The surprisingly complicated shapes of small worlds in space

Introduction

The small worlds of our solar system — asteroids, dwarf planets, and small moons — come in an astonishing variety of shapes. Many are not neat spheres but irregular, elongated, bilobed, or even weirdly angular. Their forms record the physical processes that built and reshaped them over billions of years.

Why size matters: gravity versus rigidity

The central reason small bodies have varied shapes is the balance between self-gravity and material strength. Large planets and big moons are massive enough that gravity pulls material into an approximately spherical shape; internal forces and heat also allow slow flow that smooths irregularities. Small worlds, however, have weak gravity and often are held together by the rigidity and cohesion of rock, metal, ice, or granular material.

There is a rough size threshold: bodies above a few hundred kilometers across tend toward hydrostatic equilibrium (near-spherical), while smaller bodies retain irregular shapes. Exact thresholds depend on composition: icy objects can relax toward roundness at smaller sizes than rocky ones because ice deforms more easily under stress.

Common shape categories and why they happen

Irregular lumps and potato shapes

Many asteroids and small moons look like “potatoes” — lumpy, non-spherical bodies with several irregular prominences. These shapes arise because self-gravity is too weak to erase the dents and peaks created during formation or collisions. Fracturing from impacts produces large cliffs and basins, and a low-strength surface can preserve sharp features.

Examples: The asteroids 253 Mathilde and 951 Gaspra photographed by spacecraft show clear irregularities and large craters that dominate their overall shape.

Elongated bodies

Some small worlds are highly elongated, resembling prolate spheroids or stretching into irregular rods. Elongation can result from rapid rotation, tidal forces, or the gentle sticking together of fragments. If a parent body is spun up — for example by collisions or by a process called the YORP effect (a small torque from uneven sunlight emission) — centrifugal forces can pull mass toward the equator and, for weak bodies, cause the object to elongate or shed material.

Examples: The near-Earth asteroid 1620 Geographos is notably elongated; its stretched shape probably reflects a past history of collisions and spin changes.

Bilobed and contact binaries

A striking class of shapes is bilobed bodies: two rounded lobes connected by a narrower “neck.” These are typically contact binaries, formed when two separate bodies gently collided and stuck together rather than merging violently. The neck often shows signs of weaker cohesion or past deformation.

Examples: Comet 67P/Churyumov–Gerasimenko photographed by the Rosetta mission and the Kuiper belt object 486958 Arrokoth imaged by New Horizons both show clear bilobed structures that preserve the memory of two earlier building blocks.

Highly cratered, faceted, and angular forms

Some small worlds appear angular or faceted, with flat surfaces and sharp edges. This geometry can come from catastrophic impacts that shatter a body and leave large blocks or facets, or from the accumulation of large boulders and rubble that form a blocky surface. In rubble-pile bodies (assemblies of rock fragments held by gravity and friction), large constituent blocks can create stepped or planar features.

Examples: The asteroid Ryugu and Bennu, visited by Hayabusa2 and OSIRIS-REx respectively, show boulder-strewn, angular surfaces and large-scale facets that reflect their rubble-pile interior structure.

Processes that change shapes over time

Collisions and fragmentation

Impacts are the dominant sculptors of small bodies. High-velocity collisions can crater a body, remove large chunks, or shatter it entirely. When a large object breaks up, the fragments can reaccumulate under mutual gravity into rubble-pile asteroids with irregular shapes determined by how pieces reassemble. Reaccumulation tends to produce hollow, porous structures rather than solid monoliths.

Rotational evolution and shedding

Rotation can dramatically alter shape. When a small body spins faster, centrifugal force at the equator increases. If rotation becomes rapid enough, material can migrate outward, creating an equatorial ridge or even shedding mass that forms satellites. In some cases, fast rotation causes elongation or fission, producing binary systems or contact binaries when fragments rejoin.

One pathway to rapid spin is the YORP effect, where uneven sunlight heating and re-radiation on small, irregular bodies produces tiny torques that accumulate over millions of years. YORP can speed up a body slowly but significantly, changing its shape or creating companions.

Tidal interactions

When a small body passes close to a much larger mass, tidal forces from the larger body’s gravity can stretch and distort the smaller one. Close encounters with planets sometimes produce dramatic changes: a tidal pull can elongate an object, strip material, or disrupt a weak body entirely. Repeated tidal stresses may weaken internal structure and promote reshaping over long timescales.

Sublimation and thermal effects

For icy comets and outer solar system objects, heating near the Sun causes ice to sublimate (turn from solid to gas). Outgassing can erode surface layers, forming pits, jets, and cliff retreat that change the object’s shape. Thermal cycling — the repeated heating and cooling each orbit — can fracture surface material and cause gradual breakdown, especially for porous or icy bodies.

Interior structure and its influence

Shape often reflects internal composition and structure. Two end-member types are monolithic bodies (solid rock or metal) and rubble piles (collections of fragments with significant internal porosity). Monoliths can maintain sharper features and thinner structures, while rubble piles settle into shapes set by how fragments pack and by the balance of gravity and friction.

Rubble-pile asteroids tend to have lower bulk density and may have internal voids. Their surfaces show abundant large boulders and blocky features because the larger pieces dominate the mechanical behavior. Conversely, bodies with differentiated interiors (layers formed by melting and separation of materials) may be more spherical if they were molten early in their history and later cooled.

Examples from spacecraft missions

Close-up images from spacecraft have revolutionized our understanding by revealing real shapes and surface details.

Rosetta’s visit to comet 67P/Churyumov–Gerasimenko revealed a bilobed, necked shape with cliffs and layered deposits that point to a gentle contact merger of two bodies and subsequent surface evolution by sublimation and collapse.

New Horizons flew past Arrokoth (formerly called Ultima Thule) in the Kuiper belt and found a remarkably smooth bilobed body whose lobes were clearly distinct, suggesting low-speed accretion in the early solar system rather than violent collisions.

Hayabusa2’s images of Ryugu and OSIRIS-REx’s images of Bennu showed near-Earth rubble-pile asteroids with abundant boulders, angular facets, and evidence of fast rotation and regolith motion. These missions also found signs of surprising surface activity like particle ejection events.

What shapes tell us about formation and history

Shapes are like a record of a small world’s past. Bilobed and contact-binary shapes indicate low-energy accretion in the early solar system or the gentle coalescence of fragments. Highly irregular, heavily cratered shapes point to a long history of impacts and minimal internal relaxation. Elongation and equatorial ridges suggest rotational evolution or mass movement. Angular, blocky surfaces imply a rubble-pile interior and reaccumulation after disruption.

By combining shape with measurements of rotation, density, surface composition, and crater statistics, scientists reconstruct a body’s formation timeline, collisional history, and internal properties. That helps us understand planet formation, the delivery of water and organics to early Earth, and potential hazards posed by near-Earth objects.

Implications for science and exploration

Shape matters for mission planning. A lander or sample-return mission must account for surface slopes, boulder fields, and weak gravity that complicates touchdown and mobility. The irregular gravitational field of a non-spherical body affects spacecraft orbits and navigation. For asteroid deflection or resource utilization concepts, knowing whether a target is a monolith or rubble pile is crucial because it changes how the body will respond to applied forces.

Science benefits too: surfaces preserve records of solar system processes, and bilobed or contact-binary shapes directly sample otherwise separate building blocks. Studying small-body shapes yields insight into how matter assembled in the early stages of planetary systems.

Open questions and ongoing research

Despite progress, many questions remain. The exact thresholds where objects relax to roundness depend on poorly known material properties at different temperatures and pressures. The frequency and timescales of reaccumulation after disruptions are still being refined by numerical models and observations.

New missions and surveys will improve statistics. Larger telescopic surveys detect more small bodies and measure shapes indirectly through lightcurves (changes in brightness as an object rotates). Future spacecraft visits will continue to reveal surface detail and interior structure, helping to connect physical models with real examples.

Conclusion

The surprisingly complicated shapes of small worlds arise from a mix of gravity, material strength, collisions, rotation, tidal forces, and thermal effects. Their irregular, elongated, bilobed, and faceted forms are natural outcomes of these competing processes and of the objects’ internal structure. Studying these shapes gives scientists a window into solar system formation, collisional evolution, and the practical challenges of exploring and interacting with small bodies.