An image of the asteroid Dimorphos captured by NASA’s DART mission minutes before impact last September, revealing it to be another ‘rubbish heap’ asteroid. (credit: NASA/JHUAPL) |
by Fred Jourdan and Nick Timms
Monday, January 30, 2023
A large amount of rocks and other materials are rushing around our solar system in the form of asteroids and comets. If one of them came towards us, could we succeed in preventing the collision between an asteroid and the Earth?
Well, maybe. But there seems to be one type of asteroid that could be particularly difficult to destroy.
| We have discovered that rubble heap asteroids are a type of asteroid that is extremely tough and difficult to destroy by collision. |
Asteroids are bits of rocky debris in space, remnants of a more violent past in our solar system. Studying them can reveal their physical properties, clues to the solar system’s ancient history, and the threats these space rocks may pose by impacting Earth.
In our new study published last week in the Proceedings of the National Academy of Scienceswe discovered that rubble heap asteroids are a type of asteroid that is extremely tough and difficult to destroy by collision.
Two main types of asteroids
Mainly concentrated in the asteroid belt, asteroids can be classified into two main types.
Monoliths – made from a solid block of rock – are what people usually think of when they think of asteroids. It has been predicted that monolithic type asteroids about a kilometer in diameter would have a lifetime of only a few hundred million years in the asteroid belt. That’s not long at all considering the age of our solar system.
The other types are rubble heap asteroids. These consist entirely of batches of fragments ejected during the total or partial destruction of pre-existing monolithic asteroids.
However, we don’t really know the durability, and therefore potential lifespan, of rubble heap asteroids.
Sneaky and abundant pile of rubble
In September 2022, NASA’s DART (Double Asteroid Redirection Test) mission successfully crashed into the asteroid Dimorphos. The purpose of this mission was to test if we could deflect an asteroid by impacting it with a small spacecraft, and it was a resounding success.
Like other recent asteroid missions by the Japan Aerospace Exploration Agency (JAXA) visit asteroids Itokawa and Ryuguand by NASA to the asteroid Determineclose-up images have shown that Dimorphos is yet another rubblepile asteroid.
These missions showed us that rubble heap asteroids have a low density because they are porous. Moreover, they are abundant. In fact they are very abundant, and since they are the broken pieces of monolithic asteroids, they are relatively small, and therefore difficult to spot from Earth.
Therefore, these asteroids pose a major threat to Earth and we really need to understand them better.
Learning from asteroid dust
In 2010, the JAXA-designed Hayabusa spacecraft returned from the 535-meter-long, peanut-shaped Itokawa asteroid. The probe brought with it more than a thousand rock particles, each smaller than a grain of sand. These are the very first samples brought back from an asteroid!
It then turned out that photos taken by the Hayabusa spacecraft while it was still orbiting Itokawa demonstrated the existence of rubble-pile asteroids for the first time.
Early results from the JAXA team that analyzed the returned samples showed Itokawa formed after a parent asteroid was completely destroyed which was at least 20 kilometers wide.
| Thus, Itokawa is like a giant space pillow. |
In our new study, we analyzed several dust particles sent back by the Itokawa asteroid using two techniques: the first shoots an electron beam at the particle and detects the electrons that are sent back. It tells us if a rock has been hit by a meteor impact.
The second is called argon-argon dating and uses a laser beam to measure the amount of radioactive decay that has occurred in a crystal. This gives us the age of such a meteor impact.
Giant space pillows that last forever
Our results established that the massive impact that destroyed Itokawa’s parent asteroid and formed Itokawa occurred over 4.2 billion years ago, which is almost as old as the solar system itself. -same.
This result was totally unexpected. It also means that Itokawa survived almost an order of magnitude longer than its monolith counterparts.
Such a surprisingly long survival time for an asteroid is attributed to its shock-absorbing nature. Due to being a pile of rubble, Itokawa is approximately 40% porous. In other words, almost half of it is made up of voids, so constant collisions will simply crush the gaps between the rocks, instead of shattering the rocks themselves.
Thus, Itokawa is like a giant space pillow.
This result indicates that rubble-pile asteroids are much more abundant in the asteroid belt than we once thought. Once formed, they seem very difficult to destroy.
This information is essential to prevent any potential collision of asteroids with Earth. While the DART mission succeeded in shifting the orbit of the asteroid it was targeting, the kinetic energy transfer between a small spacecraft and a rubble-pile asteroid is very small. This means that they are naturally resistant to collapsing on impact.
Therefore, if there was an imminent and unforeseen threat to Earth in the form of an incoming asteroid, we would want a more aggressive approach. For example, we may need to use the shock wave from a nuclear explosion in space, because large explosions could transfer much more kinetic energy to a naturally damped rubble pile asteroid, and thus push it back .
Should we really be testing a nuclear shock wave approach, then? That’s a whole other question.
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