Astro Bob: If you could stand on asteroid Bennu you'd sink right in!

When OSIRIS-Rex touched down to grab a sample of the asteroid in October 2020 it encountered a surface resembling a children's 'ball pit.'

Bennu blast
On October 20, 2020, NASA’s OSIRIS-REx spacecraft kicked up roughly six tons of loose rock and dust when it sampled the surface of near-Earth asteroid Bennu. The collection arm sank far deeper than expected, revealing surprising details about how loosely-packed the surface is. The spacecraft will return approximately 8.8 ounces (250 grams) of samples to Earth in September 2023.
Contributed / Goddard Space Flight Center, CI Lab, Jonathan Nort
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As kids we both feared and loved quicksand . If you've ever watched a Tarzan movie you know what I mean. Basically, quicksand is sand that looks solid but is so saturated with water it acts like a liquid. In the Hollywood depiction, the more you struggled the farther it would suck you under — the jungle's version of a black hole. Without help from a friend or a well-placed vine you were doomed.

Bennu Feb darker.png
Bennu is a dark, carbon-rich asteroid just 1,722 feet (0.3 miles) across with a diamond-like shape. It's a potentially hazardous object with a 1-in-1,800 chance of impacting Earth between 2178 and 2290. NASA's Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission explored and sampled Bennu from December 2018 to May 2021.
Contributed / NASA, Goddard, University of Arizona

It turns out that the surface of the asteroid Bennu, sampled by NASA's OSIRIS-REx mission in October 2020, shares some of quicksand's qualities. Data analysis revealed that the spacecraft would have sunk into the asteroid during sampling had it not fired its thrusters to quickly back away from its surface.

Bennu popcorn
Bennu's surface is packed together nearly as lightly as a bowl of popcorn.
Contributed / NASA Goddard

It turns out that the rocks and "dirt" comprising Bennu are so lightly packed that if you were to step onto the surface you'd feel little resistance, like walking into one of those plastic 'ball pits' in a children's play area. An easier way to experience the sensation is to stick your hand into a bowl of popcorn.

“If Bennu was completely packed, that would imply nearly solid rock, but we found a lot of void space in the surface,” said Kevin Walsh , a member of the OSIRIS-REx science team.

The latest findings about Bennu’s fluffy surface were published on July 7 in a pair of papers in the journals Science Advances (free read) and Science , led respectively by Kevin Walsh, a member of the OSIRIS-REx science team, and Dante Lauretta, principal investigator of the probe.


Bennu ejects rocks
Bennu surprised scientists by ejecting bits of rock either through thermal stress or micrometeorite (dust-sized meteoroid particles) impacts.
Contributed / NASA, Goddard, University of Arizona

From the start Bennu has been full of surprises. Before the probe's arrival, Earth-based observations indicated a relatively smooth surface, but when the spacecraft arrived in December 2018 it sent back photos of a landscape littered with boulders. Later images revealed that the asteroid was shooting rocks into space, something never before observed.

Researches believe thermal stress may be the reason. Day-night temperature changes cause rocks to expand and contract. Cracks can form, releasing small chips that fly off into space in Bennu's low gravity — less than 1/100,000 that of Earth. Micrometeorite bombardment may also be a contributor, chipping away at the surface rocks.

But back to our main story. Scientists were stunned when they saw the huge wall of debris radiating from the sample site in images returned by the probe.

“We were like, ‘Holy cow!’” said Lauretta.

The sample arm touched the surface gently at the "walking pace of an insect," then released a blast of pressurized nitrogen to lift and capture material inside its collector head. Mission scientists expected the touch-and-go to leave only a small impression. Instead it excavated a crater 26 feet (8 meters) wide and 27 inches (68 cm) deep.

Even as thrusters fired to back the probe away from Bennu once sampling was finished, it continued to plunge into the asteroid. By the time the craft began to rise, it had sunk nearly 1.6 feet (0.5 meter) into the surface. Yikes.

Bennu is what astronomers call a rubble-pile asteroid, an accumulation of rocks and debris from past asteroid collisions held together by gravity. The material is loose to begin with, but its tiny size doesn't give Bennu much tug, so it can't compress and consolidate itself into a solid body like the moon or Earth. A visitor would have to "wade" across its surface.

That's good to know. With this new information scientists can better interpret Earth-bound observations of other asteroids, which could be useful in designing future asteroid missions and for developing methods to protect Earth from asteroid collisions. After dropping off its package of goodies in 2023, OSIRIS-REx will next rendezvous with the asteroid Apophis, another potentially hazardous body, when it makes an exceptionally close approach to Earth in 2029.


If one day we should find Bennu or another loosey-goosey asteroid aiming for our planet, a mission to send it off course by direct impact might not be wise. We'd likely shatter it into a horde of smaller projectiles reeling from the scene along unpredictable trajectories. Better to alter the object's path by using the gravity tractor method, where the mass of a spacecraft orbiting the asteroid gently tugs on the object, slowly changing its orbit.

People ask why we spend money on missions to collect samples from obscure places like Bennu. For a couple reasons. One is knowledge. We want to better understand how our solar system and in particular, how life came to be. Also — at least in the case of asteroids — what you don't know can kill you.

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"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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