Comets are called rubble piles because their nuclei are not solid, dense chunks of rock and ice. Instead, they are loose collections of smaller fragments held together mostly by their own weak gravity, with enormous amounts of empty space inside. The nucleus of a typical comet is 70% or more void by volume, closer to a pile of gravel than a solid snowball.
What “Rubble Pile” Actually Means
In astronomy, “rubble” has a specific technical meaning borrowed from geology: angular rock fragments with no cement or glue binding them together. A rubble pile body is distinguished from a cracked-but-intact object by the fact that its internal pieces have shifted and rotated relative to one another. Think of the difference between a cracked windshield (still one piece, just fractured) and a bag of broken glass (loose fragments that happen to be sitting together). Comets and some asteroids fall into the second category.
These fragments range from dust grains to boulders and are held in place primarily by gravity, not by any structural strength. Because the gravitational pull of a comet nucleus is vanishingly small, the whole assembly is extremely fragile. It absorbs impacts by compressing inward rather than fracturing outward, but long-term tidal forces from a planet’s gravity can pull one apart entirely.
Why Comets End Up This Way
Comets formed in the outer solar system through a process of gentle, low-speed collisions. Small icy and dusty bodies, sometimes called cometesimals, bumped into each other at speeds slow enough that they stuck together rather than shattering. Three-dimensional impact simulations show that these low-velocity accretionary collisions produce two distinct outcomes: thin layers that splat onto an existing body, or mergers between similarly sized objects that create the bilobed, “rubber duck” shapes seen in several comets. Both processes build up a nucleus with extremely low tensile strength, lots of internal gaps, and an irregular shape.
This gentle sticking process is fundamentally different from how rocky planets form. Earth and Mars grew large enough that their own gravity compressed and heated their interiors, fusing material together. Comets never got anywhere near that threshold. Their interiors remained a loosely packed jumble of whatever collided and stuck billions of years ago.
The Evidence From Comet 67P
The strongest direct evidence comes from the European Space Agency’s Rosetta mission, which orbited Comet 67P/Churyumov-Gerasimenko from 2014 to 2016. By tracking tiny changes in the spacecraft’s radio signal as the comet’s gravity tugged on it, scientists measured the nucleus’s mass with high precision. Combining that mass with volume measurements from onboard cameras produced a bulk density of just 533 kilograms per cubic meter, roughly half the density of water. For a body made of rock and ice (both of which are individually denser than water), that astonishingly low number means the interior is 72 to 74 percent empty space.
Comet 9P/Tempel 1, visited by NASA’s Deep Impact mission, showed a similar profile: low density, high porosity, and a dusty surface that behaved more like packed powder than solid ground. These aren’t outliers. Every comet nucleus measured so far fits the same pattern.
How Comets Differ From Rubble Pile Asteroids
Asteroids can also be rubble piles. The asteroid Bennu, sampled by NASA’s OSIRIS-REx mission, is a loosely bound collection of rocky fragments. The key difference is composition. Asteroids are primarily rocky, while comets are a mix of ice and dust. That ice content matters because it makes comets even more porous: ice sublimates (turns directly to gas) when heated by the Sun, hollowing out internal cavities and weakening whatever structural bonds existed. Over many orbits, this outgassing can further loosen an already fragile interior.
The ice also explains why rubble pile comets are especially vulnerable to breakup. As a comet approaches the Sun, jets of gas can exert uneven forces that spin the nucleus faster. If it spins fast enough, the centrifugal force at the equator overcomes the feeble self-gravity, and the whole thing flies apart. Tidal disruption during close planetary encounters poses another threat, stretching the loose aggregate until fragments separate.
Peering Inside a Comet
No one has ever directly seen the interior of a comet, which is part of why the rubble pile label took decades to confirm. The internal structure “is never directly observed but belies a violent history,” as one review in the Annual Review of Astronomy and Astrophysics put it. Scientists have relied on indirect clues: density measurements, surface geology, and breakup events observed through telescopes.
Radar is emerging as the most promising tool for direct interior imaging. The CONSERT experiment aboard Rosetta transmitted radio waves through the nucleus and discovered that cometary interiors are transparent to radar signals to depths of several kilometers. That transparency itself is telling, since a solid, uniform interior would scatter or absorb those signals differently than a porous one. At megahertz frequencies, radar can reveal internal layers, vents, and structural boundaries at scales of meters to tens of meters. Future missions could use orbital radar tomography, adapted from the same techniques geologists use to map Earth’s crust, to build full three-dimensional maps of a comet’s deep interior without ever landing on it.
Why the Label Matters
Calling comets rubble piles isn’t just a colorful nickname. It changes how scientists model their behavior. A solid body responds to impacts, tidal forces, and solar heating in predictable ways. A rubble pile absorbs collisions by compressing rather than cracking, reshapes itself under tidal stress, and can shed large chunks when spun up by outgassing jets. Planetary defense calculations for any comet or asteroid on a collision course with Earth depend heavily on whether the object is solid or loosely bound, because the strategy for deflecting a rubble pile is fundamentally different from deflecting a monolith.
The rubble pile model also reshapes our understanding of how the solar system formed. These fragile, porous bodies are essentially fossils of the gentle accretion process that built the outer solar system 4.6 billion years ago. Their interiors have never been compressed or melted, preserving the original arrangement of ice and dust from the solar nebula in a way that no planet or large moon can.