The asteroids in our solar system are survivors. They have withstood collisions for billions of years. The surviving asteroids are divided into two groups: monolithic asteroids, which are intact chunks of planetesimals, and debris piles, which are fragments of shattered ancestral asteroids.
It turns out there are a lot more debris pile asteroids than we thought, and this increases the difficulty of protecting Earth from asteroid impacts.
The early days of planet formation were marked by endless collisions that shattered countless planetesimals. The fragments populate the main asteroid belt and other regions of the inner solar system. But some of these fragments have been assembled into debris pile asteroids, and surprisingly they are more resistant to collision and harder to destroy than their monolithic brethren.
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Debris pile asteroids are recognizable by their density, which is much less than monolithic asteroids. The peanut-shaped Itokawa was the first confirmed debris asteroid, and astronomers believe the well-known asteroids Bennu and Ryugu are also debris asteroids. When Japan’s Hayabusa spacecraft visited Itokawa in 2005, images showed that its surface was free of impact craters, a surefire sign that it was a loose collection of debris, since a monolithic asteroid would most likely show signs of impact.
Hayabusa brought home some samples from Itokawa, and a new research article in the Proceedings of the National Academy of Sciences is based on those samples. The article is titled “Debris pile asteroids are forever,” and the lead author is Professor Fred Jourdan of Curtin University’s School of Earth and Planetary Sciences.
Itokawa is only about 500 meters long and about 2 million kilometers from Earth. Hayabusa collected 1,500 tiny rock grains from the asteroid and they were returned to Earth in June 2010. This research article is based on examining three of these particles, and thanks to advanced analysis technologies, these three particles revealed a lot.
Scientists assume that monolithic asteroids have a lifespan of a few hundred million years. For asteroids in the main belt it is even shorter: a few hundred thousand years. There are so many opportunities for collisions in the Main Belt that few are likely to remain unscathed. But rubble piles are not as brittle and can last much longer.
“Unlike monolithic asteroids, Itokawa is not a single boulder but belongs to the debris heap family, meaning that it is composed entirely of loose boulders and stones, almost half of which is empty space,” Professor Jourdan said.
This image from JAXA’s Hayabusa spacecraft shows a boulder on the surface of Itokawa. Hayabusa’s images were the first to show the existence of debris pile asteroids. JAXA scientists wrote: “This is a very important clue for studying the asteroid’s formation history. It can be assumed that a larger celestial body originally existed before Itokawa. And upon its destruction, a fragment of it became Itokawa, while other finer fragments piled up on the asteroid’s surface.” Photo credit: JAXA
While monolithic asteroids can be shattered by collisions, debris piles are more elastic and absorb kinetic energy more easily. An impact can change the shape of a debris pile without breaking it. The new research shows that Itokawa is very old – more than four billion years old. It wouldn’t have survived this long if it hadn’t been a pile of rubble.
“The survival time of Itokawa-sized monolithic asteroids in the asteroid belt is estimated to be only a few hundred thousand years,” Jourdan said. “The massive impact that destroyed the Itokawa parent monolithic asteroid and formed Itokawa occurred at least 4.2 billion years ago. Such an astonishingly long survival time for an Itokawa-sized asteroid is attributed to the shock-absorbing nature of the debris pile material.”
“In short, we found that Itokawa is like a giant space cushion and is very difficult to destroy.”
“So such asteroids pose a major threat to Earth, and we really need to understand them better.”
Fred Jourdan, lead author, School of Earth and Planetary Sciences at Curtin University.
One of the methods the researchers used to study the three Itokawa fragments is called Electron Backscattered Diffraction. It uses an electron microscope to study the crystallographic structure and orientation of the rocks. It can detect misalignments in crystal structure caused by heat and shock. Along with other analysis techniques, the analysis showed that the three fragments “originally resided deep within the parent monolithic asteroid,” the paper said.
Deep within the asteroid, they were protected from all the bombardment and shock heating of the solar system’s early, chaotic era. These particles originated from Itokawa’s surface, and if they had been there since the early days, they would have shown signs of shock and heating. Collisions are far too common for an asteroid to avoid. The particles show only weak vibrations and heating. “To be affected or subsequently affected by impact-related thermal events at ~4.2 Ga, the particles would have to be brought close to the surface, either by complete destruction of the parent body or by deep crater excavation,” the authors explain in their paper.
The research explains the story of Itokawa. 4.6 billion years ago, a monolithic asteroid formed that was Itokaway’s mother body. Between 4.6 and 4.2 billion years ago, successive impacts led to progressive fracture. Then, 4.2 billion years ago, one of two things happened. Either an impact dug a deep crater or completely destroyed the asteroid. In a very short time, the rubble turned into Itokawa. Throughout its history, Itokawa has suffered many impacts since then, but the asteroid’s debris pile nature allowed it to absorb those impacts without cratering or being destroyed.
This character from the study explains the story of Itokawa. Photo credit: Jourdan et al. 2023
“Argon dating reveals the age of the particles to be around 4.2 billion years. “Such a long survival time for an asteroid is attributed to the shock-absorbing nature of the debris pile material and suggests that debris piles, once formed, are difficult to destroy,” the authors write.
The results don’t just apply to Itokawa. If they’re that much harder to destroy, then there’s probably a much larger population of debris pile asteroids than thought. We know that Bennu, Ryugu and others are debris pile asteroids. This has implications for our ability to protect Earth from asteroid impacts.
This image shows the boulder-strewn surface of Bennu. When NASA’s OSIRIS-REx collected samples, the sample arm sank much deeper into the asteroid than expected, suggesting it is a debris-pile asteroid. Image Credit: NASA/University of Arizona.
“We wanted to find out whether debris-pile asteroids are shock-resistant or whether they shatter at the slightest blow,” said associate professor and co-author Nicholas Timms. “Now that we’ve found that they can survive in the solar system for almost its entire history, they must be more common in the asteroid belt than previously thought, so there’s a greater chance that a large asteroid hurtling toward Earth will do so.” pile of rubble.”
In an article in The Conversation, Jourdan emphasized the threat they pose. “In fact, they are very numerous, and because they are shattered pieces of monolithic asteroids, they are relatively small and therefore difficult to see from Earth,” he writes. “So such asteroids pose a major threat to Earth, and we really need to understand them better.”
The risk for us is that these asteroids can absorb a lot of kinetic energy. That means kinetic impactors like those in NASA’s DART mission might not effectively steer them away from Earth. “Here we have shown that small debris-pile asteroids can survive billions of years against the surrounding bombardment in the inner solar system due to their resilience to collisions and fragmentation. Therefore, more aggressive approaches (e.g., nuclear blast deflection) may have a higher chance of success against debris-heap asteroids,” the authors write in their article.