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How far will NH pass by Pluto?, - and will it be possible to effect a meaningful course change?
Greg Hullender
post Apr 17 2008, 04:12 AM
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QUOTE (Oersted @ Apr 16 2008, 10:38 AM) *
Why isn't a figure 8 or just an S-shaped orbit physically possible? (as opposed to just being not desirable for coverage reasons).

I'm not sure what an S-shaped orbit would be, but there are no stable 3-body orbits except for L4 and L5. I know that the proof of this starts with Hamiltonians and Lagrangians (or at least I think it does) but I'm not able to work it out myself. However, I remember Feynman telling it to my freshman class at Caltech, and I suspect he knew what he was talking about. :-)

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mchan
post Apr 17 2008, 05:31 AM
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The key word is "stable" orbit, meaning it would repeat without any propulsive maneuvers. One may recall the free-return trajectory used by Apollo 13 was ostensibly a figure 8, but it would not have repeated if the spacecraft did not re-enter because the Moon would have moved along in its orbit around the Earth by the time the spacecraft got out to the Moon's orbit again.
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brellis
post Apr 17 2008, 05:53 AM
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To rephrase the original question:

Kuiper Belt Objects are really far apart. There's not much chance of an accidental close encounter with a KBO, but theoretically a juicy one might be discovered in time for a trajectory change to provide a targeted encounter. If such a discovery were to be made, could the trajectory be altered to point NH differently after the Pluto encounter?
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remcook
post Apr 17 2008, 10:44 AM
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QUOTE (Greg Hullender @ Apr 17 2008, 05:12 AM) *
I'm not sure what an S-shaped orbit would be, but there are no stable 3-body orbits except for L4 and L5. I know that the proof of this starts with Hamiltonians and Lagrangians (or at least I think it does) but I'm not able to work it out myself. However, I remember Feynman telling it to my freshman class at Caltech, and I suspect he knew what he was talking about. :-)

--Greg


Things seem to have moved on since Feynman's days...check this out!

http://www.ams.org/notices/200105/200105-body-ps.html

And of course spiraling orbits occur all the time if you have atmospheric drag (not stable though!).

Fascinating subject, and unfortunately off topic smile.gif Physics forum used to have a celestial mechanics bit, but I think they changed the format a bit.
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YesRushGen
post Apr 17 2008, 01:41 PM
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QUOTE (nprev @ Apr 16 2008, 02:40 PM) *
One thing I'd like to know is how much maneuvering propellent NH is currently estimated to have post-Pluto for the KBO encounter, and how large a conic section of space she can feasibly reach...


I'm certainly not the expert around here, but I think that Alan said in a past posting that NH would be able to alter it's trajectory by up to 1 degree or so.

edit: ah... here it is! (post #2)

http://www.unmannedspaceflight.com/index.php?showtopic=2328

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tasp
post Apr 17 2008, 02:11 PM
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Among several difficulties for the existence of a Plutonian ring system would be atmospheric drag effects. The low gravity allows the thin atmosphere to waft quite high during close approaches (heh, heh) to the sun. Even just a tiny bit of gas up to the top of the Roche limit would doom a ring system.

But if the wafty atmosphere is bad news for a ring, it is good news for any orbiter or lander we may wish to send there. In another thread here, decels of up to 40 Gs seem to be possible in traversing the atmosphere.


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nprev
post Apr 18 2008, 01:03 AM
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QUOTE (YesRushGen @ Apr 17 2008, 06:41 AM) *
I'm certainly not the expert around here, but I think that Alan said in a past posting that NH would be able to alter it's trajectory by up to 1 degree or so.


Thanks, YRG. Well, that ain't much if they're hoping for a target reachable in a decade or less...I'm sure the search will be fast & furious once the target zone clears the galactic center from our viewpoint...


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JRehling
post Apr 18 2008, 07:21 AM
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QUOTE (nprev @ Apr 17 2008, 05:03 PM) *
Well, that ain't much if they're hoping for a target reachable in a decade or less...


Based on some back of the envelope math I did a few years back...

The cone of possible destinations for NH is pretty large. It's skinny, but it's long. I estimated that about 1/2500 KBOs will be in it. Since the number of KBOs is considerably larger than that, the probability that at least one of them can be visited is close to 1.0. And there's a very good probability of being able to visit more than one.
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dmuller
post Apr 18 2008, 09:40 AM
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QUOTE (brellis @ Apr 17 2008, 03:53 PM) *
There's not much chance of an accidental close encounter with a KBO ... could the trajectory be altered to point NH differently after the Pluto encounter?

I recall reading somewhere (sorry memory lapse, cant recall where that was) that the search for KBOs will be focused to take place nearer to the Pluto encounter, because by then the KBO that could be reached would well be in a very small area of the sky around & beyond Pluto, guess near the 1 degree cone mentioned in an earlier post, since they move very slow as well.

Any trajectory adjustment will likely be using engine firing. Of the 3 criteria for a swing-by that I know of (orbital speed, mass and density), Pluto fares extremely bad in the first two.


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Greg Hullender
post Apr 18 2008, 04:18 PM
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QUOTE (remcook @ Apr 17 2008, 03:44 AM) *
Things seem to have moved on since Feynman's days...check this out!

http://www.ams.org/notices/200105/200105-body-ps.html


That IS impressive! Had not seen that at all. But it's not a figure-8 orbit around two objects; it's three equal bodies in a mutual figure-8 pattern, so it doesn't apply to the original question:

QUOTE (Oersted @ Apr 16 2008, 07:16 AM) *
Or maybe even - to go really out on a limb - a few high-speed figure 8's around both bodies, before the probe continues on its merry way. smile.gif Would that even be physically possible?


I suppose the cleanest answer would have been to say "NH is moving so fast, and Pluto and Charon are so small, that the only possible orbits for NH -- even using the engines -- are variations on a nearly straight line." As Alan Stern said two years ago, a one-degree deflection is the most anyone thinks we can get.

And, certainly, as Apollo 8 showed, a figure-8 trajectory is possible, if you only need to do it one time. But this puts a fairly strict upper bound on the velocity of the vehicle -- it can't have more than a very small hyperbolic excess velocity with respect to either body. Space probes aren't like cars; NASA can't just downshift into the turn. :-)

--Greg
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JRehling
post Apr 18 2008, 06:36 PM
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QUOTE (brellis @ Apr 16 2008, 09:53 PM) *
If such a discovery were to be made, could the trajectory be altered to point NH differently after the Pluto encounter?


Basically, there's some delta-v that will be left after the encounter, and that equals a budget. Any follow-up within the budget is possible. Can it cross the solar system to visit a really interesting place way over there? No. It will have a list of choices. The possible trade-offs would concern visiting one really interesting one vs. two less interesting ones.

In principle, it would be possible to widen the scope of possibilities by altering the Pluto encounter. If the delta-v of the encounter is to be 7.5 m/s in one direction, a different Pluto encounter could give you that 7.5 m/s in another direction. However, sacrificing science at Pluto is surely an unacceptable measure. Without the trajectory they now have planned, you'd lose the radio occultation and C/A to Charon.
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