OSIRIS-REx Did One Last Close Flyby of Asteroid Bennu. It’s Almost Time to Come Home

OSIRIS-REx Did One Last Close Flyby of Asteroid Bennu. It's Almost Time to Come Home

After more than two years in orbit around the asteroid Bennu, NASA’s OSIRIS-REx spacecraft is ready to come home. It brings with it a pristine sample of space rocks that geologists here on Earth are dying to study at close range. The example will arrive in September 2023, but we don’t have to wait nearly as long for new data from OSIRIS-REx. Last week, the probe made one final flyby of Bennu to photograph the sample collection site. The photos are now linked and should be here by the middle of the week.

If you’ve been on the OSIRIS-REx mission, you probably already know why scientists want to see these photos, but if you haven’t, hold on to your hats – it’s a wild story.

The mission’s first major shock came back in December 2018 when the spacecraft reached the asteroid. The team expected Bennu’s surface to be smooth and sandy, but the initial images from OSIRIS-REx showed a rough rock field dotted with large rocks and loose gravel.

The team also expected the asteroid to be geologically calm, but just six days after it arrived in orbit, they were amazed when the asteroid hurled rocks from the surface into space. Today, the leading theories regarding the source of the ejections are 1. that it could be caused by small meteorite impacts, or 2. that it could be caused by thermal fracture: As Bennu rotates, the sun heats and cools the rocks repeatedly, causing them to crack and Eruptions that allow small particles to escape the asteroid’s weak gravity.

Asteroid Bennu ejects particles from space as seen by OSIRIS-REx on March 19, 2019. Photo credit: NASA / Goddard / University of Arizona.

Bennu’s unexpectedly rough terrain and his surprising ejections made it difficult to isolate a safe place to land and take a sample. The team originally hoped for a 25-meter-wide, obstacle-free zone in which the TAG rehearsal maneuver (touch-and-go) could be carried out. However, the largest flat open areas that OSIRIS-REx could find on Bennu were only 10 meters in diameter. With the help of citizen scientists searching the asteroid’s images for safe landing targets, the team eventually selected a region called the “Nightingale” as the TAG location.

The landing took place on October 20, 2020, and OSIRIS-REx picked up so much material that the detection mechanism remained open. The team quickly stowed the capsule away before too much material was lost and did without a planned sling maneuver that would have measured the weight of the sample.

But here’s the craziest part. During the TAG maneuver, the OSIRIS-REx’s collector arm plunged 48.8 centimeters into Bennu and would probably have gone meters (yes, meters!) Lower if its engines had not reversed forward motion. In other words, touching down on Bennu was more like jumping into a ball pit than landing on solid ground.

The OSIRIS-REx TAG maneuver on October 20, 2020. Photo credit: NASA / Goddard / University of Arizona.

Bennu’s extremely low density – 60% of the asteroid could be empty space – has a significant impact on planetary defense. If Bennu were running towards Earth (it isn’t) the techniques required to deflect could be vastly different from the techniques used to deflect an asteroid out of a single boulder. Fortunately, astronomers are pretty sure that no major asteroids of any kind will be heading for Earth in the next century, so the world’s space programs have some time to test different ideas.

The photos being downlinked this week are important because they show how the TAG maneuver changed Bennu’s surface in the Nightingale region. “By examining the distribution of the excavated material on the TAG site, we can learn more about the nature of the surface and subsurface materials and the mechanical properties of the asteroid,” said Dr. Dante Lauretta, main researcher at OSIRIS-REx. The new photos should arrive on April 13th.

In a few weeks, on May 10th, OSIRIS-REx will fire its engines to earth, leaving Bennu behind for good. Scientists are pleased to compare its Bennu sample with the sample from the asteroid Ryugu, which the Japanese Space Program (JAXA) collected last December.

That may not be the end of the mission either. The sample capsule will re-enter Earth’s atmosphere, but the rest of the probe will remain in space and could hit another asteroid later in the decade. The OSIRIS-REx mission has been full of surprises so far, and there may be a few more surprises to come.

Read more: Rani Gran, “NASA’s OSIRIS-REx completes final tour of asteroid Bennu.” NASA.

Selected image source: NASA / Goddard.

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