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Moon Base baby steps



 
 
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  #101  
Old February 10th 04, 07:49 PM
Sander Vesik
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Default Moon Base baby steps

In sci.space.policy Henry Spencer wrote:
In article ,
Sander Vesik wrote:
No, *some* of them are *possibly* rubble piles. Some are definitely not;
Eros, in particular, appears to be essentially solid rock. Even for the
low-density ones, the matter is not entirely settled...


Surely also at least the double asteroids (two asteroids orbiting each other
closely) are also not rubble piles?


They might be. A close encounter with a planet can split a rubble pile
into a pair of rubble piles, by tidal interaction. (In fact, one of the
points offered in support of the rubble-pile hypothesis is precisely that
we see a suspiciously large number of double asteroids, which ought to be
fairly rare unless there is some specific mechanism that creates them.)


But if you have two piles of rubble orbiting each other, then surely
tidal forces would over time convert these to fuzzy rubble-balls that would
then coalesce into a single body?

--
Sander

+++ Out of cheese error +++
  #102  
Old February 11th 04, 11:23 AM
Paul Blay
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Default Moon Base baby steps

"Sander Vesik" wrote ...
But if you have two piles of rubble orbiting each other, then surely
tidal forces would over time convert these to fuzzy rubble-balls that would
then coalesce into a single body?


Over time they would become tidally locked to each other - and in the
process they would orbit further apart. (Or such is my back-of-cornflake-packet
understanding). Ignoring interactions with any _other_ bodies.
  #103  
Old February 13th 04, 10:24 PM
Henry Spencer
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Default Moon Base baby steps

In article ,
Sander Vesik wrote:
...A close encounter with a planet can split a rubble pile
into a pair of rubble piles, by tidal interaction. (In fact, one of the
points offered in support of the rubble-pile hypothesis is precisely that
we see a suspiciously large number of double asteroids, which ought to be
fairly rare unless there is some specific mechanism that creates them.)


But if you have two piles of rubble orbiting each other, then surely
tidal forces would over time convert these to fuzzy rubble-balls that would
then coalesce into a single body?


Tidal forces within such a system will probably fairly quickly lock the
spin of each rubble ball to their orbit around each other, but the effect
of that on the spacing between them is quasi-random. (For forward spins
it will move them outward, but there's no special tendency for such small
bodies to have forward spins.) And if they're formed by fission of a
single body, they'll probably start out nearly locked anyway, so any
effect will be small.

Once that's happened, only solar tidal effects will change the spacing of
the system, and those will be very slow -- too slow, in an environment
where close planetary encounters happen frequently.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #104  
Old February 14th 04, 03:01 PM
Dr John Stockton
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Default Moon Base baby steps

JRS: In article , seen in
news:sci.space.policy, Henry Spencer posted at
Mon, 9 Feb 2004 19:37:44 :-

A close encounter with a planet can split a rubble pile
into a pair of rubble piles, by tidal interaction. (In fact, one of the
points offered in support of the rubble-pile hypothesis is precisely that
we see a suspiciously large number of double asteroids, which ought to be
fairly rare unless there is some specific mechanism that creates them.)



Why a pair of rubble piles? Half a rubble pile is still a rubble pile,
and should itself be split (ignoring the presumably improbable case of
an encounter virtually tangent to the Roche limit).

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.
  #105  
Old February 14th 04, 10:03 PM
Sander Vesik
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Default Moon Base baby steps

In sci.space.policy Henry Spencer wrote:
In article ,
Sander Vesik wrote:
...A close encounter with a planet can split a rubble pile
into a pair of rubble piles, by tidal interaction. (In fact, one of the
points offered in support of the rubble-pile hypothesis is precisely that
we see a suspiciously large number of double asteroids, which ought to be
fairly rare unless there is some specific mechanism that creates them.)


But if you have two piles of rubble orbiting each other, then surely
tidal forces would over time convert these to fuzzy rubble-balls that would
then coalesce into a single body?


Tidal forces within such a system will probably fairly quickly lock the
spin of each rubble ball to their orbit around each other, but the effect
of that on the spacing between them is quasi-random. (For forward spins
it will move them outward, but there's no special tendency for such small
bodies to have forward spins.) And if they're formed by fission of a
single body, they'll probably start out nearly locked anyway, so any
effect will be small.

Once that's happened, only solar tidal effects will change the spacing of
the system, and those will be very slow -- too slow, in an environment
where close planetary encounters happen frequently.


Ah, I see. Thanx.

--
Sander

+++ Out of cheese error +++
  #106  
Old February 16th 04, 12:04 AM
Henry Spencer
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Default Moon Base baby steps

In article ,
Dr John Stockton wrote:
A close encounter with a planet can split a rubble pile
into a pair of rubble piles, by tidal interaction. (In fact, one of the
points offered in support of the rubble-pile hypothesis is precisely that
we see a suspiciously large number of double asteroids, which ought to be
fairly rare unless there is some specific mechanism that creates them.)


Why a pair of rubble piles? Half a rubble pile is still a rubble pile,
and should itself be split (ignoring the presumably improbable case of
an encounter virtually tangent to the Roche limit).


If memory serves -- I didn't pay close attention to the details -- the
two-way split of a rubble pile is a non-trivial interaction between tidal
forces and the rotation of the original rubble pile, and generally does
not involve further splitting of the resulting two piles.

Multi-way splits of fragile bodies are certainly possible, as witness SL9,
but they generally don't leave the fragments in orbit around each other.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |
  #107  
Old February 19th 04, 12:18 AM
Dr John Stockton
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Default Moon Base baby steps

JRS: In article , seen in
news:sci.space.policy, Henry Spencer posted at
Sun, 15 Feb 2004 23:04:56 :-
In article ,
Dr John Stockton wrote:
A close encounter with a planet can split a rubble pile
into a pair of rubble piles, by tidal interaction. (In fact, one of the
points offered in support of the rubble-pile hypothesis is precisely that
we see a suspiciously large number of double asteroids, which ought to be
fairly rare unless there is some specific mechanism that creates them.)


Why a pair of rubble piles? Half a rubble pile is still a rubble pile,
and should itself be split (ignoring the presumably improbable case of
an encounter virtually tangent to the Roche limit).


If memory serves -- I didn't pay close attention to the details -- the
two-way split of a rubble pile is a non-trivial interaction between tidal
forces and the rotation of the original rubble pile, and generally does
not involve further splitting of the resulting two piles.

Multi-way splits of fragile bodies are certainly possible, as witness SL9,
but they generally don't leave the fragments in orbit around each other.


OK, I can easily enough believe that if the spin rate gives a
centrifugal effect non-negligible in comparison with surface gravity,
then the break-up will occur at a greater distance, and that the
subsequent distribution of angular momentum from spin into spin + spin +
orbital will leave the new spins insufficient for further break-up.

"The satellite is here assumed not to be spinning." is now added to my
Roche material in gravity3.htm.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.
 




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