September 5, 2010 | David F. Coppedge

Asteroid Pi

Asteroids are much more diverse than previously imagined.  The Spitzer Space Telescope (now in its “warm” mission after the liquid coolant has dissipated) is targeting about 700 asteroids in a program called ExploreNEOs (Near-Earth Objects).  NASA’s Spitzer Space Telescope website reported results from the first hundred studied.  The scientists found that near-Earth asteroids…

…are a mixed bunch, with a surprisingly wide array of compositions.  Like a pi�ata filled with everything from chocolates to fruity candies, these asteroids come in assorted colors and compositions.  Some are dark and dull; others are shiny and bright.  The Spitzer observations of 100 known near-Earth asteroids demonstrate that the objects’ diversity is greater than previously thought.

The diversity is inferred from the asteroids’ infrared signature, the temperature disclosing something about surface size, composition, and thermal inertia (ability to hold heat).  From that data, scientists try to infer where the objects originated from, and how old they are.  One of the surprises is that numerous bright NEOs appear to be young:

Since asteroid surfaces become darker with time due to exposure to solar radiation, the presence of lighter, brighter surfaces for some asteroids may indicate that they are relatively young.  This is evidence for the continuing evolution of the near-Earth object population.
    In addition, the fact that the asteroids observed so far have a greater degree of diversity than expected indicates that they might have different origins.  Some might come from the main belt between Mars and Jupiter, and others could come from farther out in the solar system.  This diversity also suggests that the materials that went into making the asteroids — the same materials that make up our planets — were probably mixed together like a big solar-system soup very early in its history.

But extending the primordial soup metaphor to the solar system decreases the explanatory power of previous cosmogonies (stories about the origin and development of the solar system), which depended on composition as a function of radius, and complicates new explanations (cf. 09/24/2008, 05/04/2007).
    The original paper in The Astronomical Journal1 noted that an object at Earth orbit should become dark by “space weathering” in a million years.  Noting the large number of bright objects, the authors suggested that the bright (and presumably young) objects may be extinct comets or remnants of recent collisions that exposed fresh surface material.  They acknowledged, however, that much is still unknown about asteroid origin and evolution.  Evolutionary theories tend to be complicated, and some findings contradict expectations.  For instance, “The small (500 m long) NEO Itokawa was visited by the Japanese spacecraft Hayabusa, and was found to have a highly varied surface, with both regolith-free regions and regions of substantial regolith,” the paper said, referring to pulverized surface material.  “This unexpected result has greatly increased interest in the surface properties of NEOs and how they depend on an object’s history.”
    In addition, about 15% of asteroids appear to be binaries (co-orbiting objects) – a dynamically improbable phenomenon, given asteroids’ low gravity and relative motions.  Though recently split objects from a collision might tend to remain in proximity enough to remain gravitationally bound, the Yarkovsky effect (a perturbing force due to sunlight pressure) and other forces would seem to disrupt the delicate pas de deux of a binary pair in short order (see 10/25/2001, 05/24/2002, 09/12/2003, 09/28/2005).
    According to current thinking, most NEOs originated in the main asteroid belt and became perturbed into earth-vicinity orbits through collisions, the Yarkovsky effect and chaotic motions.  “We find that the distribution of albedos in this first sample is quite broad,” they said, however, “probably indicating a wide range of compositions within the NEO population.”  That diversity had to originally exist in the main asteroid belt, therefore, or requires positing a larger-than-predicted mixing of material from different parts of the solar system.  Either case complicates simplistic ideas about the origin of the solar system that preceded the space age.
    One other asteroid phenomenon is making a planetary scientist “lose sleep,” according to New Scientist: the problem of Trojan asteroids.  These are not condoms or malware, but asteroids that get locked into the stable Langrangian points of planets, making them lead or follow large planets in their orbits.  “A family of asteroids that travels in lockstep with Jupiter appears to be different in one important respect from their purported kin in the outer solar system,” New Scientist began.  “The mismatch could spell trouble for the leading theory of how our solar system evolved.”  Called the “Nice” model, the theory, which proposes that the gas giants disrupted objects as they migrated after formation and gathered some into its Trojan orbits, predicts a different size distribution than was found by Wesley Fraser of Caltech.  He said he has “lost a lot of sleep” over the puzzle; he “cannot imagine any scenario that has a chance to explain this result.
    As with most asteroid science, though, like the asteroids themselves, our knowledge is too sparse to provide solid ground for any theory of formation or evolution.  ExploreNEOs is important work in another respect all earthlings should care about:   the authors said, “Knowledge of the size distribution is critical for estimates of the Earth impact hazard.” reported that two small asteroids came pretty close to the Earth on September 8, and that such close encounters are not all that rare.  One rogue asteroid with our name on it could ruin a nice pi�ata party.

1.  Trilling, Mueller et al, “ExploreNEOs.  i.  Description and First Results from the Warm Spitzer Near-Earth Object Survey,” The Astronomical Journal, Vol 140, No 770; doi: 10.1088/0004-6256/140/3/770.

With all due regard for journalistic creative writing, pi�atas are typically created by intelligent design.  So who filled the asteroid pi�ata with such a creative variety of goodies?  Are they implying scientists are scooping up the treats without thanking the Source?  Metaphors bewitch you (07/04/2003).
    The scientific work behind the metaphors is dull but necessary, tedious but incremental.  No one can draw firm conclusions from this admittedly preliminary work, other than the common experience of scientists working in exploratory projects: (1) surprises abound, (2) predictions are made to be broken, (3) nature yields its secrets with difficulty, and (4) human hubris tends to outrun reality.
    Consider what they said about space weathering darkening objects within a million years.  That sounds like a long time, but it is a tiny, tiny fraction of the assumed age of the solar system (4.5 billion years).  Have there been that many impacts within the last .02% of the assumed age?  Liberate your mind.  The few planetary scientists not wedded to the moyboy paradigm (“millions of years, billions of years” see 09/16/2005), with its reckless drafts on the bank of time (07/02/2007), have the freedom to think outside the box and perhaps provide novel solutions to puzzles inherent in deep-time addiction (e.g., 09/05/2002).

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Categories: Physics, Solar System

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