YORP effect
The YORP effect is a slow change in an asteroid's spin caused by uneven heating and cooling by sunlight.
The YORP effect takes its name from four scientists, Yarkovsky, O'Keefe, Radzievskii and Paddack, who contributed to the idea. When sunlight warms an asteroid, its surface later radiates the heat away. If the asteroid's shape is irregular, the tiny push from the escaping heat is not balanced, producing a small twisting force.
Over millions of years, this torque can speed up or slow down an asteroid's spin and tilt its axis. The effect was first measured directly in 2007, when astronomers detected the near-Earth asteroid 54509, later named YORP, gradually spinning faster.
YORP explains several puzzles. It can spin small rubble piles up until material slides to the equator and forms a moon, creating binaries. It also explains why small asteroids tend to spin either very fast or very slowly and why their axes cluster in certain directions. It works hand in hand with the Yarkovsky effect, which changes orbits.
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