Some ‘rubble pile’ asteroids are nearly impossible to destroy, scientists say

A type of “hard to destroy” asteroid made of a pile of rocks may need new strategies like a “nuclear blast” to push it off trajectory from any potential collision course with Earth, scientists said.

The research, published in the journal PNAS on Monday, studied three tiny dust particles collected from the surface of an ancient 500-metre-long rubble pile asteroid called Itokawa.

Analysis of these dust particles, returned to Earth by the Japanese Jaxa space agency’s Hayabusa 1 probe, suggested the asteroid could be almost as old as the Solar System itself.

Scientists, including those from Curtin University in Australia, found that Itokawa – about two million km away from Earth and about the size of Sydney Harbour Bridge – was nearly impossible to destroy.

Last year, Nasa had demonstrated in its Dart mission proof-of-concept test that it is likely possible to effectively move an Earth-threatening asteroid before it reached the planet by impacting it with a spacecraft.

But the new study indicates that it may be “very hard” to change the trajectory of an asteroid like Itokawa made of a pile of rubble and dust by impacting it with spacecraft.

“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,” study co-author Fred Jourdan said.

“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,” Dr Jourdan said.

Scientists call for testing new strategies to push such rubble pile asteroids like Itokawa off trajectory from a potential collision with Earth.

“If an asteroid is detected too late for a kinetic push, we can then potentially use a more aggressive approach like using the shockwave of a close-by nuclear blast to push a rubble-pile asteroid off course without destroying it,” said Nick Timms, a study co-author.

Researchers said the impact that destroyed the asteroid’s parent rock and formed Itokawa likely happened at least 4.2 billion years ago.

The study suggests the long survival time for an asteroid the size of Itokawa may be due 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,” Dr Jourdan explained.

In the study, two complementary techniques were used to analyse the three dust particles: one method measured if a space rock was shocked by any meteor impact while another was used to date asteroid impacts.

The durability of such rubble pile asteroids was previously unknown.

The latest finding, according to scientists, “jeopardises” the ability to design strategies in case one such asteroid was hurtling toward Earth.

“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,” Dr Timms added.