The asteroids in our Solar System are survivors. They’ve withstood billions of years of collisions. The surviving asteroids are divided into two groups: monolithic asteroids, which are intact chunks of planetesimals, and rubble piles, which are made of up fragments of shattered primordial asteroids.
It turns out there are far more rubble pile asteroids than we thought, and that raises the difficulty of protecting Earth from asteroid strikes.
The early days of planetary formation were marked by endless collisions that shattered countless planetesimals. The fragments populate the main asteroid belt and other regions in the inner Solar System. But some of those fragments reassembled into rubble pile asteroids, and surprisingly, they’re more resistant to collisions and harder to destroy than their monolithic brethren.
Rubble-pile asteroids are detectable by their density, which is much lower than monolithic asteroids. Peanut-shaped Itokawa was the first confirmed rubble-pile asteroid, and astronomers think the well-known asteroids Bennu and Ryugu are both rubble-pile asteroids, too. When the Japanese spacecraft Hayabusa visited Itokawa in 2005, images showed that its surface was free of impact craters, a dead giveaway that it was a loose collection of rubble since a monolithic asteroid would most certainly show signs of impacts.
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 “Rubble pile asteroids are forever,” and the lead author is Professor Fred Jourdan from the School of Earth and Planetary Sciences at Curtin University.
Itokawa is only about 500 meters long and is about 2 million km (1.2 million miles) from Earth. Hayabusa collected 1500 tiny grains of rock from the asteroid, and they were returned to Earth in June 2010. This research article is based on the study of three of those particles, and thanks to advanced analytical technologies, those three particles revealed a lot.
Scientists think that monolithic asteroids have a lifespan of a few hundred million years. For asteroids in the main belt, it’s 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 aren’t as brittle and can last much longer.
“Unlike monolithic asteroids, Itokawa is not a single lump of rock but belongs to the rubble pile family, which means it’s entirely made of loose boulders and rocks, with almost half of it being empty space,” Professor Jourdan said.
This image from JAXA’s Hayabusa spacecraft shows a boulder on Itokawa’s surface. Hayabusa’s images were the first to show the existence of rubble pile asteroids. JAXA scientists wrote: “This is a very important clue to studying the asteroid’s formation history. It is safe to assume that a larger celestial body originally existed before Itokawa. And on its destruction, a fragment from it became Itokawa as other finer fragments piled on the asteroid surface.” Image Credit: JAXA
While monolithic asteroids can be shattered by collisions, rubble piles are more elastic and can more easily absorb kinetic energy. An impact can alter a rubble pile’s shape without shattering it. The new research shows that Itokawa is extremely ancient—more than four billion years old. It wouldn’t have survived this long unless it was a rubble pile.
“The survival time of monolithic asteroids the size of Itokawa is predicted to be only several hundreds of thousands of years in the asteroid belt,” Jourdan said. “The huge impact that destroyed Itokawa’s monolithic parent asteroid and formed Itokawa happened at least 4.2 billion years ago. Such an astonishingly long survival time for an asteroid the size of Itokawa is attributed to the shock-absorbent nature of rubble pile material.”
“In short, we found that Itokawa is like a giant space cushion and very hard to destroy.”
“Hence, such asteroids represent 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 misalignment in the crystal structure that results from heat and shocks. Along with other analytical techniques, the analysis showed that the three fragments were “initially located deep in the monolithic parent asteroid,” the paper states.
Deep inside the asteroid, they were protected from all the bombardment and shock heating in the Solar System’s early, chaotic era. These particles were from the surface of Itokawa, and if they’d been there since the early days, they would’ve shown evidence of shock and heating. Collisions are far too plentiful for an asteroid to avoid them. The particles show evidence of only weak shocks and heating. “In order to be affected or subsequently affectable by impact-related thermal events at ~4.2 Ga, the particles would need to be brought near the surface, either by total disruption of the parent body or by deep crater excavations,” the authors explain in their paper.
The research explains Itokawa’s history. 4.6 billion years ago, a monolithic asteroid formed that was Itokaway’s parent body. Between 4.6 and 4.2 billion years ago, successive impacts created progressive fracturing. Then 4.2 billion years ago, one of two things happened. Either an impact excavated a deep crater, or else it totally destroyed the asteroid. In a very short period of time, the debris reformed into Itokawa. Throughout its history since then, Itokawa’s suffered many impacts, but the asteroid’s rubble-pile nature allowed it to absorb those impacts without being cratered or destroyed.
This figure from the study explains Itokawa’s history. Image Credit: Jourdan et al. 2023.
“Argon dating reveals the age of the particles as about 4.2 billion years. “Such a long survival time for an asteroid is attributed to the shock-absorbent nature of rubble pile material and suggests that rubble piles are hard to destroy once they are created,” the authors write.
The results apply to more than only Itokawa. If they’re so much harder to destroy, then there’s likely a much higher population of rubble-pile asteroids than thought. We know that Bennu, Ryugu, and others are rubble-pile asteroids. That has implications for our ability to defend Earth from asteroid strikes.
This image shows Bennu’s boulder-strewn surface. When NASA’s OSIRIS-REx collected samples, the sampling arm sank much deeper into the asteroid than expected, indicating that it’s a rubble-pile asteroid. Image Credit: NASA/University of Arizona.
“We set out to answer whether rubble pile asteroids are resistant to being shocked or whether they fragment at the slightest knock,” Associate Professor and co-author Nicholas Timms said. “Now that we have found they can survive in the solar system for almost its entire history, they must be more abundant in the asteroid belt than previously thought, so there is more chance that if a big asteroid is hurtling toward Earth, it will be a rubble pile.”
In an article in The Conversation, Jourdan emphasized the threat they pose. “In fact, they are very abundant, and since they are the shattered bits of monolithic asteroids, they are relatively small and thus hard to spot from Earth,” he writes. “Hence, such asteroids represent 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 that kinetic impactors like in NASA’s DART mission might not effectively direct them away from Earth. “Here, we showed that small rubble pile asteroids can survive billions of years against the ambient bombardment in the inner solar system due to their resistance to collisions and fragmentations. Therefore, more aggressive approaches (e.g., nuclear blast deflection) might have a higher chance of success against rubble pile asteroids,” the authors write in their paper.