Confirmed: Ryugu is a rubble pile


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Close-up of Ryugu's surface
Click for source paper [pdf].

At a special session today dedicated to results from the Hayabusa-2 probe to the asteroid Ryugu at the 50th Lunar and Planetary Science Conference in Texas, scientists confirmed from numerous data and images that the asteroid has a low density, is covered with boulders and pebbles, is very porous, and is thus a rubble pile that is held together by gravity, barely.

From their lead presentation [pdf]:

The estimated total porosity is even higher than that of rubble-pile asteroid Itokawa (44 ± 4%), indicating that asteroid Ryugu is also a rubble pile. This is consistent with a theory arguing that all Solar System bodies with diameter of ~1 km should be rubble piles and might have formed from reaccumulation of fragments generated by catastrophic disruption events of ~100-km sized parent bodies.

They also posit that the asteroid’s diamond shape is caused by the asteroid’s 3.5 hour rotation, which causes its weak rubble pile structure to be easily pulled to the equator, and then outward.

Another paper [pdf] did crater counts, and found that there are fewer large craters than one would expect.

The density of large craters (D>100 m) on Ryugu is lower than the empirical saturation level and its slope is steeper than that of the saturated distribution, suggesting that craters larger than 100 m are not saturated and the size distribution reflects the crater production function. However, craters smaller than 100 m are significantly under-saturated, suggesting that some crater erasure processes such as seismic shaking and armoring effect are active on the Ryugu surface. Based on cratering chronology model for the main belt, the surface age of Ryugu is estimated to be 5–200 [million years] from the size–frequency distribution of craters larger than 100 m.

In other words, this rubble pile is constantly being shaken by its rotation and time and later impacts, which steadily rewrites the surface.

If this asteroid was headed to Earth, I imagine the only safe solution to prevent disaster would be to slowly and gently deflect it so it only flies past. To do this will require an arrival far in advance of the schedule impact, to give time for the deflection process to work.

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5 comments

  • John

    Bennu also seems to be a rubble pile. I wonder how many asteroids are rubble piles and how many are solid chunks of rock.

    Is a loosely bound, low density, porous rubble pile less of an impact threat than a solid asteroid?

    Maybe piles would tend to break up into their constituent small rubbles- which may not make it through the atmosphere and would spread less damage over a wider area.

  • jburn

    I once observed concrete poured into a hole with rebar to reinforce it. Immediately afterward a vibrating tool was shoved into the mix causing the concrete to become less viscous (in appearance?) and flow around everything more consistently, eliminating most of the air pockets.

    I can’t help wondering if a spacecraft landed on one of these objects and were vibrated, would it sink below the surface? A potential natural shielding for a habitation or method to submerge an explosive device, or shake it apart to be scattered like sand in solar wind.

    Yeah, sci-fi stuff but fun to imagine…..

  • fred K

    What would be the effect would be of a very large gamma ray pulse you’d produce in a stand-off nuclear bomb detonation? Seems like a thin surface of a hemisphere would vaporize resulting in a push to the rubble pile.

    It is not clear if this would tend to push the asteroid more or less as a whole, or if it would tend to disrupt the asteroid into lots of little fragments.

    I doubt my intuition, and I tend to doubt any opinion on the subject. It would be interesting to test this. Too bad we don’t live in the 1950s anymore.

  • MDN

    It seems to me that a penetrating nuke (bunker buster style) should be quite effective in dispersing a body such as this, rendering it essentially harmless as a diffuse cloud of rubble with no constituent pieces big enjough to present a significant threat should they strike Earth. That is not to say you might not suffer some number of Chelyabinsks, but you’d avoid a Tanguska or worse, even with a limited lead time to react. And think of the meteor showers we’d get to enjoy for decades+ thereafter.

    If further exploration proves rubble piles to be the norm and not exceptions, I suggest a test case would be warranted as we have all the tech, and could field a standing defense system quickly should it prove effective.

  • Vic

    About the near-Earth asteroids’ rubble-pile composition:
    “We think they’re very loose aggregates. They’re not solid through and through,” Melissa Morris, OSIRIS-REx deputy program scientist at NASA Headquarters in Washington, D.C.
    So it’s unlikely that the kinetic impact method will work – such composition will prevent shock wave propagation and proper impulse transfer.

    But in any case deflecting potentially hazardous bodies by evaporating their material using highly concentrated sunlight is the only method that meets all of the following criteria: scalability up to global-threat NEO sizes, sufficient thrusting power, environmentally friendliness, and low cost.

    An improved concept for such solar-based deflection using an innovative concentrating collector was proposed and substantiated in 2013 (see: https://link.springer.com/article/10.1007%2Fs11038-012-9410-2; also a short demo-video: https://www.youtube.com/watch?v=9u7V-MVeXtM).

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