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Eight more moons for Saturn
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post Jun 27 2006, 05:31 AM
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The Minor Planet Center today issued an MPEC that listed eight more outer moons for Saturn. They are all very small, in retrograd orbits, and typically have orbital periods of two to three years. They were discovered with the eight meter Subaru telescope in Hawaii, by Jewitt and company.

Edit: As Jyril points out below, there is a ninth satellite that the MPC, for some reason, listed in a seperate posting.
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Jyril
post Jun 27 2006, 11:50 AM
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Nine new moons, actually. S/2004 S 19 in the previous MPEC is also a new satellite.

That makes 56 Saturnian moons versus 63 Jovian moons. At this rate, Saturn will soon regain the planet with most satellites title. In addition, new moons of Saturn tend to be a couple of times bigger than Jupiter's (4-6 km vs. 1-2 km). Based on that, Saturn may have many more irregular satellites than Jupiter not yet detected. It is obvious that Uranus and Neptune are understudied in this respect, so there's probably many more satellites yet to be discovered.


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elakdawalla
post Jun 27 2006, 05:15 PM
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Can anyone help me? I've forgotten how to read these orbital elements from the MPECs to find out basic information (in particular, the semimajor axis). Here's one example set of elements:

CODE
S/2004 S 19
Epoch 2006 Mar. 6.0 TT = JDT 2453800.5                  MPC
M  89.58888              (2000.0)            P               Q
n   0.39374725     Peri.  268.52521     -0.67401577     +0.66054458
a   0.1214475      Node    47.33793     +0.71661425     +0.69335597
e   0.3604062      Incl.  153.27216     -0.17935092     +0.28799020
P   2.50           H   14.3           P/d   914.29

--Emily


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ugordan
post Jun 27 2006, 05:34 PM
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You can find an explanation of the orbital elements here.

The 3 elements you're probably most interested are:

a -- semimajor axis in AU,
e -- orbital eccentricity and
P -- orbital period in years.


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elakdawalla
post Jun 27 2006, 06:02 PM
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Thanks ugordan! smile.gif I thought that's what "a" meant but it seemed very surprising on the face of it for the semimajor axes to be 10% of an AU or bigger. These outer planets have really huge systems! No wonder Cassini can't spot these moons.

--Emily


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volcanopele
post Jun 27 2006, 06:12 PM
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Well Neptune has a couple of moons that have semi-major axes at around 0.3 AU


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Jyril
post Jun 27 2006, 08:21 PM
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Neptune's outer irregular satellites have very large eccentricities (around 0.5) which allow them to reach distances of almost 0.5 AU from Neptune! Furthest of them spends over 25 years making one orbit around Neptune.

Given how poorly Neptune's irregular satellites are known, it wouldn't be surprising if even more distant satellites are found. Because of the great distance from the Sun, Neptune's Hill sphere extends over 0.75 AU so there's plenty of room left.


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Rob Pinnegar
post Jun 27 2006, 09:43 PM
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QUOTE (Jyril @ Jun 27 2006, 02:21 PM) *
Neptune's Hill sphere extends over 0.75 AU so there's plenty of room left.

The Hill spheres of outer solar system bodies are pretty impressive -- I worked out Pluto's once and it came to something like 700,000 kilometres, not bad for such a small body.
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elakdawalla
post Jun 27 2006, 10:13 PM
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So what are the Hill spheres of Jupiter, Saturn, and Uranus?

--Emily


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Jyril
post Jun 27 2006, 10:38 PM
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Jupiter: 53.2 million km = 0.35 AU
Saturn: 65.5 million km = 0.44 AU
Uranus: 70.1 million km = 0.47 AU
Neptune: 116 million km = 0.77 AU

In comparison,

Earth: 1.50 million km
Mercury: 221,000 km
Pluto: 7.58 million km

So it is really the distance from the Sun that matters most.

The actual equation is

r ~ a * ( m/3M ) ^ 1/3

where r is the approximate radius of the Hill sphere, a orbital distance, m the mass of the orbiting body and M the mass of the parent body (in SI units). The mass of the satellite within the Hill sphere must be negligible compared to the other masses.


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Jyril
post Jun 27 2006, 10:54 PM
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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.


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tasp
post Jun 27 2006, 11:47 PM
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QUOTE (Jyril @ Jun 27 2006, 05:54 PM) *
Add various disturbances and it becomes clear that no large satellite can have own moons in our Solar System.



I am not smart enough to set up a simulation to see if this is possible, but there may be a congenital (if that is the right word) reason why we don't see satellites of moons.

Upon formation, let's say a moon like Dione, could be expected to be rotating non-synchronously with the host planet. Seems I have seen figures around 8 to 10 hours. Over time, the moon will tide lock to the planet, but during the period when it is not, any satellites of the moon will be subject to tidal effects from the moon, and will be accelerated in their orbit around the satellite, just as our moon is being accelerated today by tidal effects.

The orbital evolution of such satellites should be interesting. As they 'loft' away from their host moon, the effect weakens and the satellite may be expelled ever so gently from the 'Hill sphere' of the moon.

What then?

Well the little satellite is still in orbit around the host panet, and not moving away very quickly from the original 'host' moon.

I feel such objects are quite likely to wind up in a Trojan relationship with the moon.

Objects such as Pandora and Calypso may be old satellites of moons, conveniently left for us to explore, time capsules of the materials swept up by the original host moon, and never processed thermally or tectonically.


In the case of a larger moon such as Titan, we might expect the 'spin off' process to be rather more energetic and the cast off satellite might wind up in a higher orbit around the host planet, rather than in a Trojan relationship.

Hyperion might be such a body. It's crater devestated surface recording the influx of debris coalescing into Titan, back in the time when Saturn's moon's (and their satellites) were forming.
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mchan
post Jun 29 2006, 06:17 AM
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QUOTE (Jyril @ Jun 27 2006, 03:38 PM) *
r ~ a * ( m/3M ) ^ 1/3

where r is the approximate radius of the Hill sphere, a orbital distance, m the mass of the orbiting body and M the mass of the parent body (in SI units).

Uh, SI units are good, but are they required in this equation (as long as consistent units are used)?
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Jyril
post Jun 29 2006, 02:00 PM
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Umm... no, if you don't want an SI result. rolleyes.gif


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helvick
post Jun 29 2006, 04:09 PM
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QUOTE (Jyril @ Jun 29 2006, 03:00 PM) *
Umm... no, if you don't want an SI result. rolleyes.gif

Provided you measure the masses using the same units and are consistent for those two the result will be in whatever standard you choose for the orbital distance. So we could measure the masses in MegaElephants and use the recently discovered GigaCellphone for orbital distance and it would still work fine yielding results that are quite neat since a GigaCellphone is around 10 million km.
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