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As far as I understand, both the Yarkovsky effect and the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect exert forces upon bodies as a result of thermal photon radiation. How do both effects differ from one another?

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  • $\begingroup$ As Rob's answer shows, it's a rather arbitrary. The Yarkovsky was described first, in terms of affecting translational behavior. The YORP effect was described later, in terms of affecting rotational behavior. In truth, they're one and the same. An applied force whose action does not go through the center of mass results in both translational and rotational acceleration. $\endgroup$ – David Hammen Aug 23 '18 at 1:48
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The Yarkovsky effect is the thrust on a small object in space that has been heated by sunlight, created from radiant energy. The YORP effect is when the thrust produces rotation, likely due to uneven heating or differences in surface projections (IE: A mountain acts as a better solar collector than a flatter portion of a spherical object on the opposite side).

See: "The Yarkovsky and YORP Effects" (Feb 4 2015), by D. Vokrouhlicky, W. F. Bottke, S. R. Chesley, D. J. Scheeres, and T. S. Statler:

"The Yarkovsky effect describes a small but significant force that affects the orbital motion of meteoroids and asteroids smaller than 30 − 40 kilometers in diameter. It is caused by sunlight; when these bodies heat up in the Sun, they eventually re-radiate the energy away in the thermal waveband, which in turn creates a tiny thrust. This recoil acceleration is much weaker than solar and planetary gravitational forces, but it can produce measurable orbital changes over decades and substantial orbital effects over millions to billions of years.

The same physical phenomenon also creates a thermal torque that, complemented by a torque produced by scattered sunlight, can modify the rotation rates and obliquities of small bodies as well. This rotational variant has been coined the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. During the past decade or so, the Yarkovsky and YORP effects have been used to explore and potentially resolve a number of unsolved mysteries in planetary science dealing with small bodies.".

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Both effects are the result of sunlight being absorbed and re-radiated as heat. As this re-radiation (cooling) is not uniform, it results in kinematic effects (unbalanced recoil). However, the defining physical difference boils down to the Yarkovsky effect being a force and YORP effect being a torque. The former is applied to the objects barycenter, whilst the latter is applied to the object at some distance r from its center. So we may view Yarkovsky as a push destabilizing the orbit and YORP as a spin destabilizing the structure of the body itself.

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