On 13/11/2012 8:48 AM, Bill Pragnell wrote:
> Not a wobble, if it encircled a star there would be no orbital motion - an
> outright crash! This is the scenario overlooked by Niven in Ringworld, which he
> later retconned with the rim stabilising thrusters.
>
> However, I do not believe the present artifact is supposed to encircle a star,
> or spin like a ringworld (at least not enough to generate significant
> centrifugal forces). I like the idea that it is planet-scaled, generating
> gravity by virtue of its own mass. It is difficult to see how it could form
> naturally of course...

Fair point Bill. But Ed said "It should spin in about 12 hours or 24 
hours or so."

Personally I prefer the Culture Orbitals. ;-)

-- 
Regards
     Stephen

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Le 2012-11-13 10:42, Le_Forgeron a écrit :
> Le 13/11/2012 13:04, Thomas de Groot a écrit :
>> On 13-11-2012 9:48, Bill Pragnell wrote:
>>> However, I do not believe the present artifact is supposed to encircle
>>> a star,
>>> or spin like a ringworld (at least not enough to generate significant
>>> centrifugal forces). I like the idea that it is planet-scaled, generating
>>> gravity by virtue of its own mass. It is difficult to see how it could
>>> form
>>> naturally of course...
>>
>> My idea too. A torus with Earth's diameter at least as cross-section to
>> generate the necessary gravity at the surface; the same composition,
>> with a molten metallic core (inner torus). The whole structure orbiting
>> a star... which would probably tear it apart, or free in the
>> interstellar medium, with a need for a light and warmth source...
>>
>> Interesting musing... ;-)
>
> Oh yes, what about the magnetosphere ? (magnetotorus ??)
> Is there a north pole too ?
> (probably, assuming the inner torus has its own revolution speed)
>
> Would the magnetic field looks like the tokamak helicoidal field ?
>
> And for added fun: could such planet have a moon or more ?
>
>

A planetary magnetic field is not needed to protect life on the planet's 
surface, you only need a thick enough athmosphere. After all, if it was 
/realy/ needed to protect us from solar particles, those do hit us in 
the polar regions and would whipe out all life in those areas...

It could have a, or a few, moons if it's far enough and small enough. A 
natural satellite the size of our moon would probably need to orbit in 
about 4 to 7 months and it may spell doom whatever it's distance. If 
it's small enough, like Phobos and Diemos of Mars, they could be much 
closer.



Alain

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Am 14.11.2012 00:00, schrieb Alain:

> A planetary magnetic field is not needed to protect life on the planet's
> surface, you only need a thick enough athmosphere.

Veto! A thick atmosphere means high pressure; high pressure means high 
temperature; high pressure plus high temperature means hell... see Venus.

> After all, if it was
> /realy/ needed to protect us from solar particles, those do hit us in
> the polar regions and would whipe out all life in those areas...

I'm not so sure about that. The solar particles come in from the side, 
where the magnetic field does deflect the particles. They end up in the 
Van Allen belts, where due to the shape of the magnetic field they are 
confined high above the atmosphere, traveling back and forth between the 
poles. AFAIK only a small portion of the particles does make it down to 
the atmosphere to create polar lights, and I'd assume they've already 
been slowed down a good deal by then.

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Stephen <mca### [at] aolcom> wrote:
> Fair point Bill. But Ed said "It should spin in about 12 hours or 24
> hours or so."

Which is not fast enough for significant centrifugal effects - but just right
for an earthlike day/night cycle. Although depending on its inclination and
rotation axis, the inhabitants might be in for some wacky seasons, and there
could be a lot of secondary light at night.

> Personally I prefer the Culture Orbitals. ;-)

Me too. Mostly for the amenities :)

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"Bill Pragnell" <bil### [at] hotmailcom> wrote:
> Stephen <mca### [at] aolcom> wrote:
> > Fair point Bill. But Ed said "It should spin in about 12 hours or 24
> > hours or so."
>
> Which is not fast enough for significant centrifugal effects -

Are you sure, are my calculations wrong?


> but just right
> for an earthlike day/night cycle. Although depending on its inclination and
> rotation axis, the inhabitants might be in for some wacky seasons, and there
> could be a lot of secondary light at night.
>
> > Personally I prefer the Culture Orbitals. ;-)
>
> Me too. Mostly for the amenities :)

And the snappy repartee. ;-)

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"Stephen" <mcavoys_AT_aolDOT.com> wrote:
> "Bill Pragnell" <bil### [at] hotmailcom> wrote:
> > Stephen <mca### [at] aolcom> wrote:
> > > Fair point Bill. But Ed said "It should spin in about 12 hours or 24
> > > hours or so."
> >
> > Which is not fast enough for significant centrifugal effects -
>
> Are you sure, are my calculations wrong?

Um, I didn't check. Quick calculation of my own, assuming a torus minor diameter
the same as the earth, which gives a major radius of about 6 earths (~77,000km),
spinning once per 12 hours, I get a centripetal acceleration of about 1.6
ms-2... ok, a bit more significant than I thought, that's about 1/6th of a g
more on the inner surface and less on the outer surface.

Athletes from the inner provinces would trounce their opponents from the outer
lands :)

> > > Personally I prefer the Culture Orbitals. ;-)
> >
> > Me too. Mostly for the amenities :)
>
> And the snappy repartee. ;-)

and let's not forget the parties!

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"Bill Pragnell" <bil### [at] hotmailcom> wrote:
> "Stephen" <mcavoys_AT_aolDOT.com> wrote:
> > "Bill Pragnell" <bil### [at] hotmailcom> wrote:
> > > Stephen <mca### [at] aolcom> wrote:
> > > > Fair point Bill. But Ed said "It should spin in about 12 hours or 24
> > > > hours or so."
> > >
> > > Which is not fast enough for significant centrifugal effects -
> >
> > Are you sure, are my calculations wrong?
>
> Um, I didn't check. Quick calculation of my own, assuming a torus minor diameter
> the same as the earth, which gives a major radius of about 6 earths (~77,000km),
> spinning once per 12 hours, I get a centripetal acceleration of about 1.6
> ms-2... ok, a bit more significant than I thought, that's about 1/6th of a g
> more on the inner surface and less on the outer surface.
>

Oops! I forgot to divide bt 9.81 and called m/s^2 g :-(


> Athletes from the inner provinces would trounce their opponents from the outer
> lands :)
>
> > > > Personally I prefer the Culture Orbitals. ;-)
> > >
> > > Me too. Mostly for the amenities :)
> >
> > And the snappy repartee. ;-)
>
> and let's not forget the parties!

If you can remember the parties then you wern't there. ;-)

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On 15-11-2012 13:22, Stephen wrote:
> "Bill Pragnell" <bil### [at] hotmailcom> wrote:
>> "Stephen" <mcavoys_AT_aolDOT.com> wrote:
>>> "Bill Pragnell" <bil### [at] hotmailcom> wrote:
>>>> Stephen <mca### [at] aolcom> wrote:
>>>>> Fair point Bill. But Ed said "It should spin in about 12 hours or 24
>>>>> hours or so."
>>>>
>>>> Which is not fast enough for significant centrifugal effects -
>>>
>>> Are you sure, are my calculations wrong?
>>
>> Um, I didn't check. Quick calculation of my own, assuming a torus minor diameter
>> the same as the earth, which gives a major radius of about 6 earths (~77,000km),
>> spinning once per 12 hours, I get a centripetal acceleration of about 1.6
>> ms-2... ok, a bit more significant than I thought, that's about 1/6th of a g
>> more on the inner surface and less on the outer surface.
>>
>
> Oops! I forgot to divide bt 9.81 and called m/s^2 g :-(
>
>
>> Athletes from the inner provinces would trounce their opponents from the outer
>> lands :)
>>
>>>>> Personally I prefer the Culture Orbitals. ;-)
>>>>
>>>> Me too. Mostly for the amenities :)
>>>
>>> And the snappy repartee. ;-)
>>
>> and let's not forget the parties!
>
> If you can remember the parties then you wern't there. ;-)


I see that on my travels, jumping through hoops with my spacecraft, I 
missed all the fun... :-)

Thomas

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Alain <kua### [at] videotronca> wrote:
> A planetary magnetic field is not needed to protect life on the planet's
> surface, you only need a thick enough athmosphere.

However, a magnetic field may be needed to protect the /atmosphere/ itself.
Mars once had a much thicker atmosphere, and the going hypothesis is that it was
blown away by the solar wind after Mars' core solidified.

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> Alain <kua### [at] videotronca> wrote:
>> A planetary magnetic field is not needed to protect life on the planet's
>> surface, you only need a thick enough athmosphere.
>
> However, a magnetic field may be needed to protect the /atmosphere/ itself.
> Mars once had a much thicker atmosphere, and the going hypothesis is that it was
> blown away by the solar wind after Mars' core solidified.
>
>
>

Mars is also about half the Earth's radius, making it 1/8th the mass and 
a much lower surface gravity. Been farther from the sun makes it cooler 
and reduce the athmospheric evaporation that was greatly increased by 
the lower mass. Mars DOES slowly loose some of it's athmosphere, mostly 
light gases like oxygen and nitrogen, at a greater rate, leaving about 
only carbon dioxide.

Now, take a planet with an average density of about 3, compared to 
around 5 for the Earth, and increase it's radius to get a surface 
gravity of 0.9g. Take it slightly farther from the Sun.
That planet could have a surface athmosphere similat to Earth, but it 
would extent much higher, making it thicker. The planet been farther 
from the sun receive less energy, compensating the higher thermal retention.
The Solar wind effect on it would be rather negligeable even without any 
magnetic field. At the same time, that planet can actualy collect stray 
gases, and some mass from the solar wind, to replentish itself.
On such a planet, nights would be warmer than on Earth and, for the same 
axis tilt, you'd expect less contrasted seasons as the athmosphere would 
have more thermal inertia.



Alain

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