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#101
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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
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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
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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
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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
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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
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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
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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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