Asteroids in our solar system survive. They have survived billions of years of collisions. The surviving asteroids are divided into two groups: monolithic asteroids, which are intact masses of planetesimals, and piles of rubble, made up of shattered protoasteroid fragments.
It turns out that there are much more piles of rubble than we thought, which increases the difficulty of protecting the Earth from asteroid impacts.
The early days of planet formation were characterized by endless collisions that shattered countless planetesimals. Debris lives in the main asteroid belt and other regions within the solar system. However, some of these fragments re-aggregate into piles of rubble, surprisingly more crash-resistant and harder to destroy than their monolithic brethren.
Rubble-pile asteroids are detectable at much lower densities than monolithic asteroids.peanut mold Itokawa was the first confirmed Rubble Mountain asteroid, and astronomers believe that the famous asteroids Bennu and Ryugu are also both Rubble Mountain asteroids.Japanese spacecraft Hayabusa visited Itokawa in 2005 At that time, images showed no impact craters on its surface. As monolithic asteroids most reliably show signs of impact, it turns out to be a loose collection of rubble.
Hayabusa brought back several 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”A mountain of rubble Asteroid foreverThe lead author is Professor Fred Jourdan, Department of Earth and Planetary Sciences, Curtin University.
Itokawa is only about 500 meters long and about 2 million kilometers (1.2 million miles) from Earth. Hayabusa returned to Earth in June 2010 after collecting 1,500 small rock particles from the asteroid. This research article is based on her studies of three of those particles, which, thanks to advanced analytical techniques, reveal a lot.
Scientists believe monolithic asteroids have a lifespan of hundreds of millions of years. Main belt asteroids are even shorter, hundreds of thousands of years. There are so many opportunities for collisions on the main belt that few are likely to survive unscathed. But piles of rubble are less fragile and last much longer.
“Unlike monolithic asteroids, Itokawa is not a single mass of rock, but belongs to the rubble mountain family, meaning that it is made entirely of loose rocks and boulders, and nearly half of it is empty space. said Professor Jordan.
A monolithic asteroid can shatter upon impact, but a pile of debris is more resilient and can absorb kinetic energy more easily. Impact can change shape without shattering a pile of rubble. New research shows that Itokawa is very old, over 4 billion years old. If it weren’t for the pile of rubble, it wouldn’t have survived this long.
“The lifetime of a monolithic asteroid the size of Itokawa is predicted to be only a few hundred thousand years in the asteroid belt,” Jordan said. “The giant impact that destroyed Itokawa’s monolithic parent asteroid and formed Itokawa occurred at least 4.2 billion years ago. It’s because of the shock absorption.”
“In short, Itokawa is like a giant space cushion, and it turned out to be very difficult to destroy.”
One of the methods the researchers used to study the three Itokawa fragments is called electron backscatter diffraction. Electron microscopy is used to examine the crystal structure and orientation of rocks. It can detect crystal structure shift due to heat or impact. The analysis, along with other analytical techniques, showed that the three fragments were “originally located deep within the monolithic parent asteroid,” the paper said.
Deep inside the asteroid, they were protected from all the bombardment and shock heating of the early chaotic days of the solar system. If so, it would have shown evidence of impact and heating. Too many collisions for the asteroid to avoid collisions. Particles show evidence of only weak impact and heating. “To be affected or become affected by an impact-related thermal event of 420 million gallons or less, the particles must be moved closer to the surface by either completely destroying the matrix or drilling deep into the crater. should be allowed,” the authors explain. their papers.
This research explains the history of Itokawa. 4.6 billion years ago, the monolithic asteroid that is the mother of Itokaway formed. Between 4.6 and 4.2 billion years ago, successive collisions caused progressive fragmentation. Then he happened 4.2 billion years ago, one of two things. The impact dug a deep crater or completely destroyed the asteroid. The debris quickly turned into Itokawa. Throughout its subsequent history, Itokawa has suffered many impacts, but the nature of the asteroid’s heap of debris has allowed it to absorb those impacts without being cratered or destroyed.
“Argon dating reveals that the particles are about 4.2 billion years old.” , suggesting that once the rubble piles are built, they are difficult to destroy,” the authors write.
This result also applies to objects other than Itokawa. If it’s very hard to destroy, there could be a lot more piles of rubble than you think. We know that Bennu, Ryugu, etc. are piles of rubble. This affects our ability to protect the Earth from asteroid impacts.
Associate professor and co-author Nicholas Timms said, “We set out to figure out whether piles of rubble are shock-resistant or whether they break apart at the slightest shock.” Now that we know we can survive, the asteroid belt should be more abundant than previously thought, so if a large asteroid is flying toward Earth, it’s a pile of rubble.
of article of conversation, Jordan emphasized the threat they pose. “Indeed, they are so abundant, they are the shattered fragments of monolithic asteroids, so they are relatively small and difficult to find from Earth,” he writes. “Such asteroids are therefore a major threat to Earth and we need to better understand them.”
The risk for us is that these asteroids can absorb a lot of kinetic energy. That means a dynamic impactor like NASA’s DART mission may not be able to effectively keep it away from Earth. “Here we show that asteroids, small piles of rubble, can survive billions of years against ambient shocks inside the solar system because they are resistant to collisions and breakup. “Nuclear blast deflection, for example, may be more likely to be successful against rubble-heap asteroids,” the authors write in their paper.