What Lies Inside Asteroid Bennu & Why Doesn’t It Have a Moon? Scientists May Have the Shocking Answer

Asteroids may look like solid rocks from space, but many are actually loose collections of rocks, boulders and dust held together mainly by gravity and weak forces. These objects are known as rubble-pile asteroids, and understanding what lies inside them is crucial for explaining how they formed, changed over time and respond to forces such as rotation.

Now, a study led by Yun Zhang and colleagues has used advanced computer simulations to investigate the hidden interior of Bennu, the 500-meter-wide asteroid explored in detail by NASA’s OSIRIS-REx mission. The results suggest that Bennu is much weaker internally than a solid rock, with a complex structure containing several stronger regions.

The findings could also explain a major mystery: why Bennu does not have a moon, despite rotating fast enough for material to potentially escape its surface.

Bennu Is Not a Solid Rock

Scientists believe that many asteroids between roughly 200 meters and tens of kilometers across are rubble piles. Instead of being single solid objects, they may have formed when the remains of a larger asteroid were shattered by a massive collision and later came back together under their own gravity.

Bennu is one such asteroid.

Its distinctive shape resembles a spinning top, with a wider equatorial region and a narrower body toward its poles. Several other asteroids have similar shapes, including Ryugu, which was visited by Japan’s Hayabusa2 mission.

But there is a major problem: scientists cannot directly drill deep into an asteroid to examine its interior.

Instead, researchers have to work backwards. They study an asteroid's surface, rotation, gravity field and movement of rocks, and then use physics-based simulations to determine what kind of internal structure could produce those observations.

That is exactly what Zhang and the team attempted with Bennu.

The Asteroid Is Being Spun Up

One of the most important forces affecting Bennu is the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect.

This effect occurs because an asteroid absorbs sunlight and later releases heat back into space. Because the asteroid has an irregular shape, the outgoing thermal radiation can produce a tiny torque.

Over very long periods, this torque can gradually change an asteroid's rotation.

Bennu is currently experiencing rotational acceleration. According to observations, its spin rate is increasing by about 3.63 × 10⁻⁶ degrees per day².

That may sound extremely small, but over millions of years it can significantly change the asteroid.

If Bennu's current spinup continued steadily, its rotation rate could double in about 1.5 million years in its current near-Earth orbit. In the inner asteroid belt, the estimated timescale would be around 6 million years.

This creates an interesting problem.

Bennu has large craters near its equator that appear to be much older than these rotational timescales. If increasing rotation can destabilize an asteroid, how did these surface features survive?

The answer may lie deep inside Bennu.

A Weak Interior With Stronger Hidden Regions

To investigate this, Zhang and colleagues used a numerical technique called the Soft-Sphere Discrete Element Method (SSDEM).

The method allows researchers to model an asteroid as a huge collection of interacting particles and study how those particles respond as the asteroid's rotation increases.

The team tested different combinations of material strength and friction and compared the simulated behavior with observations made by OSIRIS-REx.

Their results point toward a surprisingly weak asteroid.

The simulations suggest that Bennu's surface cohesion is likely below about 0.78 pascals, while the interior cohesion is generally below about 1.3 pascals. Its friction angle is also likely below approximately 35 degrees.

However, Bennu does not appear to be uniformly weak.

The researchers found that several locally stronger internal regions can exist inside the generally weak rubble pile. These stronger areas could correspond to large boulders or concentrations of cohesive fine material.

This combination can reproduce several of Bennu's observed characteristics at the same time.

Why Doesn't Bennu Have a Moon?

This is one of the most interesting parts of the study.

When a rapidly rotating rubble-pile asteroid becomes unstable, material can move toward its equator. If enough material escapes the asteroid, it can potentially gather into a smaller body—a moon.

Many top-shaped asteroids are accompanied by satellites, so scientists have long considered rotational spinup an important pathway for creating asteroid-moon systems.

But Bennu has no known moon.

The simulations offer a possible explanation.

If Bennu were stronger throughout its interior, increasing rotation could cause material to shed from its surface. That escaping material could eventually form a satellite.

But a weak, deformable interior behaves differently.

Instead of simply losing material into space, parts of the asteroid can rearrange and deform internally as its rotation increases. This allows Bennu to respond to rotational stress without necessarily throwing large amounts of material away.

In other words, its weakness may actually help it survive rotational instability.

What Happened to Bennu's Surface?

OSIRIS-REx also observed evidence of relatively recent mass movement on Bennu. Some material appears to have moved downslope within roughly the past 200,000 years, when the asteroid was already rotating at a rate close to today's value.

The asteroid also contains long linear surface features that may indicate regions with greater internal stiffness.

At the same time, gravity measurements show that Bennu has a heterogeneous mass distribution, suggesting that its interior is not uniform and may contain lower-density regions.

The new simulations provide a framework that can connect these seemingly different observations.

A globally weak structure could allow some areas to deform and move, while stronger internal zones could preserve certain structures and influence how the asteroid responds to increasing rotation.

A Window Into Bennu's Violent Past

The researchers say the inferred weak interior is consistent with Bennu's possible origin.

The asteroid may have formed when a much larger parent body was catastrophically shattered. The resulting fragments eventually came back together under gravity, creating a loosely packed rubble pile.

This process, known as gravitational reaccumulation, could naturally produce an asteroid with low resistance to internal deformation.

The stronger regions inside Bennu could have formed from large surviving boulders or concentrations of cohesive material during this reaccumulation process. They might also have developed later.

This means Bennu's present-day structure could preserve clues about the violent collision that created it.

The Mystery of Bennu's Missing Fine Dust

Another interesting clue comes from Bennu's surface.

Scientists expected fine dust and tiny particles to be present, but Bennu's surface appears to lack large amounts of such material. Yet fine grains were detected beneath the surface during the OSIRIS-REx sampling operation.

Several processes could explain this.

Impacts and thermal stress may produce fine particles, while electrostatic forces could lift them from the surface. Because Bennu has large empty spaces between its rocks, some particles may also move downward into the subsurface during impacts or seismic shaking.

The study suggests that the weak and highly porous structure of Bennu may make it difficult for fine material to remain distributed across the entire surface.

A Bigger Lesson About Asteroids

The importance of this research goes beyond Bennu.

The study demonstrates that scientists can potentially use surface observations to infer an asteroid's hidden interior.

Different combinations of strength and friction can lead to completely different evolutionary outcomes. Some asteroids may deform internally as their rotation increases, while others may lose material and potentially create moons.

For carbon-rich rubble-pile asteroids with properties similar to Bennu, the researchers suggest that weak internal structures could promote deformation while reducing the likelihood of satellite formation.

This could eventually help scientists understand why some spinning-top asteroids have moons while others do not.

More importantly, it shows that an asteroid's surface is not simply a collection of rocks sitting on the outside. Its craters, boulders, slopes, rotation and gravity field can act as clues to what is happening hundreds of meters below.

Bennu may look like a simple pile of rocks, but its behavior reveals a surprisingly complex interior—and that hidden structure could hold the story of how the asteroid was born, reshaped and continues to evolve through space.

Reference: Zhang, Y., Michel, P., Barnouin, O.S. et al. Inferring interiors and structural history of top-shaped asteroids from external properties of asteroid (101955) Bennu. Nat Commun 13, 4589 (2022). https://doi.org/10.1038/s41467-022-32288-y