Neptune Orbiter, Another proposed mission Options

[tasp]

Wikipedia has stats on Nereid and reports its' orbital plane is tilted to the ecliptic ~5 degrees. This would make an encounter for an incoming spacecraft easier. I am still having trouble visualizing the orientation of the entire Neptune/Triton/Nereid system.

With Triton 157 degrees retrograde, and Nereid 27 degrees, we wind up at 185 degrees total, so aren't they 5 degrees off of coplanar?

{you can tell I don't have Celestia on my PC . . . .}

[JRehling]

Wikipedia has stats on Nereid and reports its' orbital plane is tilted to the ecliptic ~5 degrees. This would make an encounter for an incoming spacecraft easier. I am still having trouble visualizing the orientation of the entire Neptune/Triton/Nereid system.

With Triton 157 degrees retrograde, and Nereid 27 degrees, we wind up at 185 degrees total, so aren't they 5 degrees off of coplanar?

{you can tell I don't have Celestia on my PC . . . .}

The math doesn't work there because there are three dimensions in which the axes can be tipped.

Imagine two drunk men with hula hoops around their waists. One man leans forward at 45 degrees. The other man leans to his left at 45 degrees. The number 45 is equal, but their hula hoops are not coplanar.

Just playing around with Solar System simulator, I see that Nereid's next close approach to Triton will be mid-April, and will bring it within about 1.25 million km -- no closer. Other Nereid periapsises may bring the two closer, but they can't get too much closer. Think about Cassini imaging Iapetus from a million km away -- and Iapetus is rather large.

[tasp]

Thanx, appreciate the clarification very much.

Some days I just shouldn't try to do anything without coffee . . .

But . . . .

Over time, it seems the orientations of these orbits might change (while maintaining their respective inclinations) and at various epochs (not ours, unfortunately) the orbital planes might turn in to relatively coplanar alignment.

Do we have any idea how fast this might happen at Neptune?

[Rob Pinnegar]

The orbits will certainly precess -- but at that distance from the Sun, tidal effects have got to be tiny. The changes would happen over a very long period of time.

[TritonAntares]

Hi,
any ideas for an illustration, report, paper, etc. of Triton's seasonal changes during its 176 year trip
together with Neptune around the sun combined with its 157.35 deg retrograde orbit?
Would be very interesting to visualize how polar night and day are changing and which latitudes are affected...
Triton should be the strangest world in the solar system concerning seasonal changes, even if the seasons are quite long out there...

THX & Bye.

[mchan]

There is a graph in "The New Solar System" by Beatty, et al, which shows, IIRC, Triton's sub-solar latitude over a thousand years or so.

[TritonAntares]

Hi again,
here a link to a french website dealing with the question of latitude changes of Triton's subsolar point over historical time :
http://bugle.imcce.fr/fr/observateur/support/Triton/

There seems to be some disagreement on the calculation of those variations... ?
Anyhow, Triton's seasons don't look that simply predictable as Earth's....

Bye.

[tedstryk]

It also has some of the strangest seasons, due to its tilt, its odd orbit, and Neptune's tilt and long orbit. By the way, I am glad to see new work is still being done at Pic du Midi. The French nearly made a tragic decision to close it a few years ago, due to the fact that their observatories in other parts of the world have larger telescopes, and, given the nature of this sharp peak, there really isn't anywhere else to build on Pic du Midi. Still, it is unbeaten in its steady skies for planetary astronomy. Fortunately, I think the French powers that be realized that.

[Rob Pinnegar]

Yeah, looking at some of the references cited above, the precession period for Triton's orbit around Neptune is on the order of 600 years or so -- a lot shorter than I would have thought.

So basically Neptune's equatorial plane has about a 28-degree tilt relative to Neptune's orbit around the Sun, and Triton's orbit is inclined 22 degrees to that (retrograde of course). The result of this is that Triton's axial tilt, relative to Neptune's orbit around the Sun, can be as large as (28+22) degrees, or as small as (28-22) degrees; generally it's somewhere in between. (If we take things relative to the Earth, there's also the matter of the tilt of Neptune's orbit relative to the ecliptic, but that's only a couple of degrees or so.)

[tasp]

Just throwing out some ideas here:

Modify a follow on New Horizons craft to orbit Neptune. Take advantage of Neptune's huge Hill sphere by having the craft execute big, leisurely orbits of Neptune, perhaps keeping the orbital period around Neptune at ~6 months or so. The advantage of this is commonality with the current NH design, gather data during periapsis and transmit during the 5 months or so when your not near the planet. NH is apparently anticipated to be a long lived craft, having a follow on craft study Neptune for a significant portion of it's arc about the sun would be useful. The large orbit might also allow observations of the interesting apparent KBO style outie satellites. We can have a Triton pass every orbit and time the encounters to give us a close pass by an inner ring satellite and/or an outie too.

I'm still thinking having the craft start in either a prograde or retrograde orbit and then orbit shaping to the other is possible and useful in studying Neptune's magnetic field.

[djellison]

Where do you get the delta V to break into orbit around Neptune? We're talking a SIGNIFICANT difference in spacecraft design to add that capacity. Also - NH is designed for short, brief encounters - not a long-life orbiting mission. You would want to move away from the RCS to reaction wheels.

What about downlink? you would want more than the 1k/sec that NH will probably manage from that range.

Once you've done that - you have very little that is still NH.

Doug

[nprev]

Good points, Doug.

It's sure easy to forget the downlink bitrate in particular when talking about these deep outer system missions. Lasercomm really needs to be developed for future Flagship-class forays beyond Saturn. Cassini may well be the last one of these that relies on RF...

[tasp]

The ~6 month orbit about Neptune is driven by the low data rate. Taking advantage of Neptune's huge Hill sphere, we can have the craft loiter for months to send back data between periapsis at Neptune. If the 6 month orbit is still to short for the data rate, bump it up to a year. I don't consider the low data rate a problem, it is the driver for the longest possible mission time at Neptune.

Advantages to this include :

* potentially very long probe life at Neptune, studying Neptune over a significant arc of its' orbit about the sun is one of the mission goals.

* reuse of the NH design, thus saving $ and increasing odds mission can be flown

* long excursions away from Neptune may allow study of Nereid (apparently it is ~coplanar to the ecliptic, and at least a few probe orbits about Neptune should be ~ in the ecliptic to facilite ring studies) and/or some of the other (potentially KBO derived) outie satellites (the NH design is , after all, optimized for studying such objects, and 'parking a NH follow on craft in Neptune's vicinity might get us several flybys of these objects.

* not mentioned in my post, but adding a Dawn style ion thruster and some RTG's to power it, could still result in a probe that, with a variation of the VEEGA trajectory, still reach and orbit Neptune as cheaply as possible



Cheap, interesting missions seem to have the best chance of making launch. I am just trying to come up with something that is as off the shelf as possible, and can do an interesting mission for the bare minimum $.

[helvick]

Tasp - NH is a flyby craft not an orbiter and its design is totally unsuited for an orbiter mission. There is no way to get something like it into orbit around anything without so much rework that it would end up being something completely different.

[djellison]

RTG powered ion propulsion, that's VERY VERY off the NH design - as would be the need for reaction wheels. It's not a re-use of the NH design at all. It's something very very different.

Doug