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Exploring the unknown: NASA’s epic journey to Bennu and back

Why did NASA’s probe nearly get ‘swallowed’ while bringing back a 250g ‘dirt clod’ from a place 6.2 billion km away from Earth?

On September 24th, NASA made history by bringing back a sample collected from an asteroid located much farther away from Earth than ever before.

This asteroid is named Bennu, with a diameter of 262 meters, situated approximately 6.2 billion kilometers from Earth. The sample collection was carried out by a NASA spacecraft in 2020, after which the spacecraft spent three years returning to Earth.

As it approached Earth at a distance of about 108,000 kilometers, the OSIRIS-REx spacecraft released a capsule containing the sample into the Earth’s atmosphere. The capsule was then carefully inspected by NASA personnel upon retrieval. Initial examinations revealed that the sample capsule was intact and undamaged during re-entry. Estimates indicate that approximately 250 grams of material were collected, surpassing the initial target of 60 grams. This represents the largest amount of space material returned to Earth since the Apollo program.



The Challenging Descent

Prior to this mission, NASA launched the OSIRIS-REx spacecraft in 2016 with the goal of exploring the asteroid Bennu and bringing back samples for research. It wasn’t until 2018 that the spacecraft arrived at the asteroid.

Before OSIRIS-REx, there were other missions to explore asteroids, such as Japan’s Hayabusa 1 and the U.S. mission NEAR-Shoemaker. These missions confirmed what asteroid researchers had hypothesized for some time: many asteroids are not solid rocks but rather conglomerates of rocks, sand, and pebbles, formed from collisions between larger objects throughout the solar system’s history.

Collective Image of a Boulder on the Surface of the Asteroid Bennu.

Regarding OSIRIS-REx’s mission, when observed through telescopes, astronomers believed that Bennu’s geological structure closely resembled that of Itokawa, based on information gathered by Japan’s Hayabusa mission in 2010. Thus, Bennu was thought to be composed of rocky boulders interspersed with smoother, sandy regions where the spacecraft could safely touch down.



However, NASA researchers encountered an unexpected challenge. When OSIRIS-REx approached Bennu for a close-up view, it discovered a rocky terrain with sharp boulders protruding, defying Bennu’s weak gravity. This rendered the original landing plan, which relied on a laser altimeter for altitude measurements, ineffective.

To find a suitable landing spot on Bennu, scientists had to operate OSIRIS-REx remotely, using the spacecraft’s onboard camera to meticulously map the surface features down to the centimeter.

With the special assistance of renowned astrophysicist, musician, and singer Brian May, a 3D mosaic image (created by stitching two images together to provide depth perception) was generated. This allowed the OSIRIS-REx team to assess the safety of potential landing sites.

A Narrow Escape from an Asteroid’s Clutches

After 22 months of careful consideration, researchers decided on a landing site they named “Nightingale,” where OSIRIS-REx touched down on October 20, 2020. Instead of landing on a 50-meter-wide patch as initially planned, the spacecraft, roughly the size of a pickup truck, had to attempt landing within a narrow 10-meter crater. Moreover, the operations team had to remotely reprogram the spacecraft to complete this mission.



However, even after selecting what appeared to be the smoothest landing spot, the spacecraft encountered unexpected challenges. Upon landing, OSIRIS-REx sank about 50 centimeters deep into the asteroid’s surface, as if it were “swallowed” in a muddy puddle. Only when the spacecraft quickly released a stream of nitrogen gas from its TAGSAM propulsion system did it escape from Bennu’s grasp.

However, the forceful gas release caused a significant amount of rocks and debris to be ejected into space, surrounding the spacecraft in flight and threatening its safety. Fortunately, no incidents occurred with OSIRIS-REx.

During this time, the NASA operations team experienced a mix of relief and perplexity. According to their calculations, Bennu’s surface should have been robust enough to support the weight of the spacecraft, similar to a pile of rocks.



However, the descent process revealed that Bennu’s surface had a much lower density than the rest of the asteroid. Scientists are still searching for an explanation.

The low-density surface may be due to the presence of voids between rock fragments on the surface. Over billions of years of traveling through the solar system, these fine particles might have settled inside the asteroid, resulting in a much denser interior compared to the surface.

To confirm this, scientists will need to gather information about the interiors of asteroids through the upcoming HERA mission by the European Space Agency. This mission will investigate the effects of NASA’s DART spacecraft impact experiment on the asteroid Dimorphos, which took place in September 2022.