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Eight more moons for Saturn
Rob Pinnegar
post Jul 15 2006, 07:32 PM
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QUOTE (Jyril @ Jun 27 2006, 04:54 PM) *
Add various disturbances and it becomes clear that no large satellite can have own moons in our Solar System.

It would be interesting, though, if some of these outer-irregular satellites of the giant planets were of the double-asteroid type, i.e. two similarly sized bodies in close orbit.

Cassini might be able to resolve this sort of thing (albeit just barely).
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Jyril
post Jul 15 2006, 08:35 PM
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If the irregular satellites originate from captured and disrupted asteroids, it wouldn't be surprising if some of them are close (or contact) binaries.

Aren't they way too far to be detectable by Cassini?


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tasp
post Jul 16 2006, 02:46 AM
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Seems like Iapetus would have a uniquely large Hill sphere for a moon . . . .


wink.gif
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Themisto
post Aug 5 2006, 11:39 AM
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QUOTE (Jyril @ Jun 28 2006, 12:54 AM) *
Titan probably has larger Hill sphere than any other satellite. Still, the radius of its Hill sphere is mere 52,400 km. Callisto, whose orbital radius is larger, has slightly smaller Hill sphere (50,100 km). Jupiter's greater mass and smaller orbital distance makes Ganymede's Hill sphere much smaller, only 31,700 km in radius. Add various disturbances and it becomes clear that no large satellite can have own moons in our Solar System.

Hi,

using the Hill sphere formula given in this chat, it becomes indeed obvious that the larger satellites have Hill spheres about 1-2 orders of magnitudes smaller than the planets:
Ganymede 0.00021 AU
Callisto: 0.00033 AU
Titan: 0.00035 AU
Iapetus: 0.00025 AU

Mercury, in comparison, has 0.0015 AU.

However, tiny asteroid (243) Ida has only 0.00005 AU, or 8200 km, but owns a satellite. How does this work?

T.
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Rob Pinnegar
post Aug 5 2006, 03:07 PM
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QUOTE (Themisto @ Aug 5 2006, 05:39 AM) *
However, tiny asteroid (243) Ida has only 0.00005 AU, or 8200 km, but owns a satellite. How does this work?

Simple... the satellite has to stay closer (in practice, substantially closer) than 8200 km from Ida. Otherwise the sun will snatch it away.

Staying within the Hill sphere, though, is no guarantee of a stable orbit when we're talking about satellites of satellites. The Galilean satellites all have Hill spheres, but that doesn't mean they can have moons of their own. For example, anything going into orbit around Callisto (within Callisto's Hill sphere) would probably be able to stay in that orbit for a year or two. Eventually, though, the huge gravitational perturbations from Jupiter would destabilize it. It would most likely end up crashing into Callisto.

This is one of the problems that has to be considered for the Europa Orbiter; it will need to constantly stabilize itself, and that's going to eat up fuel.
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Jyril
post Aug 6 2006, 02:55 PM
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Dactyl's orbit is poorly constrained, but it is roughly 100 km.


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