Quirks and Quarks

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

Rubble pile asteroids are extremely shock resistant which may explain how they've stayed together for almost as long as the solar system is old. It also has important implications for how we might deal with one that's heading towards the Earth.

Asteroid dust that made it back to Earth in 2010 reveals tough 'rubble piles' are common

A single grain of asteroid sample the width of a human hair is shown in the middle of a small white circle where the material it was transported in was cut out.
Scientists studying three tiny grains from the asteroid Itokawa, like the one barely visible in this picture, have discovered that the asteroid is at least 4.2 billion years old. (Royal Holloway/ISAS/JAXA)

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.

In this artistic rendition of the spacecraft attempting to capture an asteroid sample from the surface of Itokawa.
Illustration of the Hayabusa spacecraft catching samples from the surface of the asteroid Itokawa that didn't go as planned. Instead of bringing grams of asteroid material back to Earth, it only captured 1,500 tiny particles. (Akihiro Ikeshita/JAXA)

"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.

This black and white image of the asteroid Itokawa has the appearance of a bumpy peanut made up of different sized rocks.
The asteroid Itokawa is a rubble pile asteroid with rocks loosely held together by their mutual gravity. The telltale signs it's made from rubble is its varied terrain and lack of impact craters, presumably because any impact energetic enough to create a crater shakes rocks loose and they fill the hole. (JPL/NASA/JAXA)

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. 

Two black and white images that are shown here side by side show different size rocks on the surface of the Ikotawa rubble pile asteroid.
Two close-up images of rubble on the asteroid, Itokawa, taken in 2005. ( JAXA)

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?

NASA captured this video that looks like a comet overhead that's breaking up of Hayabusa re-entering the Earth's atmosphere.
Though the original Hayabusa mission faltered early on, the mission is still considered a success after it returned to Earth with 1,500 asteroid particles that scientists have been studying ever since. (Greg Merke/Jesse Carpenter/NASA Ames Research Center)

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