Primordial asteroids are like giant space pillows and could be harder to destroy than previously thought

Dust collected from the surface of an ancient, peanut-shaped and "potentially-hazardous" asteroid has revealed that some space rocks are much bouncier and harder to destroy than first thought — posing concerns about Earth’s long-term safety. 

The analysis of three tiny dust particles — gathered from the surface of the 1640-foot-long (500 meters) rubble pile asteroid Itokawa — shows that the cosmic wanderer has survived in space despite numerous collisions for at least 4.2 billion years. This means that not only are asteroids of the same type more likely to come into contact with our planet, but that smashing into them will probably not be the best way to deflect or destroy such space rocks. 

Rubble piles are smashed up former asteroids birthed in the wake of giant impacts and consist of stones and boulders that are loosely gathered and bound together by gravity. Typically, almost half of a rubble pile asteroid’s volume is made up of empty space, leaving scientists curious about these space rocks’ shock-absorbing capabilities. Now, a new study, published Jan 23. in the journal Proceedings of the National Academy of Sciences, has revealed that the heaps of space rock have survived for nearly as long as the solar system has existed.

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"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," lead author Fred Jourdan, a geochemist at the University of Curtin in Perth, Australia, said in a statement. "In short, we found that Itokawa is like a giant space cushion, and very hard to destroy."

The dust samples were collected from Itokawa in 2005 during the Japanese Space Agency’s Hayabusa 1 mission, which sent a probe on a 3.8 billion mile (6 billion kilometers) round-trip to land on the asteroid — scraping the tiny grains from its surface before safely returning them to Earth in 2010.

To analyze the samples, the researchers used two methods. In the first, called electron backscattered diffraction, the scientists blasted the dust grains with a beam of electrons, allowing the team to study the grains’ crystalline structure from the way the electrons diffracted off their surfaces. The second method, argon-argon dating, fired another beam at the grains — this time from a laser — to release argon gas, which revealed the asteroid’s age based on the extent of the gas’s radioactive decay.

The researchers found that Itokawa had been drifting around space for eons, easily outliving single-boulder asteroids which only have predicted survival times in the hundreds of thousands of years.

"We set out to answer whether rubble pile asteroids are resistant to being shocked or whether they fragment at the slightest knock," co-author Nick Timms, a geologist at the University of Curtin, said in the statement. "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."

NASA tracks the locations and orbits of roughly 28,000 asteroids, following them with the Asteroid Terrestrial-impact Last Alert System (ATLAS), an array of four telescopes that can perform a scan of the entire night sky every 24 hours. The space agency flags any space object that comes within 120 million miles (193 million km) of Earth as a "near-Earth object" and classifies any large body within 4.65 million miles (7.5 million km) of our planet as "potentially hazardous."

If a rubble pile asteroid were to plummet towards us, the researchers say that understanding how the rock heaps differ from their single-chunked kin could enable humans to deploy better planetary defense systems against them.

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Space agencies around the world are already working on possible ways to deflect dangerous asteroids. On Sept. 26, the Double Asteroid Redirection Test (DART) spacecraft redirected the nonhazardous asteroid Dimorphos by ramming it off course, altering the asteroid's orbit by 32 minutes in the first test of Earth's planetary defense system.

However, this new research shows that merely bumping an asteroid off course will not always be an option — especially with shock absorbent rubble pile asteroids.

"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," Timms said.

This method is, so far, untested. Future research will be required to prove whether asteroid deflection by nuclear blast is viable.