As recently as the 1970s, dynamicists scoffed at the notion that some asteroids have moons, even though observers had reported seeing occasional double blink-outs when asteroids passed in front of stars.

They aren't scoffing any more. More than 100 bodies in the asteroid and Kuiper belts are now known to be binaries. In fact, by some estimates roughly 15% of small objects in the asteroid belt and in near-Earth orbits are paired. It's an embarrassment of riches: far too many exist to have been formed by bodies simply crashing into one another.

Simulation of splitting asteroid

In this computer simulation, an asteroid with a rigid core (gray particles) and a rubbly exterior has spun so fast that it's formed a small moon nearby.

Nature / K. J. Walsh and others

A new study, published by a trio of theorists in the July 10th issue of Nature, finds that some asteroids can spin so rapidly that they literally fly apart. The researchers invoked a process known as the YORP effect that causes small bodies to spin up or slow down when exposed to sunlight.

YORP stands for Yarkovsky, O'Keefe, Radzievskii, and Paddack — the four scientists who identified the effect. The phenomenon occurs when a body absorbs sunlight and reradiates it as heat. For a spherical object, this is a zero-sum exchange of the two forces. But if the asteroid is faceted, as most small ones are, then some heat gets radiated at an angle instead of radially, causing a tiny torque that can either speed up its rotation or slow it down.

The Nature authors focused on different types of "rubble-pile" asteroids, chunky collections held loosely together by gravity. In their computer simulations, rubbly asteroids behave much like a fluid. As their spin rates quicken to just a few hours, the asteroids first become elongated footballs as mass slides from their poles to their equators. Eventually chunks of matter fly off these whirling dervishes — sometimes reaccumulating onto the main bodies and sometimes creating moons close by.

These YORP-driven cleavings work so well, say the authors, that the process likely is the source of many, if not most, binary asteroids. The University of Maryland's press release about the new finding includes an animation showing how a rubble pile can spin itself to pieces.


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Scott C. Schumacher

July 11, 2008 at 1:57 pm

Then I suppose that YORP effects may not be entirely responsible for secular variation in asteroid orbits as some or most of the momentum transfered by YORP effect may instead be transfered into angular momentum. On the other hand, there would, I suppose, be a limit to the amount of angular momentum which could be transfered so that older rubble piles may have had the maximum amount of transfered angular momentum and thus would begin to experience YORP effect-driven secular orbit variation.

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Norm Johnson

July 11, 2008 at 4:33 pm

isn't this the same effect demonstrated by our little Physics Sets, sould to tens of thousands of boys and girls in the 1950s? Gilbert was the manufacturer though the company probably sold a lot more Chemistry Sets and even more Erector Sets.
The tiny glass globe contained a free-spinning pinwheel on a spindle. The instruction was to set it in the sun and watch it spin. Each facet of the pinwheel was dark on one side and light on the other.

And it surely Did Spin!

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Someone who works for a living

July 14, 2008 at 6:01 pm

Isn't this pretty much identical results to those reported by D. P. Rubincam and S. J. Paddack, Science 316 211 (2007)? Wow, I wish I could have that job -- duplicating research and getting paid for it. Even Wikipedia has comments on the possible relationship between the YORP effect and binary asteroids. Get a job, people!

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Enrico the Great

July 15, 2008 at 9:53 pm

This is a FRIENDLY forum!

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Scott C. Schumacher

July 15, 2008 at 10:51 pm

It has occurred to me that YORP modelling may be incomplete.

YORP is a function of thermal inertia: incident sunlight is reradiated at a later time, possibly causing the object to spin up. However, as the spin rate increases, the time between incidence and reradiation remains the same. This causes the angle between incidence and reradiation to increase. Eventually, this increase in angle could cause the reradiation to occur at an angle which may nullify any spinup. (For instance, much of the reradiation might be on the night side of the object.)

Thus the amount of spinup may likely be a function of size and thermal inertia.

I suspect that the smallest, oldest objects may be the ones with the greatest spin rate.

RE: (Not new): I guess that this poster is ignorant of the role refereed scietific journals have in the advance of scientific thought. It is not enough to just say that a particular scientific idea has merit: it must also have "pedigree". Anyone can say something. Only those who say it in a refereed journal are listened to by the scientific body.

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