How Dark Asteroids Die When They Wander Too Close to the Sun

For years, astronomers have noticed something strange about the region close to the Sun: there are far fewer asteroids there than expected. Even more puzzling, dark asteroids, the kind with low reflectivity, are almost completely missing. A new study published in the journal Icarus finally provides strong experimental evidence explaining why this happens. The answer turns out to be surprisingly dramatic: when dark asteroids get too close to the Sun, they don’t slowly erode or gently fall apart over millions of years — they can rapidly disintegrate under intense solar heating.

The research comes from a team led by Georgios Tsirvouils of Luleå University of Technology in Sweden, along with collaborators including Mikael Granvik from the University of Helsinki. Their work supports a theory first proposed in 2016, suggesting that near-Sun asteroids are destroyed by a process known as instantaneous thermally-driven erosion. In simpler terms, the Sun’s radiation heats these objects so violently that they essentially break themselves apart.

The Mystery of Missing Near-Sun Asteroids

Astronomers have long known that asteroids are scarce close to the Sun, especially inside Mercury’s orbit. Observations show that this shortage is most pronounced for low-albedo asteroids, which are darker and absorb more sunlight than brighter, reflective ones.

Several explanations had been proposed over the years. One idea suggested that tidal forces from the Sun slowly stretch and tear asteroids apart. Another proposed sublimation, where asteroid material gradually vaporizes due to intense heat. While both processes do occur, they operate on timescales of millions or even billions of years, which doesn’t fully explain how quickly dark asteroids seem to disappear from near-Sun regions.

This gap between theory and observation is what led researchers to explore more extreme mechanisms — and ultimately to the idea that solar heating alone might be far more destructive than previously believed.

Revisiting a Bold 2016 Theory

In 2016, Mikael Granvik and colleagues proposed that asteroids near the Sun don’t slowly fade away. Instead, they undergo rapid thermal breakdown caused by extreme temperature gradients. As the asteroid’s surface heats up faster than its interior, mechanical stresses build up until the material fractures violently.

At the time, this idea was largely theoretical, based on computer models and indirect observations. What the new study adds is direct experimental evidence showing that this kind of destruction really can happen — and happen fast.

Simulating the Sun in a Laboratory

To test the theory, the research team used a specialized facility known as the Space and High-Irradiance Near-Sun Simulator (SHINeS) at Luleå University of Technology. This chamber can recreate the vacuum of space and expose materials to solar radiation levels similar to those experienced extremely close to the Sun.

The scientists created pellets made from CI carbonaceous chondrite simulants, materials chosen because they closely resemble the composition of dark, carbon-rich asteroids. These samples were then placed in the chamber and subjected to increasing levels of solar-like radiation.

What happened next was anything but subtle.

Watching Asteroids Fall Apart in Real Time

As the simulated solar intensity increased, the asteroid samples went through three clearly defined stages of destruction, all captured on camera.

The first stage was initial heating, during which the surface temperature rose and small amounts of dust were released. This phase looked relatively mild, suggesting that the asteroid was still structurally intact.

The second stage was far more violent. Known as explosive ejection, this phase involved millimeter-sized fragments being forcefully expelled from the surface. These weren’t slow crumbles — they were sudden bursts driven by internal stress.

The final stage, called subsurface degradation, occurred when heat penetrated deeper into the material. Internal expansion caused the structure to weaken from within, leading to rapid cracking and collapse.

At distances equivalent to about 0.22 astronomical units (AU) from the Sun, some samples managed to survive for a few hours. But when the radiation was increased to simulate conditions at 0.1 AU, the destruction became almost instantaneous. For context, Mercury orbits at about 0.39 AU, meaning these conditions are far more extreme than anything most planets experience.

Why Dark Asteroids Are Especially Vulnerable

The experiments also explain why dark asteroids are the ones that disappear first. Their low albedo means they absorb a much larger fraction of incoming solar energy. This leads to faster heating, stronger thermal gradients, and more intense internal stress.

Brighter asteroids, by contrast, reflect more sunlight and heat up more slowly. This makes them better able to survive closer to the Sun, at least for longer periods.

This difference neatly explains why surveys show such a strong lack of dark asteroids in near-Sun orbits.

Solving Puzzles in the Solar System

The findings also shed light on several long-standing astronomical mysteries.

One of them is 322P/SOHO 1, an object that passes extremely close to the Sun — as close as 0.05 AU. It brightens dramatically during its close approaches, much like a comet, but it doesn’t display a classic cometary tail. This odd behavior has led scientists to debate whether it’s a comet, an asteroid, or something in between.

The new study suggests that 322P/SOHO 1 could be a dark asteroid undergoing thermal erosion. The brightening may be caused by clouds of millimeter-sized dust fragments being explosively ejected from its surface as it heats up, rather than by ice sublimation like in traditional comets.

Another intriguing case is (3200) Phaethon, the rocky object responsible for the Geminid meteor shower. Laboratory results show erosion rates about 430,000 times faster than the mass loss observed from Phaethon. This discrepancy likely exists because Phaethon has already been “baked” by repeated close passes to the Sun, making it more resistant to cracking, or because debris ejected from a larger body may not escape its gravity as easily as in lab samples.

What This Means for Asteroid Science

This research strongly supports the idea that thermal processes alone can rapidly destroy asteroids near the Sun. It challenges the long-held assumption that such destruction must always be slow and gradual.

It also helps astronomers better interpret observations of near-Sun objects, some of which blur the line between asteroids and comets. Understanding how and why these bodies break apart improves models of asteroid populations, dust production near the Sun, and even meteor showers observed on Earth.

More broadly, the study highlights how extreme environments in the solar system can drive unexpected physical processes, reminding us that even seemingly solid rocks can behave violently under the right conditions.

Looking Ahead

The authors note that further work is needed to refine erosion models and test additional asteroid compositions. Future observations, especially of near-Sun objects during close approaches, could allow scientists to watch solar destruction in action, validating these laboratory results on a cosmic scale.

For now, the conclusion is clear: when dark asteroids stray too close to the Sun, they don’t just fade away — they are thermally torn apart, piece by piece, by the Sun’s relentless energy.

Research paper:
https://doi.org/10.1016/j.icarus.2026.116942