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© Copyright 1996, Jim Loy
1980 S1 and
1980 S3 are the names of two of the moons of Saturn. They are called
"co-orbital moons," because they orbit Saturn in almost the same
exact orbit. The orbits are so nearly identical that if the two moons moved
totally independently of each other, they would occasionally collide. But they
do not collide. How do we know they don't? The answer is that physics (orbital
dynamics) says that they don't. This article is a brief explanation.
Both moons are moving in nearly circular orbits. They are not moving at exactly the same speed. The faster moon slowly gains on the slower moon. The faster moon is faster because it is in a slightly lower orbit. They approach each other very slowly, and are close to each other for a long period of time.
The gravity of the trailing moon pulls on the leading moon, and the gravity of the leading moon pulls on the trailing moon. This adds energy to the trailing moon and takes energy from the leading moon. This does not speed up the trailing moon, but instead tugs it into a higher orbit. In this higher orbit, the trailing moon has more energy, and a slower speed. Similarly, the leading moon loses energy and is pulled into a lower, faster orbit.
And so, instead of colliding, they change speeds, and move apart. They act as if they repell each other.
There are other co-orbital moons around Saturn. Tethys has four tiny moons in its orbit, and Dione has two smaller moons in its orbit. These two larger moons are not greatly affected by the smaller moons. But the smaller moons are tugged around by the larger moons and by each other. And they may never collide.