Poynting-Robertson effect


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Poynting–Robertson effect

(poin -ting rob -ert-sŏn) An effect whereby interplanetary particles slowly spiral toward the Sun. The particles interact with solar radiation (by absorbing and reradiating energy) so that they lose orbital momentum; this causes a decrease in orbital velocity so that they move into progressively smaller orbits around the Sun. For a particle moving in a circular orbit that is d astronomical units from the Sun, the time taken to fall into the Sun is given by
t = 7 × 107 ρrd 2 years
where r and ρ are particle radius (in m) and density (in kg m–3), respectively. The equation shows that the effect gets progressively more important as smaller particles are considered, and is particularly important in the 10–6 to 10–2 meter range.

Radiation pressure on the particles opposes the Poynting–Robertson effect and can overcome it for very small particles (radius less than 10–6 m). These particles are blown out of the Solar System by the action of the radiation pressure aided by the solar wind. Although these two processes remove dust from the Solar System, the dust level is constantly replenished by disintegrating comets, etc.

Poynting-Robertson Effect

 

the decrease in the angular momentum, and consequently in the orbital dimensions, of a small body moving around the sun or around some other intense source of radiation and isotropically reemitting solar radiation. This effect was discovered by the English physicist J. H. Poynting in 1903, and a precise relativistic theory was provided by Poynting’s compatriot H. Robertson in 1937.

The Poynting-Robertson effect is related to the fact that prior to absorption by a body, solar photons move radially and possess zero angular momentum with respect to the sun. But the body reemits solar radiation isotropically in a coordinate system moving with the body, so that the mean specific angular momentum of the radiated photons is equal to the specific angular momentum of the body. Partial transfer of the body’s angular momentum to the reemitted photons occurs, and the body approaches the sun in a spiral.

A spherical body of radius a (cm) and density δ (g/cm3) located in a quasi-circular orbit of radius R astronomical units theoretically “falls” into the sun in T = 7 × 106αδ R2 years. In actuality, the body vaporizes in the vicinity of the sun and unites with its atmosphere in the form of a vapor cloud. For a body moving in an elliptical orbit, the contraction of its orbital dimensions is accompanied by a decrease in the orbital eccentricity.

In 1950 the Soviet astronomer V.V. Radzievskii revealed the existence of a planetocentric Poynting-Robertson effect, that is, of a decrease in the orbit of a body moving about a planet, again owing to reemission of solar radiation.

B. IU. LEVIN

Poynting-Robertson effect

[′pȯint·iŋ ′räb·ərt·sən i‚fekt]
(astronomy)
The gradual decrease in orbital velocity of a small particle such as a micrometeorite in orbit about the sun due to the absorption and reemission of radiant energy by the particle.