Zero Genie

Those missions showed us that rubble pile asteroids have a low density because they are porous. Also, they are abundant. 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.

Hence, such asteroids represent a major threat for Earth and we really need to understand them better.

Learning from asteroid dust

In 2010, the Hayabusa spacecraft designed by JAXA returned from the 1,750-foot-long (535 meters), peanut-shaped asteroid Itokawa. The probe brought with it more than a thousand particles of rocks, each one smaller than a grain of sand. Those were the first-ever samples brought back from an asteroid!

As it then turned out, the pictures taken by the Hayabusa spacecraft while it was still orbiting Itokawa demonstrated the existence of rubble pile asteroids for the first time.

Early results by the team at JAXA who analysed the returned samples showed Itokawa formed after the complete destruction of a parent asteroid which was at least 12.4 miles (20 km) large.

In our new study, we analyzed several dust particles returned from asteroid Itokawa using two techniques: the first one fires an electron beam at the particle and detects electrons that get scattered back. It tells us if a rock has been shocked by any meteor impact.

The second one is called argon-argon dating and uses a laser beam to measure how much radioactive decay happened in a crystal. It gives us the age of such a meteor impact.

Giant space cushions that last forever

Our results established that the huge impact that destroyed Itokawa’s parent asteroid and formed Itokawa happened more than 4.2 billion years ago, which is almost as old as the solar system itself.

That result was totally unexpected. It also means Itokawa has survived almost an order of magnitude longer than its monolith counterparts.

Such an astonishingly long survival time for an asteroid is attributed to its shock-absorbent nature. Due to being a rubble pile, Itokawa is around 40 percent porous. In other words, almost half of it is made of voids, so constant collisions will simply crush the gaps between the rocks, instead of breaking apart the rocks themselves.

So, Itokawa is like a giant space cushion.

This result indicates rubble pile asteroids are much more abundant in the asteroid belt than we once thought. Once they form, they appear to be very hard to destroy.

This information is critical to prevent any potential asteroid collision with Earth. While the DART mission was successful in nudging the orbit of the asteroid it targeted, the transfer of kinetic energy between a small spacecraft and a rubble pile asteroid is very small. This means they are naturally resistant to falling apart if impacted.

Therefore, if there was an imminent and unforeseen threat to Earth in the shape of an incoming asteroid, we’d want a more aggressive approach. For example, we may need to use the shockwave of a nuclear blast in space, since large explosions would be able to transfer much more kinetic energy to a naturally cushioned rubble pile asteroid, and thus nudge it away.

Should we actually test a nuclear shock wave approach, then? That is an entirely different question.

Fred Jourdan, Professor, Curtin University and Nick Timms, Associate Professor, Curtin University

This article is republished from The Conversation under a Creative Commons license. Read the original article.