According to your 鈥淭his is the end鈥� feature (4 June, p 26): 鈥淲hen sunlight strikes asteroids they spin faster and faster, and many will centrifuge themselves into smithereens.鈥� What causes this ever-increasing speed?
鈥� If ever there was a great name then YORP is it. The (no wonder it was abbreviated) causes an irregularly shaped asteroid to spin faster and faster under the action of incoming sunlight.
The irregular shape is crucial, in the same way that blowing on a flat sheet of paper does nothing, but blowing on the same sheet folded into a windmill shape causes it to spin. The 鈥渂lades鈥� deflect the air, causing a net turning effect, or moment, and a change in angular momentum.
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With a perfectly spherical asteroid, photons are absorbed and reradiated perpendicular to the surface. There is zero moment as the photons reradiate radially.
鈥淥ver millions of years, an irregular asteroid gains angular momentum and its spin gets ever faster鈥�
But photons reradiating from an irregularly shaped asteroid each generate a tiny moment and the sum of all these will not be zero. Over millions of years, the asteroid gains angular momentum and because there is no friction in space, the spin gets ever faster. Eventually, it blows up due to the centrifugal force.
There are a few videos online of wind turbines going too fast and disintegrating, and this is much the same thing.
Hugh Hunt, Reader in engineering dynamics and vibration, Trinity College, Cambridge, UK
鈥� Asteroids鈥� spin can speed up or slow down as a result of incoming sunlight because these rocks heat up when illuminated but, due to a thermal lag, don鈥檛 give off the resulting radiation until they have rotated. However, the process usually affects asteroids鈥� orbits more than their rotation.
will study the YORP effect on asteroid 101955 Bennu when it reaches it in 2018, with plans to collect and return samples to Earth. The effect is of special interest because it could influence whether Bennu strikes Earth in 150 years or so.
Jay M. Pasachoff, Visitor in planetary sciences, California Institute of Technology, Pasadena, California, US
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This article appeared in print under the headline 鈥淰icious spin鈥�