Asteroid sample shows just what we might need to deflect a surprise killer impactor

Saving the Earth from an incoming planet-killing asteroid may one day come down to what we learned from three tiny grains of material collected from the asteroid Itokawa.

The tiny grains, which are invisible to the naked eye, are among the rarest scientific samples that exist.

"When you handle them there's a chance you can lose them. As you can imagine, you're really, really afraid this can happen," said Fred Jourdan, a professor of geochronology from Curtin University in Perth, Australia.

In 2003 the Japanese Space Agency launched the Hayabusa spacecraft on a mission to become the world's first space probe to visit an asteroid and return samples to Earth.

The asteroid, discovered in 1998 and named after Japanese rocket engineer Hideo Itokawa, formed in the asteroid belt between Mars and Jupiter, but then was ejected into its Earth-crossing orbit about two million kilometres away.

A malfunction that occurred with the probe's sample catcher meant that, much to the dismay of scientists on the ground, it returned to Earth with only a few micrograms — about 1,500 particles of asteroid dust — instead of the several grams they were hoping to retrieve.

"So every single one of [the grains] is so priceless," Jourdan told Quirks & Quarks host Bob McDonald.

When the spacecraft visited the asteroid in 2005, one of the first things scientists realized is that this asteroid Itokawa is what's known as a "rubble pile" instead of a solid chunk of rock.

"The name says it all. It's just a pile of rubble, so there's boulder, gravel, pebbles, dust and a lot of void spacing inside it," Jourdan said.

They form when one asteroid in the asteroid belt slams into and destroys another that's made of solid rock — what Jourdan refers to as a "monolith" asteroid — and the fragments from the impact reassemble.

According to Jourdan, the million dollar question then became: When did the impact occur that created Itokawa? 

Previously, researchers hypothesized from theoretical models that most rubble pile asteroids are likely only a few hundred million years old.

But Itokawa appears to be an exception to that rule. "The result we got is it's at least 4.2 billion years old, so almost as old as the solar system itself which is 4.56 billion [years old]," said Jourdan. 

His study of the asteroid sample appeared in December in the journal PNAS.

Shock resistant 'void space'

The asteroid belt is a place where asteroids obliterate each other, then reform new asteroids out of the rubble, so the fact Itokawa has been around so long suggests these types of asteroids may be shatterproof.

"Because they can take a beating, as demonstrated by their age in the asteroid belt," he said, "they're really resistant to shock." 

Forty per cent of the asteroid Itokawa is void space, which explains how it absorb shocks so well. 

Pummeling a solid chunk of rock with a sledgehammer smashes it into pieces; slamming it into a pile of rubble, on the other hand, doesn't have the same effect.

"You look at the pile [after]; it looks exactly the same. Why is that? Because the energy has completely been absorbed by the void space in it," Jourdan explained.

A bigger push for surprise shock resistant asteroids

The implications of this could help us better deflect an Earth-bound asteroid like this in the future, especially if we get very little notice that it's on the way. 

In the fall of 2022, NASA nudged a different rubble pile asteroid, named Dimorphos, from its orbit as part of their Double Asteroid Redirection Test (DART), by slamming an unmanned spacecraft directly into it.

The test went well, in large part, because scientists had years to plan for this mission, and it was sufficiently far enough away. If the asteroid really was on a collision course with the Earth, that subtle change in its direction might be enough avoid a disastrous impact if it's done early enough.

Jourdan said that's great when we get a lot of notice but what if we only have a month's notice that an asteroid is on a crash course for our planet?

His findings suggest a rubble pile asteroid would be shock resistant enough to survive us giving it a big push, perhaps with a nuclear explosion detonated beside it. It would then be redirected without shattering into smaller, but still potentially dangerous, chunks that could rain down on Earth.

These types of asteroids made from rubble seem to be more common than we once thought. 

Jourdan said every asteroid humans have sent spacecraft to, like Ryugu, Bennu, Dimorphos and Didymos, are all rubble pile asteroids, so it's time to start specifically preparing for this kind of threat. 

"We need to learn to defend ourselves against rubble piles, not monolith asteroids."

Produced and written by Sonya Buyting