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More Moons Around Pluto?
DEChengst
post Nov 1 2005, 12:03 PM
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QUOTE (jamescanvin @ Nov 1 2005, 06:45 AM)
Recently on this board we have discussed how NH would have to be very lucky to fly by two Kuiper Belt objects after Pluto-Charon. Now we get two for free! Should get a sample of five now, NH just became even better value!
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I'm not sure the two moons are as interesting for learning more about differences between Kuiper belt objects as two seperate objects would be. The two moons may have been formed together with Pluto itself so may be very much like Pluto composition wise. The other options ofcourse is that it are captured objects which would be just as interesting as flying by two seperate objects smile.gif


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odave
post Nov 1 2005, 04:04 PM
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Add my congrats to the pile as well. This is fantastic news!

I also greatly appreciate Alan and John's participation here in UMSF. As we all well know, the Internet is full of kooks, trolls, and all sorts of other miscreants. The fact that members of NH and other mission teams (Jason Perry, Mike Caplinger, et. al.) actively post here is a testament to the quality of the members, moderators, and discussion found in this forum!


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Ames
post Nov 1 2005, 04:22 PM
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QUOTE (DEChengst @ Nov 1 2005, 01:03 PM)
I'm not sure the two moons are as interesting for learning more about differences between Kuiper belt objects as two seperate objects would be. The two moons may have been formed together with Pluto itself so may be very much like Pluto composition wise. The other options ofcourse is that it are captured objects which would be just as interesting as flying by two seperate objects smile.gif
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Now let's see - Hmmmm. Name just one object in the solar system that when we looked a little closer at it, nobody said "Woa!" "Wow!" "What The?"
Everything we look at is amazing, and the closer we look the better it gets.
The sheer multitude of landforms, colours, shapes, alignments, hotspots... seem endlessly beautiful and strange.

I can guarantee Pluto et al will not disappoint.

Nick
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john_s
post Nov 1 2005, 04:47 PM
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QUOTE (Rob Pinnegar @ Nov 1 2005, 02:54 AM)
I'm confused here: On the website cited above, the estimates for the diameter of S/2005 P1 are 160 km if its albedo is 0.04, and 110 km if its albedo is 0.35.

But shouldn't the estimated diameters of the new moons vary roughly as 1/sqrt(albedo)? I would've thought that a ninefold increase in reflectively would decrease the estimated diameter by a factor of about three, since the total brightness has to stay the same.

Using 16.8 as apparent M_v for Charon, and 23.0 as apparent M_v for S/2005 P1 as given on the website, and assuming both bodies have the same albedo, I get a diameter about one-seventeenth of Charon's, or ~70 km, for 2005 P1. Have I got something wrong?
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No, you don't have anything wrong- we goofed on the web site, though we got the diameter right for the lowest albedo. Thanks for catching the error- we'll fix it! The correct numbers should be something like 160 km for an albedo of 0.04 and 52 km for an albedo 0.35.
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Rob Pinnegar
post Nov 1 2005, 05:03 PM
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QUOTE (john_s @ Nov 1 2005, 10:47 AM)
No, you don't have anything wrong- we goofed on the web site, though we got the diameter right for the lowest albedo.  Thanks for catching the error- we'll fix it!

Glad I could help. And by the way -- congratulations to your team.
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Rakhir
post Nov 1 2005, 10:18 PM
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QUOTE (JRehling @ Oct 31 2005, 09:51 PM)
  New, larger, light-bucket telescopes on the ground should be able to outperform HST in gathering more data on these bodies long before NH arrives...
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If NH is launched during the secondary launch window and if the OWL project is accepted and meet the current schedule, a 60-m class telescope would be available as soon as 2016-2017. It could then be used for the arrival of NH in 2019-2020.
The full 100-m OWL capability would be available too late for NH, in 2020.

Rakhir
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RNeuhaus
post Nov 2 2005, 03:00 PM
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QUOTE (JRehling @ Oct 31 2005, 05:16 PM)
I respectfully suggest that you really ought to familiarize yourself with some of the basics here -- there are at least four reasons why this scenario is science fiction.

One, there is no way to change the spacecraft at this point -- add mini-thrusters?! The time to make major design alterations came and went a long time ago.
...
...
...

I know that the spirit of this board is in a lot of good fun, but it wouldn't hurt to do a *LITTLE* fact checking before dispensing "good advise".
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Many thanks to JRehling with your very good explanation. Now, I can feel it more realistic after knowing your details. Sorry of my first reaction since I am a novel of astronomy science. Little by little I will be better off on that.

Taking the advantage of this post, congratulations to Alan and his team for this great news.

Rodolfo
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Rob Pinnegar
post Nov 2 2005, 04:04 PM
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I just ran through some simple calculations concerning the orbit of the inner new satellite (the one for which a~49000 km).

If this moon were traversing a perfectly circular path around the system's barycentre, with the orbit in the same plane as Charon's, then the total acceleration due to gravity it would experience at closest approach to Charon should be about 5% stronger than the acceleration it experiences when it is farthest away from Charon. So one could conclude from this that the orbit can't really be a perfect circle.

I'm no expert at classical dynamics, but from an intuitive standpoint it's difficult to see how the orbit could be stable under these circumstances unless the moon were in the 4:1 resonance.

My semi-educated guess is that, to first approximation, the moon's distance from Pluto probably varies with an angular frequency that is exactly three times the angular frequency of its revolution about the system's barycentre. For the outer moon it would be five times the revolution frequency.

The only other satellite in the solar system that is in a similar position is Hyperion, but somehow I doubt that the effects are anywhere near as large in Hyperion's case since Saturn's mass is so much greater than Titan's. It will be an interesting problem to solve.

[Edit: ... If it hasn't been solved already, which I'm guessing it has.]
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JRehling
post Nov 2 2005, 05:22 PM
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QUOTE (Rob Pinnegar @ Nov 2 2005, 09:04 AM)
I just ran through some simple calculations concerning the orbit of the inner new satellite (the one for which a~49000 km).

If this moon were traversing a perfectly circular path around the system's barycentre, with the orbit in the same plane as Charon's, then the total acceleration due to gravity it would experience at closest approach to Charon should be about 5% stronger than the acceleration it experiences when it is farthest away from Charon. So one could conclude from this that the orbit can't really be a perfect circle.
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A fun additional consideration: The other "moon" that the new ones can synchronize against is Pluto! You calculated a 5% "surge" when the moons are opposite Charon, but 180-deg opposite, there will be another "surge" as Pluto passes, somewhat closer than otherwise.

The Pluto "surge" felt by each outer satellite should be less than the Charon surge by precisely the ratio of their masses. Since that ratio is about 7:1, the Charon surge will be the dominant perturbation, with the Pluto surge lesser but non-negligible.
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Rob Pinnegar
post Nov 2 2005, 05:38 PM
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QUOTE (JRehling @ Nov 2 2005, 11:22 AM)
A fun additional consideration: The other "moon" that the new ones can synchronize against is Pluto! You calculated a 5% "surge" when the moons are opposite Charon, but 180-deg opposite, there will be another "surge" as Pluto passes, somewhat closer than otherwise.

This is correct and I actually did take it into account -- the calculation referred to above assumed that the moon was circling the Pluto-Charon barycentre, not Pluto itself. Without the Pluto "surge" the difference would be greater than 5%; when the inner new moon reaches closest approach to Charon, the gravitational influence of Charon is about a third of Pluto's!
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Rob Pinnegar
post Nov 2 2005, 06:06 PM
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The other neat thing is that the total acceleration vector experienced by the new moons due to the combined effects of Pluto and Charon, usually doesn't point straight at the barycentre. When the line connecting Pluto and Charon is perpendicular to the line connecting S2005 P2 to the barycentre, for example, the acceleration vector leans slightly toward Pluto.

My _suspicion_ is that the two new moons reach their greatest distances from the barycentre simultaneously with closest approach to Charon. After that, they start to fall inwards and also start to lead a bit in their orbit. Closest approach to the barycentre coincides with their closest approach to Pluto, after which they trail in their orbits and move farther away. This is however conjecture and I've not got it supported by mathematics. I could have the leading/trailing bit backwards, for example.

Of course, the two new moons will interact with each other a bit, but that ought to be a higher-order effect. [Edit: Actually it wouldn't be higher-order, just smaller in magnitude.]

[Edit: A quick back of the envelope calculation indicates that these effects could cause the inner new moon's distance from the system's barycentre to change by up to 250 kilometres, three times per orbit. Hope I haven't made any major blunders here -- I've assumed a simple harmonic oscillator, in the radial direction only, and have neglected the skew in the acceleration vector.]
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tasp
post Nov 3 2005, 04:29 AM
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QUOTE (alan @ Nov 1 2005, 12:28 AM)
New moons in resonance with Plutos rotation?
Charon's period is 6.387 days: 6.387 * 4 = 25.6 days ; 6.387 * 6 = 38.3 days
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If correct, all the satellites of Pluto will essentially repeat their relative positions every 77 days or so. This will limit the number of satellite configurations for the mission designers to evaluate. There won't be any point in moving the arrival date beyond a couple of orbits of the outer satellite. The NH craft will have the same views of all the objects.
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JRehling
post Nov 3 2005, 12:29 PM
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QUOTE (tasp @ Nov 2 2005, 09:29 PM)
If correct, all the satellites of Pluto will essentially repeat their relative positions every 77 days or so.  This will limit the number of satellite configurations for the mission designers to evaluate.  There won't be any point in moving the arrival date beyond a couple of orbits of the outer satellite.  The NH craft will have the same views of all the objects.
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Well, it would allow for 24 distinct configurations of the outer satellites, or whatever fraction of those 24 the "arrival window" allows for -- that seems like a considerable variety of options.
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Rob Pinnegar
post Nov 3 2005, 03:27 PM
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QUOTE (tasp @ Nov 2 2005, 10:29 PM)
If correct, all the satellites of Pluto will essentially repeat their relative positions every 77 days or so.

If we're lucky, the (hypothesized) resonances will turn out to be similar to the Jupiter-Io-Europa-Ganymede system, where periodically all three moons and the central planet arrange themselves in a straight line.

If we're _really_ lucky, this configuration will occur when the moons are all close to the plane of Pluto's orbit around the Sun. If this were the case, New Horizons could make close flybys of all four bodies, concentrating exclusively on one body at a time, during its flyby in '15.

We'll have to see. I guess one approach would be to take two snapshots of the system, 77 days apart. If they turn out to look the same, well, there you have it.
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tasp
post Nov 3 2005, 04:11 PM
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QUOTE (JRehling @ Nov 3 2005, 12:29 PM)
Well, it would allow for 24 distinct configurations of the outer satellites, or whatever fraction of those 24 the "arrival window" allows for -- that seems like a considerable variety of options.
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Like in the Jupiter system, due to the resonance of Io, Europa, and Ganymede, certain configurations of the satellites never occur. Like all three in a straight line on the same side of Jupiter. I was annoyed a little when the mission designers of Voyager II's Uranus flyby stated a particularly appealing configuration of the Uranian moons occured just a few days prior to the earliest possible flyby date that preserved the Neptune option. Hence the fuzzy picture of Umbriel.

Should be easier for the analysts to examine ~77 days of moon configurations than potentially years worth.
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