For decades, scientists have had a relatively simple picture of how asteroids are built. Small asteroids were thought to be loose collections of rocks and dust, often called “rubble piles,” while larger asteroids were expected to be more solid, fractured bodies that had survived billions of years of collisions.
Now, a new analysis of 433 Eros, one of the most extensively studied near-Earth asteroids, is challenging that picture.
Researchers led by Ballouz have studied how impact craters on Eros have been damaged and erased over time. Their results suggest that the asteroid's deep interior behaves much more like a rubble pile than a single, solid but fractured rock.
The finding could change our understanding of how medium-sized asteroids form, evolve and respond to impacts — and may even influence how humanity plans to defend Earth from a potentially hazardous asteroid.
Why Eros Is So Important
Asteroid 433 Eros is unusual because it is relatively large but still small enough to study as a single asteroid. It measures approximately 34 × 11 × 11 kilometres and was visited by NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft.
The mission produced detailed images of Eros, revealing a heavily cratered, dusty and rocky surface.
Three enormous craters dominate the asteroid's shape: Himeros, Shoemaker and Psyche. Among them, the approximately 7.5-kilometre-wide Shoemaker crater is particularly important for the new study.
Why?
Because a collision powerful enough to create such a huge crater would have sent seismic waves travelling through the asteroid. Those waves could shake the entire body and modify or erase smaller craters on its surface.
By examining which craters survived — and which disappeared — scientists can effectively use Eros's surface as a record of what happened inside it.
A Hidden Interior Revealed Through Craters
Scientists cannot simply drill into an asteroid to see its interior. Instead, they have to infer its structure from indirect evidence such as its density, rotation, gravity, surface features and impact craters.
Earlier studies suggested that Eros was a “fractured shard.”
In that picture, Eros would essentially be a large, coherent piece of rock that had been heavily damaged and cracked by collisions over billions of years. Although fractured, the different pieces would still remain strongly connected.
But the new analysis tells a different story.
The researchers modelled how seismic energy from the Shoemaker impact could have travelled through Eros and affected its surface. Their results indicate that seismic waves inside Eros are strongly scattered and lose energy relatively quickly.
The estimated seismic scattering length is about 0.5 ± 0.1 kilometres.
In simple terms, seismic energy does not travel cleanly through Eros over long distances. Instead, it is repeatedly scattered by structures inside the asteroid.
That behaviour resembles what scientists observe in loose, highly broken-up material such as planetary regolith.
And that leads to a surprising possibility: Eros may be a rubble pile.
What Exactly Is a Rubble-Pile Asteroid?
A rubble-pile asteroid is not simply a giant rock.
Imagine a huge asteroid being destroyed by a catastrophic collision. Its fragments could spread through space and later come back together under their own gravity. The resulting object would contain countless pieces of rock, boulders and smaller material held together mainly by gravity, with some internal friction and weak cohesion.
That is essentially a cosmic pile of debris held together as an asteroid.
Several smaller near-Earth asteroids studied by spacecraft have already shown evidence of this type of structure.
However, Eros is much larger.
Asteroids between roughly 0.2 and 10 kilometres are generally expected to contain rubble-pile structures, while objects around 10 kilometres and larger have traditionally been considered more likely to retain fractured but coherent interiors.
Eros sits right in this interesting transition zone.
If it really is a rubble pile, then the boundary between “small rubble pile” and “large fractured asteroid” may not be as clear as scientists once thought.
The Shoemaker Impact May Have Shaken the Whole Asteroid
One of the most fascinating parts of the research is the role of the Shoemaker crater.
When the enormous impact occurred, it would have generated seismic waves that travelled through Eros.
If Eros were a relatively solid body, these waves could potentially travel farther and behave differently. But if the interior were made of many large blocks separated by gaps and weak contacts, the waves would scatter much more strongly.
This could explain why Eros has an unusual shortage of small craters.
The asteroid is heavily cratered at larger scales, yet craters smaller than roughly 200 metres appear to be significantly depleted compared with what scientists would expect.
The new study suggests that powerful impacts may have produced seismic shaking capable of degrading or erasing smaller craters.
In other words, Eros's surface may be telling us what is happening deep beneath it.
But Eros May Not Look Like Smaller Rubble Piles
There is an important distinction.
Calling Eros a rubble pile does not mean that it must look exactly like smaller asteroids visited by spacecraft.
The researchers suggest that Eros could contain very large internal blocks, potentially hundreds of metres across. Its estimated seismic scattering length of about 500 metres supports this possibility.
Smaller kilometre-scale rubble-pile asteroids may contain much smaller blocks and therefore scatter seismic energy even more strongly.
So there may be a whole spectrum of rubble-pile structures rather than one standard type.
This could mean that as asteroids grow larger, their internal blocks become larger too, while the overall body can still behave mechanically like a rubble pile.
What Does This Tell Us About the Solar System?
The implications go far beyond Eros.
Many asteroids are thought to be fragments created when much larger parent bodies were catastrophically destroyed.
If Eros is also a rubble pile, it supports the idea that even intermediate-sized asteroids can form through this process.
That means an asteroid's present-day structure could preserve evidence of ancient collisions that occurred billions of years ago.
Studying these objects therefore gives scientists a way to reconstruct the collisional and dynamical history of the Solar System.
The structure of an asteroid is essentially a geological record — except instead of being buried underground, that record is floating through space.
Why This Matters for Planetary Defense
Understanding asteroid interiors is not only about solving scientific mysteries.
It could become extremely important if humanity ever needs to deflect a dangerous asteroid.
A spacecraft striking a solid rock and striking a loosely bound rubble pile could produce very different results. The asteroid's internal structure determines how impact energy is absorbed, transmitted and redistributed.
A rubble-pile asteroid might absorb an impact differently from a coherent rock. Its fragments could move relative to one another, dissipating energy throughout the body.
Knowing this structure could therefore help scientists design more effective asteroid-deflection strategies.
It could also improve our understanding of binary asteroid systems, where two bodies interact gravitationally and tidally.
The Mystery Is Not Completely Solved
The researchers emphasize that their results do not provide an absolutely definitive picture of Eros's interior.
There is currently no simple numerical value that can be measured remotely and immediately translated into “this asteroid is definitely a rubble pile.”
Their conclusion is based on comparisons with laboratory experiments and seismic observations from planetary bodies such as the Moon and Mars.
So there remains an important possibility: Eros could still be an extremely fractured but coherent asteroid whose interior behaves mechanically like a rubble pile.
That uncertainty is precisely why future asteroid missions will be so valuable.
Direct seismic measurements on an asteroid could provide much stronger evidence about how waves travel through its interior.
A New Picture of Eros
For years, Eros was viewed as an example of a medium-sized asteroid with a heavily fractured but fundamentally coherent interior.
The new research suggests something more surprising.
Beneath its dusty, cratered surface, Eros may be a gigantic gravitationally bound collection of rocks and blocks — a rubble pile on a much larger scale than previously expected.
If this interpretation is correct, Eros could represent an important missing link between the large parent bodies that formed early in Solar System history and the small rubble-pile asteroids we see today.
And perhaps the most fascinating lesson is that scientists did not need to see inside Eros directly.
They read its interior through the scars left on its surface.
That makes every crater, crack and missing crater on an asteroid more than just a surface feature — it can be a clue to the hidden architecture of an entire world.
Reference: Ballouz, RL., Ernst, C.M., Barnouin, O.S. et al. Seismic resurfacing of 433 Eros indicative of a highly dissipative interior for large near-Earth asteroids. Nat Astron 9, 347–357 (2025). https://doi.org/10.1038/s41550-024-02411-8
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