| Poynting-robertson Effect |
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Information AboutPoynting-robertson Effect |
| CATEGORIES ABOUT POYNTING-ROBERTSON EFFECT | |
| celestial mechanics | |
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From the perspective of the grain of dust, the Sun's radiation appears to be coming from slightly forward of the direct line to the centre of its orbit because the dust is moving perpendicular to the radiation's movement. This Angle Of Aberration is extremely small since the radiation is moving at the Speed Of Light and the dust grain is moving many orders of magnitude slower than that. From the perspective of the solar system as a whole, the dust grain absorbs sunlight entirely in a radial direction. However, the dust grain's motion relative to the Sun causes it to ''re-emit'' that energy unevenly (more forward than aft), causing an equivalent change in Angular Momentum (a sort of Recoil ). The Poynting-Robertson force is equal to: : where is the power radiated from the particle (equal to the incoming radiation), is the particle's velocity, is the Speed Of Light , the object's radius, is the universal Gravitational Constant , the Sun 's mass, is the solar luminosity and the object's orbital radius. Since the gravitational force goes as the cube of the object's radius (being a function of its Volume ) whilst the power it receives and radiates goes as the square of that same radius (being a function of its Surface ), the Poynting-Robertson effect is more pronounced for smaller objects. Also, since the Sun's gravity varies as one over whereas the Poynting-Robertson force varies as one over , the latter gets relatively stronger as the object approaches the Sun, which tends to reduce the Eccentricity of the object's orbit in addition to dragging it in. Dust particles sized a few micrometers need a few thousand years to get from 1 AU distance to distances where they evaporate. There is a critical size at which small objects are so affected by Radiation Pressure that the latter actually cancels the Sun's gravitation altogether. For rocky particles, this size is about 0.1 ยต M in diametre {Link without Title} . If the particles are already in motion at their creation, radiation pressure does not need to cancel gravity completely to move the particles out of the solar system, so the critical size gets a bit larger. The Poynting-Robertson effect still affects these small particles, but they will be blown out of the solar system by the Sun's light before the Poynting-Robertson force works any significant change in their motion. Poynting conceived of the force in the then-dominant " Luminiferous Aether " view of the propagation of light; Robertson is the one who redid the demonstration from a Relativistic standpoint and confirmed the result. REFERENCES
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