# why are the moons of satuarn(epimetheus & janus) in the same orbit on the same plane?

i'm looking at national geographic solar system--are the orbits correct?

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The info I found is:

Epimetheus:

mean orbital radius = 151,400 km

eccentricity = 0.010

orbit inclination = 0.35 degree (almost 0)

orbital period = 0.694 Earth days (59,960 seconds)

Janus:

mean orbital radius = 151,500 km

eccentricity = 0.007

orbit inclination = 0.17 degree (even closer to 0)

orbital period = 0.695 Earth days (60,050 seconds)

Because the numbers are rounded (e.g., orbital radius is to the nearest 100 km), it could be that the differences are even less. The numbers shown here are "mean" values. The real orbital radius changes a tiny bit from one orbit to the next.

When the satellites get close enough to each other, they will perturb each other's orbit so that collisions are avoided.

For example, if one satellite is slightly closer to the planet than the other, it will catch up to the other. As they approach each other, the faster satellite (behind the slow one) is attracted by the slower one and gains energy. It now has too much energy for its orbit: it moves away from the planet (by a small amount) and slows down.

Meanwhile, the slow satellite loses energy to the fast one. It now has insufficient energy for its orbit and "falls" to a lower orbit where, to maintain equilibrium, it goes faster.

After the encounter, the satellite that was faster and closer to the planet becomes the slow satellite, and vice versa.

Source(s): 2007 Observer's Handbook; Royal Astronomical Society of Canada
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