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Unmanned Spaceflight.com _ New Horizons _ How far will NH pass by Pluto?

Posted by: Oersted Apr 16 2008, 02:16 PM

So, from what I gather, NH will pass by Pluto at a speed of around 14 km/s. I know it won't be able to slow down in any way, and will continue into deep space afterwards, but is a substantial course change envisaged at Pluto, or will it fly by so far away that the course change will be negligible?

I realise that the instruments must be optimised for a pass at a certain distance, but what distance is that and could it possible by reduced with no ill effects?

It would be mindblowing if a low pass over Pluto could be planned so that NH will fly close by Charon as well, wouldn't it? - 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?

At least I'm not suggesting aero-braking if Pluto turns out to have a tenuous atmosphere. blink.gif

Posted by: djellison Apr 16 2008, 02:32 PM

QUOTE (Oersted @ Apr 16 2008, 03:16 PM) *
Would that even be physically possible?


No.

It's fly in, fly out, at high speed, and balancing range for time and coverage.

Doug

Posted by: ugordan Apr 16 2008, 02:38 PM

QUOTE (Oersted @ Apr 16 2008, 04:16 PM) *
... if Pluto turns out to have a tenuous atmosphere.

This is already an established fact. The atmosphere is currently (or should be in the not-so-distant future) in the process of re-freezing to the surface as Pluto moves away from the Sun.

Posted by: jamescanvin Apr 16 2008, 02:50 PM

NH trajectory through the Pluto system (from NH website)



Posted by: john_s Apr 16 2008, 02:58 PM

Last fall we changed this slightly, updating the closest approach distance from 10,000 km to 12,500 km. This buys us a bit more time near close approach, at the expense of a little spatial resolution. Going much closer wouldn't improve our resolution much because images would be smeared, and we wouldn't have time to cover much territory.

I guess we need to update that graphic!

John.

Posted by: Alan Stern Apr 16 2008, 03:03 PM

QUOTE (john_s @ Apr 16 2008, 03:58 PM) *
Last fall we changed this slightly, updating the closest approach distance from 10,000 km to 12,500 km. This buys us a bit more time near close approach, at the expense of a little spatial resolution. Going much closer wouldn't improve our resolution much because images would be smeared, and we wouldn't have time to cover much territory.

I guess we need to update that graphic!

John.



Here's the updated version.



 

Posted by: Greg Hullender Apr 16 2008, 03:04 PM

QUOTE (Oersted @ Apr 16 2008, 06:16 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


I remember how disappointed I was when I learned that a figure 8 isn't a possible orbit. (Neither is a spiral, for that matter.) If we could just post equations in here, maybe we could have a thread on orbital mechanics . . .

--Greg

Posted by: Alan Stern Apr 16 2008, 03:04 PM

QUOTE (Alan Stern @ Apr 16 2008, 04:03 PM) *
Here's the updated version.



This is better.

 Encounter_Flyby.ppt ( 105.5K ) : 860
 

Posted by: jamescanvin Apr 16 2008, 03:12 PM

QUOTE (Oersted @ Apr 16 2008, 03:16 PM) *
but is a substantial course change envisaged at Pluto, or will it fly by so far away that the course change will be negligible?


Clearly the deflection by Pluto (+moons) is going to be tiny - but I'd be interesting to know just how tiny is it?

Posted by: jamescanvin Apr 16 2008, 03:21 PM

I hope Alan doesn't mind but here is that Powerpoint image in png format.



 

Posted by: Oersted Apr 16 2008, 06:38 PM

QUOTE (Greg Hullender @ Apr 16 2008, 05:04 PM) *
I remember how disappointed I was when I learned that a figure 8 isn't a possible orbit. (Neither is a spiral, for that matter.) If we could just post equations in here, maybe we could have a thread on orbital mechanics . . .

--Greg


Why isn't a figure 8 or just an S-shaped orbit physically possible? (as opposed to just being not desirable for coverage reasons).

Thanks for all the replies and graphics, enlightening indeed! - And not just for me, I'm happy to say, since we seem to have an update on the pass parameters.

Posted by: JRehling Apr 16 2008, 06:44 PM

QUOTE (jamescanvin @ Apr 16 2008, 07:12 AM) *
Clearly the deflection by Pluto (+moons) is going to be tiny - but I'd be interesting to know just how tiny is it?


Well, Pluto has 0.2% the mass of Earth, and New Horizons will pass 12,500 km away. That means the max acceleration due to gravity will be 0.0005 g, or about 0.005 m/s^2. Calculus or modeling are required to get a true answer, but if we assume that that acceleration were in effect for 1500 seconds, it would impart a total delta-v of 7.5 m/s. That should be the right order of magnitude. That is 0.0005 of the velocity of NH (more or less to right angles to its path), which would alter NH's heading by 0.03 degrees. Close enough to zero for you?

Posted by: ugordan Apr 16 2008, 07:21 PM

My ballpark calculation assuming the impact parameter B is ~= C/A distance and v(c/a) ~= v(inf) and that there's no Charon around gives a deflection angle of 0.02 deg which agrees with JRehling's calculation nicely. For all practical purposes you can neglect Pluto has any mass.

Posted by: nprev Apr 16 2008, 07:40 PM

Kinda figured as much...it's not very large, and is mostly made of not-so-massive stuff.

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 (by this I mean total potential delta-V vs. power supply endurance, comm range, etc. if there are any other limiting factors). Also, are there any tentative plans to conduct a retargeting maneuver to reach a KBO prior to the Pluto encounter, or will all this take place afterwards? (I'm guessing that the latter is far more likely based on current planning as shown, plus there's no gravitational assist potential at Pluto to speak of).

A Plutino would presumably be the ideal target in terms of feasiblity; IIRC, most of these are approximately coplanar with Pluto's orbit?

Posted by: siravan Apr 17 2008, 01:35 AM

Looking at the encounter image, it seems that the trajectory is designed such that at closest distance to Pluto, NH is essentially 180 degree off Charon. I guess the design has scarified imaging resolution for the chance of doing radio science during occlusions. Is this the main reason or are there other considerations that forced the trajectory to be where it is? Also, is it possible for Pluto to have any rings (vis-a-vis Rhea)? And if so, is there any planning to look for them (long exposures and such)?

Posted by: Greg Hullender Apr 17 2008, 04:12 AM

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. :-)

--Greg

Posted by: mchan Apr 17 2008, 05:31 AM

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.

Posted by: brellis Apr 17 2008, 05:53 AM

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?

Posted by: remcook Apr 17 2008, 10:44 AM

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.

Posted by: YesRushGen Apr 17 2008, 01:41 PM

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


Posted by: tasp Apr 17 2008, 02:11 PM

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.



Posted by: nprev Apr 18 2008, 01:03 AM

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...

Posted by: JRehling Apr 18 2008, 07:21 AM

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.

Posted by: dmuller Apr 18 2008, 09:40 AM

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.

Posted by: Greg Hullender Apr 18 2008, 04:18 PM

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

Posted by: JRehling Apr 18 2008, 06:36 PM

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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