A Final Look At Bennu Before OSIRIS-REx Heads Home

This artist's concept shows the planned flight path of NASA's OSIRIS-REx spacecraft during its final flyby of asteroid Bennu, which is scheduled for April 7. NASA/Goddard/University of Arizona

By Brittany Enos, OSIRIS-REx - April 1, 2021

On April 7, OSIRIS-REx will get one last close-up of asteroid Bennu as the spacecraft performs a final flyover to capture images of the asteroid's surface. While performing the flyover, the spacecraft will observe Bennu from a distance of about 2.3 miles – the closest it has been since the Touch-and-Go, or TAG, sample collection event on Oct. 20.

The University of Arizona-led OSIRIS-REx mission is the first NASA mission to visit a near-Earth asteroid, survey the surface and collect a sample to deliver back to Earth. Analysis of the sample could shed more light on the origins of the solar system and life on Earth.

The OSIRIS-REx team decided to add this final flyover after Bennu's surface was significantly disturbed by the TAG sample collection event. 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. The spacecraft's thrusters also mobilized a substantial amount of surface material during the back-away burn. Because Bennu's gravity is so weak, the various forces from the spacecraft had a dramatic effect on the sample site – launching many of the region's rocks and dust in the process. This final flyby of Bennu will provide the mission team an opportunity to learn how the spacecraft's contact with Bennu's surface altered the sample site and the region surrounding it.

"Given the amazing response of the asteroid surface to TAG, I directed the team to design one final observation sequence to document the state of the surface prior to our departure. These data provide scientific value to the returned sample and will allow for a much better understanding of the surface properties," said mission principal investigator and UArizona planetary sciences professor Dante Lauretta. The university leads the science team and the mission's science observation planning and data processing.

The single flyby will mimic one of the observation sequences conducted during the mission's Detailed Survey phase in 2019. The OSIRIS-REx science instruments will observe Bennu for 5.9 hours, more than one full rotation of Bennu. Within this timeframe, the PolyCam imager will obtain high-resolution images of Bennu's northern and southern hemispheres and its equatorial region. The team will then compare these new images with the previous high-resolution imagery of the asteroid obtained in 2019.

Several other science instruments will collect data during the flyover, including the MapCam imager, the OSIRIS-REx Thermal Emission Spectrometer, the OSIRIS-REx Visible and Infrared Spectrometer and the OSIRIS-REx Laser Altimeter. In addition to studying the disturbed surface, exercising these instruments will give the team a chance to assess the current state of each instrument, as dust coated their optics during the sample collection event. Understanding the health of the instruments is also part of NASA's evaluation of possible extended mission opportunities after the sample is delivered to Earth.

After the flyby, it will take one week for the data to be downlinked to Earth. The team will then inspect the images and other data to understand how OSIRIS-REx disturbed Bennu's surface material. At this point, the team will also be able to evaluate the performance of the science instruments.

The spacecraft will remain in asteroid Bennu's vicinity until May 10, when the mission will fire its main rocket engines and enter the Return Cruise phase, beginning the two-year journey back to Earth. As it approaches Earth, the spacecraft will jettison the Sample Return Capsule that contains the rocks and dust collected from Bennu. The capsule will then travel through the Earth's atmosphere and land under parachutes at the Utah Test and Training Range on Sep. 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 documented and a portion distributed to laboratories worldwide, including at the University of Arizona, to allow scientists to study the formation of our solar system and Earth as a habitable planet.

NASA's Goddard Space Flight Center 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.