawlessly executed the most mission’s critical operation, collecting more than 2 ounces of soil from Bennu’s surface.

OSIRIS-REx is the third mission in NASA’s New Frontiers Program, managed by Marshall for the agency’s Science Mission Directorate.

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