Scientists Say ‘Rubble Pile’ Asteroids Are Surprisingly Hard to Kill

Scientists Say ‘Rubble Pile’ Asteroids Are Surprisingly Hard to Kill

Rubble pile asteroids are more common and durable than previously thought, according to new research. The scientists behind the study say this could pose a problem for planetary defence measures. But there may be reason for optimism, given recent insights gleaned from NASA’s successful DART mission to deflect an asteroid.

Once just a hypothesis, rubble pile asteroids appear to be a common fixture of the solar system, as evidenced by missions to asteroids Itokawa, Ryugu, Bennu, and Dimorphos, the latter asteroid not yet officially confirmed as such but very likely is. As the name suggests, rubble pile asteroids are loosely bound conglomerations of rock and dust held together by exceptionally weak gravity. And by weak, I mean weak; the forces involved at the surface are comparable to the weight imposed by a couple of pieces of paper held in your hand.

Rubble pile asteroids are quite porous and are differentiated from monolithic-type asteroids — intact and dense chunks of rock. Large monolithic asteroids measuring 1 kilometre or more in size likely last for hundreds of millions of years, but the nature and lifespans of rubble pile asteroids are less clear. The new PNAS paper, led by planetary scientist Fred Jourdan from Curtin University in Australia, seeks to fill in some of these gaps.

For the study, Jourdan, along with an international team, looked into the origins, composition, and durability of rubble pile asteroids, while also considering these objects from the perspective of planetary defence. Like monolithic-type asteroids, rubble pile asteroids also pose a threat to life on Earth.

The researchers analysed dust particles brought back to Earth in 2010 by the Japanese Space Agency’s Hayabusa 1 probe, which extracted surface samples from the near-Earth asteroid Itokawa five years earlier. Using a technique called electron backscattered diffraction, the team was able to determine if the particles were shocked by previous impacts, and using argon dating, they were able to date these asteroid impacts.

Results showed that Itokawa — a rubble pile asteroid — formed 4.2 billion years ago. That’s a very long time ago, and the scientists attribute this longevity to the asteroid’s ability to survive collisions with other asteroids. “Such a long survival time for an asteroid is attributed to the shock-absorbent nature of rubble pile material and suggests that rubble piles are hard to destroy once they are created,” the scientists wrote in their study. “Our results suggest that rubble piles are probably more abundant in the asteroid belt than previously thought.”

Or as Jourdan explained in a Curtin press release: “In short, we found that Itokawa is like a giant space cushion, and very hard to destroy.” And because rubble pile asteroids are hard to destroy, the solar system is likely chock full of them.

Accordingly, the new paper carries implications for planetary defence strategies to protect against threatening asteroids. That rubble pile asteroids are more durable and abundant than we thought is an obvious cause for concern. According to previous work, the amount of energy needed to completely disrupt or shatter rubble pile asteroids is around four times higher than for monolithic asteroids. What’s more, porous asteroids are more difficult to deflect with kinetic impactors because the porosity of these objects decreases the “efficiency of the transfer of momentum,” the new paper argues. Basically, rubble pile asteroids are gigantic shock absorbers.

The scientists acknowledged a recent NASA experiment on the matter, saying “much remains to be learned from the successful impact of the Double Asteroid Redirection Test (DART) spacecraft on the rubble pile asteroid Dimorphos.”

“The recent DART mission was a resounding success!” Jourdan wrote to Gizmodo in an email. “It showed that we can push rubble pile asteroids by impacting spacecraft into them. The problem is that it requires to detect the asteroids very early on since the push will be very small. So if the asteroid starts to be pushed by kinetic impact, say, three years before it collides with Earth, no problem. DART-like devices can do it. But what if we don’t have enough time?”

Not having confidence in kinetic impactors to deflect or destroy rubble pile asteroids in a hurry, the researchers suggest “more aggressive approaches,” like nuclear blasts.

To recap, the DART spacecraft deliberately smashed into the 163.07 m-long (163-metre) Dimorphos on September 26, 2022, shortening its orbit around its larger partner, Didymos, by around 33 minutes. This was an incredible result, as DART scientists had predicted an orbital adjustment of around 73 seconds. One possible factor for DART’s surprisingly big shove? The recoil effect. And, in my view, it’s this recoil effect that may provide some hope for dealing with rubble pile asteroid in the future.

More on this story: A Powerful Recoil Effect Magnified NASA’s Asteroid Deflection Experiment

DART’s impact with Dimorphos released more than 2 million pounds of ejecta from the surface, resulting in a significant debris tail. Early results released late last year suggest that the resulting ejecta plume, like air rushing out from a balloon, created an extra push; the momentum transferred into Dimorphos was nearly four times greater than an impact event that produced no plume. It’s very likely that a plume of this bulk would not form on a monolithic asteroid, and that the observed effect is a distinct consequence of Dimorphos’s porous nature.

The Didymos-Dimorphos binary asteroid system after the DART impact, revealing an elongated debris plume.  (Image: Magdalena Ridge Observatory/NM Tech)
The Didymos-Dimorphos binary asteroid system after the DART impact, revealing an elongated debris plume. (Image: Magdalena Ridge Observatory/NM Tech)

“The result of that recoil force is that you put more momentum into the target, and you end up with a bigger deflection,” Andy Cheng, Johns Hopkins lead investigator for DART, explained to reporters back in December. “If you’re trying to save the Earth, this makes a big difference.” Jourdan’s team doesn’t have much confidence in kinetic impactors as a means to protect our planet from rubble pile asteroids on short notice, but factoring in the recoil effect may mean there’s hope for the strategy yet, particularly for scenarios in which we have advanced notice of a potential impact on the order of several years.

The efficacy of kinetic impactors aside, Jourdan posits the nuclear option for rubble pile asteroids in cases when planetary defenders have very little time to respond. It’s possible, for example, to detect an inevitable impact within a few months, rather than years or decades. But to be clear, Jourdan isn’t advocating for the destruction of threatening rubble pile asteroids — a feat his team’s research suggests is practically impossible. Rather, planetary defenders should consider the possibility of blasting a nuclear device near the asteroid in an attempt to deflect it, he said.

That’s because “the the shock wave would be much more energetic than small kinetic impactors like DART,” and it would shove the incoming asteroid to a greater degree, and “could therefore could get the job done,” he told Gizmodo. “The fact that they are resistant would play to our advantage so the blast would not destroy it,” he added, because “exploding an asteroid is really not the way to go since all the debris would rain down and cause similar devastation.” Jourdan said “this kind of thing should be tested in real life, similarly to DART before we are fully confident that it works as intended.” Fair point, but that would require some serious conversations beforehand, as the 1967 Outer Space Treaty currently bans the use of nuclear devices in space.  

Andrew Rivkin, the DART investigation team lead, told Gizmodo that, when it comes to deflecting asteroids, “there’s more than one factor involved, and it can get complicated very quickly, rather than just being able to say that porosity is the most important factor.” It could very well be true, he said, that a gigantic pile of gravel won’t be as easy to deflect as a single rock owing to its porosity.

“However, if you look at size rather than mass, and you have a 100-metre [328-foot] ball of rock in space, the single rock will have a lot more mass than the 100-metre pile of gravel, again because the pile of gravel is more porous, and so the gravel will be easier to move,” Rivkin explained. “Other effects, so tiny that we don’t have to worry about them in everyday life, will also affect the results. Even in the most pessimistic case, a kinetic impactor will be able to make some minimum, predictable deflection.”

Rivkin said he and his colleagues are still working through the DART data, and “whether or not people are ready to classify Dimorphos as a rubble pile, it all shows that we still have a lot to study about the formation and evolution of asteroids from laboratory, telescopic, and spacecraft studies.”

No doubt, it’s still early days in our attempt to develop an effective and reliable means for protecting Earth against threatening asteroids. The new paper sheds light on the nature and long-term resilience of rubble pile asteroids — information that will be of great use to planetary defenders.

More: A Last-Minute Nuke to Shatter an Incoming Asteroid Could Actually Work, Study Suggests

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