Asteroid Formation: From Cosmic Dust to Small Rocky Bodies

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Asteroid Formation

Asteroids as witnesses of the primitive Solar System

Asteroids (small rocky and metallic bodies of the Solar System) are the fossil witnesses of the formation of the Solar System. They represent primordial bodies (primitive, undifferentiated fragments of the protoplanetary disk) that have undergone little thermal and mechanical transformation since their birth. Their chemical and isotopic composition provides valuable clues about the initial conditions of the protoplanetary disk and the early accretion processes.

Dust aggregation and planetesimal formation

In the protoplanetary disk, cosmic dust interacts through electrostatic forces and weak collisions. Micron-sized grains aggregate to form millimeter-sized, then centimeter-sized aggregates, up to the size of planetesimals (solid bodies from 1 to 100 km in diameter). Growth is not uniform: it is modulated by effects such as gravitational instability of the disk (local gravitational instabilities in the protoplanetary disk favoring matter concentration) and turbulent eddies (zones of turbulence and vortices in the protoplanetary gas) that concentrate particles.

Accretion, collisions, and diversity of asteroids

Recent numerical simulations indicate that these mechanisms can accelerate the formation of bodies several kilometers in size in just a few thousand years. Collisions can be both constructive, allowing matter accretion, and destructive, partially fragmenting bodies to form re-accreted debris (secondary fragments that can fall back onto other bodies). This cycle of coalescence and fragmentation has led to the diversity observed today in the main belt and in the populations of Trojans (asteroids captured at Jupiter's Lagrange points).

The velocity paradox: how can fast objects assemble weakly?

A legitimate question arises: if asteroids move at relative velocities of several kilometers per second, how can they agglomerate into such weakly cohesive rubble piles? The answer lies in several factors:

• Gas drag: in the primitive protoplanetary disk, the gas (hydrogen, helium) had not yet completely disappeared. It exerted an aerodynamic drag (aerodynamic drag force slowing down small bodies) that progressively reduced the relative velocities of planetesimals, allowing them to "meet" at speeds of only a few meters per second, well below the fragmentation threshold.
• Low relative velocities in accumulation zones: turbulent eddies and local gravitational instabilities concentrate matter in regions where orbital velocities are nearly identical. Two neighboring bodies can then have relative velocities on the order of a few centimeters per second, allowing gentle accretion (gentle merging without destruction).
• Very weak but sufficient internal gravity: for a few-kilometer asteroid, surface gravity is less than \( 10^{-3} \, \text{m/s}^2 \). This is 1,000 times weaker than on Earth, but in a frictionless environment with near-zero relative velocities, this weak attraction is enough to hold fragments together, even with minimal cohesion.

This combination explains why house-sized boulders can pile up without being welded, forming porous and fragile edifices. A useful analogy: imagine two cars driving at 100 km/h on a highway, but with a speed difference of only 1 km/h. They can make contact almost gently, without violent shock. Similarly, in the protoplanetary disk, planetesimals often move in orbital "platoons," where relative velocities are minimal.

Abundance of small asteroids

The asteroid population is strongly dominated by small bodies, from a few meters to a few kilometers in diameter. These small asteroids are extremely numerous compared to large planetesimals and large asteroids. This distribution results both from continuous fragmentation due to collisions and from limited accretion for the most massive bodies. The main belt thus contains billions of small asteroids, forming a veritable "cloud" of rocky and metallic fragments. Their abundance partly explains the high frequency of meteorites originating from these objects when they cross Earth's orbit.

N.B. :
Data obtained by space missions NEAR Shoemaker (nASA Near Earth Asteroid Rendezvous – Shoemaker) and OSIRIS-REx (eSA Rosetta and asteroid mission) have validated these models by providing direct measurements of composition, density, and internal structure of asteroids. Radar observations have also confirmed that asteroids like Itokawa or Bennu are indeed agglomerates of disjointed blocks, with significant internal voids.

Radiography of small asteroids

Stacked blocks under low gravity

Small asteroids are not monolithic blocks but rubble piles (bodies composed of rock and dust aggregates held together by gravity), agglomerates of fragments ranging from pebble-sized to house-sized. This high porosity means that internal cohesion is very weak, maintained primarily by gravity and sometimes by electrostatic forces between grains.

A fragile balance between velocity and low gravity

How can these rubble piles form and persist without disintegrating under impacts? The key lies in the fact that relative velocities between constituent blocks are, in most cases, extremely low (a few centimeters per second). These blocks settled onto one another during slow accretion, and gravity, although modest, is sufficient to keep them in equilibrium. Electrostatic forces (van der Waals) also play a role for the finest grains, creating additional cohesion at very small scales.

However, this equilibrium is fragile: a collision with a faster object or a gravitational perturbation (like a planetary flyby) can destabilize the whole, causing internal rearrangement or partial fragmentation. This explains why rubble pile asteroids often exhibit loose regoliths (debris formations on the surface) and impact craters with rounded edges, signs of poorly consolidated material.

Porosity and fragility of small asteroids

The asteroid belt hosts a great diversity of these small bodies. Their structure is such that numerous internal fractures and cavities exist, making these objects fragile and sensitive to collisions. The average density of some rubble piles (small S-type and C-type asteroids measured by space missions) can be lower than that of solid rocks, indicating that up to 50% of the volume may be porous void.

In such asteroids, a human hand could theoretically push into them without encountering significant resistance. This weak cohesion also explains why many small asteroids can easily fragment during collisions or gravitational flybys, generating debris and altering the dynamics of the main belt.

N.B. :
Radar observations and density measurements from missions such as NEAR Shoemaker (nASA Near Earth Asteroid Rendezvous – Shoemaker) on Eros and Hayabusa (jAXA Hayabusa and Hayabusa2 missions on asteroids) on Itokawa have confirmed the "rubble pile" structure for many small asteroids. Data from the OSIRIS-REx probe on asteroid Bennu also show a surface so loose that the spacecraft almost sank into it during sample collection.

Comparison of main-belt asteroids

This table presents the ten most massive asteroids in the main belt, ranked by decreasing diameter. It highlights the spectral diversity of these primitive bodies, from carbonaceous types (C, B, F, D) to silicate types (S, V) and metallic types (M, X). Mass and density data, from the Dawn and Gaia space missions, allow constraining models of internal structure and evolution of the early Solar System.

N.B. :
Mass and density values are from the most recent data (missions Dawn, Gaia and shape models from light curves). Typical uncertainties are on the order of ±5% for masses and ±10% for densities, due to variable internal porosity. Links to dedicated articles allow for deeper exploration of each object. Psyche and Hektor have similar diameters (~225 km) but very different compositions (metallic vs primitive carbonaceous). Sylvia is remarkable for its triple system.