Asteroid Defies the Laws of Physics! Or Not

According to a recent Nature World News report, “Potentially Dangerous Asteroid is Defying the Laws of Gravity”, experts have determined the asteroid (29075) 1950 DA isn’t a real asteroid but is “actually composed of rubble” and that “[s]omehow, these surface rocks aren’t tearing away from the whole to fly off into space, and astrophysicists are wondering why.” Amazing! And, so we are told, “[n]ow researchers…are suggesting that the asteroid…is held together by van de Waals forces – cohesive forces never before detected on an asteroid.” Exciting!

Of course, it’s kind of hard to determine how “exciting” this news is without additional context. For example, what might the research paper that the Nature World News report is based on say about “van de Waals forces”? Not much, but the primary citation they use is from the 1974 “Apollo soil mechanics experiment S-200 final report”. Another primary source used concerns…well…asteroids “actually composed of rubble” that are held together in much or precisely the same way that the asteroid referred to in the Nature World News article is: Walsh et al’s (2012) “Spin-up of rubble-pile asteroids: Disruption, satellite formation, and equilibrium shapes”. This study wasn’t some groundbreaking study showing that some “asteroids” are actually more like sandpiles than rocks. The realization that what might appear to be asteroids are actually composed of multiple bodies rather than a single one is hardly new.

In fact, the research paper the news article is based upon explicitly compares this astronomical phenomenon to…wait for it…sand. More specifically, granular physics, but we see far more complex dynamics in granular mechanics:
“granular configurations cannot relax spontaneously in the absence of external perturbations. This leads typically to the generation of a large number of metastable configurations; it also results in hysteresis, since the sandpile carries forward a memory of its initial conditions…The above taken together, suggest that sandpiles show complexity; that is, the occurrence and relative stability of a large number of metastable configurational states govern their behaviour.” (p. 2 of Granular Physics by Anita Mehta).

The above is another way of saying that sandpiles we find here on earth at any given beach are so far beyond explanation by the “laws of gravity” that we use the term “memory” to describe how their initial state/form results in their final state/form. We refer to the ways pieces of sand in a sandpile reconfigure as “complex” because we can’t determine how they do this (other than probabilistically). Basically, the ways in which components of rubble-pile asteroids are able to cohere are far easier to understand and far simpler than clumps of sand on Earth.

This shocking news doesn’t reveal anything about asteroids we didn’t know. It doesn’t reveal anything about physics we didn’t know. And the only seemingly physics-defying aspects of it are explained by the dynamics of sand and similar granular media that aren’t just found on Earth but can be much more complicated.

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One Response to Asteroid Defies the Laws of Physics! Or Not

  1. Leta says:

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    to share my know-how here with colleagues.

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