Asteroid Bennu is so loosely packed that a spacecraft sank into it like plastic balls
When NASA's OSIRIS-REx spacecraft touched down on the asteroid Bennu in 2020 to collect a sample, it encountered virtually zero resistance. Bennu is not a solid rock but a porous "rubble pile" held together by faint gravity. The probe's sampling arm plunged nearly half a meter into the surface, behaving as if the asteroid were a children's ball pit, and kicking up a giant plume of debris.
The Unexpected Plunge at Nightingale
In October 2020, NASA's OSIRIS-REx spacecraft reached the climax of its mission at the near-Earth asteroid 101955 Bennu. The goal was to perform a delicate Touch-and-Go (TAG) maneuver: gently touch the asteroid's surface for mere seconds, fire a burst of nitrogen gas to stir up surface regolith, capture the dislodged material inside a collector head, and immediately back away. Mission planners targeted a relatively small, boulder-rimmed crater in the northern hemisphere designated Nightingale. They expected the contact point to offer moderate resistance, behaving like compact gravel or firm soil.
Instead, the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) arm met virtually no mechanical resistance. When the circular collector head contacted the regolith, it sank directly into the surface, penetrating nearly half a meter deep without slowing down. The spacecraft's thrusters fired within seconds to arrest the descent and push the probe away into space. Had the thrusters not fired immediately, the spacecraft might have continued plunging straight into the asteroid. The encounter left a noticeable excavation crater and threw up a colossal plume of dust and boulders, demonstrating that Bennu's outer layer was astonishingly soft.
The Anatomy of a Rubble Pile
The reason OSIRIS-REx sank so easily is that Bennu is not a solid rock. It is a classic 'rubble-pile' asteroid—a collection of shattered rock fragments, boulders, pebbles, and dust held together only by faint mutual gravity. Tracking data and navigational measurements revealed that Bennu has an exceptionally low bulk density. Between the individual stones lies an enormous amount of empty void space, making the entire asteroid porous and sponge-like in its overall architecture.
Planetary scientists determine that Bennu originated from the catastrophic breakup of a much larger, carbon-rich parent asteroid in the main asteroid belt between Mars and Jupiter. When a massive collision shattered that parent body billions of years ago, a portion of the expelled fragments slowly gathered together under the subtle pull of their own gravity. Because the gravitational field was so weak, the rocks were never compressed or fused into a solid geological mass, preserving huge gaps throughout the asteroid's interior.
Granular Physics in Microgravity
The dramatic sinking behavior during the TAG maneuver comes down to how granular materials behave in an environment with almost no gravity. On Earth, gravity continually pulls soil, sand, and gravel downward, locking the grains together with frictional force and giving the ground structural load-bearing capacity. On Bennu, the gravitational pull is thousands of times weaker than on Earth, leaving the individual rocks and dust particles resting against one another with virtually zero compressive force.
Under such weak confinement, granular material exhibits almost no shear strength and behaves more like a fluid than a solid. Pushing an object against the surface does not pack the material tightly underneath; instead, the particles effortlessly slide past each other and scatter outward. Scientists frequently compare this response to a children's ball pit or a container filled with plastic beads: applying downward pressure instantly displaces the surrounding balls, allowing an incoming object to sink effortlessly until an external force stops it.
Surprises Upon Arrival
The loose, treacherous nature of Bennu's surface was one of several major surprises the mission encountered. Before OSIRIS-REx arrived at Bennu in late 2018, ground-based radar and thermal observations had led scientists to anticipate smooth, broad expanses dominated by fine regolith resembling sandy beaches. When the spacecraft entered orbit and began close-range imaging, it revealed a rugged landscape heavily covered in massive, jagged boulders, some towering dozens of meters high.
This rugged terrain forced the navigation team to discard their original touchdown plans and search for tiny, hazardous clearings among the boulder fields. Furthermore, OSIRIS-REx observed that Bennu was dynamically active, regularly ejecting small pebbles and rocks into space from its surface. Some of these particles entered temporary orbits around the asteroid before escaping into deep space or falling back, confirming that rubble piles are dynamic environments constantly undergoing subtle surface evolution.
Sample Return and Solar System Origins
While the loose surface complicated navigation, it proved to be an immense advantage for sample collection. When the TAGSAM arm fired its nitrogen gas canister into the regolith, the absence of cohesion allowed an enormous quantity of dust and rock to be swept into the collector. In fact, so much material entered the head that large pebbles jammed the collection flap slightly open, allowing a small stream of particles to drift out into space before the arm was successfully sealed inside the sample return capsule.
In September 2023, the OSIRIS-REx capsule landed safely in the Utah desert, returning pristine, uncontaminated fragments of Bennu to Earth. Laboratory analysis confirmed that the carbonaceous asteroid contains abundant organic molecules and water-bearing clay minerals. Because Bennu formed in the earliest stages of the solar system and was never altered by intense geological heating, these samples provide direct evidence regarding the ancient reservoirs of water and prebiotic carbon that existed before planets fully formed.
Implications for Planetary Defense
Beyond unlocking the early history of the solar system, understanding Bennu's rubble-pile structure is essential for planetary defense. Bennu is tracked as one of the most potentially hazardous known near-Earth asteroids, with small, calculated possibilities of an impact with Earth in the late 22nd century. Designing a mission to deflect or alter the trajectory of an asteroid requires an accurate understanding of how its material responds to kinetic impacts or explosive standoff blasts.
A rubble-pile asteroid reacts very differently to deflection techniques than a solid monolithic rock. Because Bennu's interior is porous and loosely bound, an impactor could have much of its energy absorbed by crushing voids and displacing loose rubble rather than transferring momentum cleanly to shift the asteroid's orbit. Alternatively, an overly aggressive strike could shatter the asteroid into a cluster of smaller, independently hazardous fragments. The OSIRIS-REx encounter provides valuable empirical data on how granular asteroids behave, informing future strategies to protect Earth from potential impacts.
Key takeaways
•Bennu is a porous rubble pile held together by extremely weak gravity, lacking the cohesive strength of solid rock.
•During sample collection, OSIRIS-REx sank nearly half a meter into Bennu's surface because the loose regolith offered virtually zero mechanical resistance.
•The returned Bennu samples contain water-bearing minerals and organic carbon compounds preserved from the early solar system.
•Understanding the loose, granular physics of rubble-pile asteroids is critical for designing effective planetary defense and deflection strategies.