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shepherd satellite

noun
1.
a small moon orbiting near a planetary ring, whose gravitational pull helps confine the ring and the ring's extent.
Also called shepherd moon.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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shepherd-moon in Science
shepherd satellite
  (shěp'ərd)   
A moon that orbits near the edge of a planetary ring, stabilizing the ring's particles through gravitational pull and confining the ring to a sharply defined band. For example, the moons of Uranus known as Cordelia and Ophelia are shepherd satellites that constrain Uranus's rings to a narrow band. It is believed that the gravity of the fast-moving inner satellite Cordelia causes the particles of the inner ring to speed up and move to outer orbits, while at the same time the gravity of the slower, outer satellite Ophelia decelerates outer particles, pushing them inward. The result is a compressed, well-defined ring. Also called shepherd moon.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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