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Neptune Orbiter, Another proposed mission
Guest_Analyst_*
post Jul 20 2006, 10:54 AM
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Before the "nuclear problem" comes the money problem. RTGs may be expensive, but they are very reliable, long lasting with predictable slow degration. Solar arrays at Neptune? There you have 1/36 the light level compared to Jupiter, this means 36 times larger arrays (or collector space). I guess this will be more expensive than RTGs (at least to develop), much less reliable and a problem for navigating/pointing.

Save the money problem first.
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antoniseb
post Jul 21 2006, 11:52 PM
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QUOTE (Analyst @ Jul 20 2006, 04:54 AM) *
Before the "nuclear problem" comes the money problem. RTGs may be expensive, but they are very reliable, long lasting with predictable slow degration. Solar arrays at Neptune? There you have 1/36 the light level compared to Jupiter, this means 36 times larger arrays (or collector space). I guess this will be more expensive than RTGs (at least to develop), much less reliable and a problem for navigating/pointing.

Save the money problem first.


Your mention of the factor of 36 difference in Sun light compared to Jupiter seems to have ignored the previous post. It would be fairly cheap and reliable to have thousands of square meters of mylar reflecting light back to a small PV array.
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Stephen
post Jul 25 2006, 04:35 AM
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QUOTE (antoniseb @ Jul 21 2006, 11:52 PM) *
Your mention of the factor of 36 difference in Sun light compared to Jupiter seems to have ignored the previous post. It would be fairly cheap and reliable to have thousands of square meters of mylar reflecting light back to a small PV array.

But is it a practical solution?

Once you got to Neptune and started orbiting the planet you would need to ensure that all that mylar stayed at a fixed attitude with respect to the Sun. If it functioned simply as a solar sail the easiest solution would be to eject the sail. If you have to keep it to power your PV array then you would surely be complicating matters.

For which attitude would you want to use? To have the mylar facing the Sun would be best for reflecting light back into PV array. However, doing so would probably have all that mylar acting like a solar sail again, trying to drag the probe in directions its masters back on Earth would (probably) not want it to go in. The latter could be minimised by having the mylar face edge-on with respect to the Sun, but doing that would also minimise the amount of light being reflected into the PV array.

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Greg Hullender
post Jul 25 2006, 05:01 AM
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But surely at the distance of Neptune the force applied by light to a mirror can't amount to much -- especially when the point of the mirror is to collect about as many photons as the solar panels on a Mars probe receive all the time! (Or do Mars probes have a big problem coping with the force of the photons falling on their solar cells?)
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helvick
post Jul 25 2006, 05:34 AM
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QUOTE (Greg Hullender @ Jul 25 2006, 06:01 AM) *
But surely at the distance of Neptune the force applied by light to a mirror can't amount to much -- especially when the point of the mirror is to collect about as many photons as the solar panels on a Mars probe receive all the time! (Or do Mars probes have a big problem coping with the force of the photons falling on their solar cells?)


That's true enough but a more fundamental problem would be deploying and then keeping a 400m^2 mirror tensioned and pointed to a sufficient accuracy.

One thing to remember is that while concentrator type solar panels allow you to increase the effective area of a panel cheaply the system becomes significantly more sensitive to pointing accuracy. The main reason we don't generally see small (<1m^2) panels + 10x+ refractor\reflector concentrators on EO or MO spacecraft is that the concentrator accuracy\pointing problem is not trivial.
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ljk4-1
post Jul 25 2006, 02:58 PM
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Would a Neptune orbiter be able to utilize the planet's magnetic field in
some way for power and manuevering, or would that just complicate
matters even more than a solar sail?

Maybe it could utilize the geothermal energy from Triton's geysers. cool.gif


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TritonAntares
post Jul 25 2006, 11:17 PM
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QUOTE (ljk4-1 @ Jul 25 2006, 03:58 PM) *
Maybe it could utilize the geothermal energy from Triton's geysers. cool.gif

Or maybe Triton's natural resources could be used -
let's built up a chemical plant there to synthesize hydrazine... blink.gif tongue.gif
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qraal
post Jul 26 2006, 01:40 PM
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Hi All

Solar concentrators are such a cool concept for powersats, and I've seen Neptune probe designs with concentrators, but the pointing issue is pretty much mission critical, especially since batteries are too damned heavy to use when the collectors aren't pointing at the Sun. Supercapacitors are getting better all the time and the claimed energy density of some are better than batteries too.

Yet nukes are a proven technology and not the potential catastrophe that excessively strident environmentalists carryon about. To me a proper nuclear reactor is a better option than RTGs. Thermoelectric conversion technology is getting better all the time - the old Russian Topaz reactors had a mere 3% efficiency, yet 18% efficiency is being developed, and even higher efficiencies are possible in theory. So why throwaway the option to avoid a minor risk? And a reactor is even safer if it is deactivated until high orbit is achieved.

It just seems insane to me that NASA dumped the original reactor program so long ago, and then to have a half-revival with Prometheus, to then see that program in limbo... Nuts!

Adam
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mimile
post Feb 11 2007, 11:38 AM
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There have been many good answers to the question of prograde / retrograde orbit for the spacecraft. Let me add one point :

If the spacecraft is in a retrograde orbit (same direction as Triton), it will fly by Triton with a smaller relative velocity. Thus Triton is able to deflect it by a larger angle, and there is a larger maneuvering capacity : Triton can send the spacecraft on a wider set of orbits.

On another point, visiting Nereid, I think there is no need to do it only once before aerobraking. Nereid has a VERY eccentric orbit around Neptune (0.75) : its minimum approach to Neptune is about 1.4 million km, only 4 times the size of Triton's orbit. this can easily be reached using Triton flybys. For example, at Saturn, recall Iapetus orbit radius is 3.6 million km and Titan's 1.2 million. And there are Two flybys, one distant already done, one close this year. True : it is easier in this case because Iapetus orbit is circular. But I'm pretty sure at least one close flyby of Nereid could be arranged.
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Rob Pinnegar
post Feb 11 2007, 05:04 PM
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Those are good points, but it may be worth keeping in mind that the relevant distance from Neptune for a Nereid pass won't be Nereid's periapsis. It'll be more likely Nereid's distance from Neptune at the two points when Nereid crosses Triton's orbital plane. This will make it more than 1.4 million kilometres (though probably not by an enormous amount).

Of course, a Neptune orbiter would not be completely constrained to Triton's orbital plane, but it would tend to make things easier. Iapetus has only a 15-degree inclination to Titan's orbital plane, and this is still cited as being a substantial problem in setting up Iapetus flybys. I don't know what the angle is between the orbital planes of Triton and Nereid, but it wouldn't surprise me if it was larger than 15 degrees.
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nprev
post Feb 11 2007, 05:37 PM
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Nereid: 27.6 deg with respect to Neptune's equator, orbital period 360 days, eccentricity 0.75(!)
Triton: 157.35 deg

Not easy, but not impossible if it's done as a one-time-only good deal at system entry like Cassini's Phoebe encounter, I think; arrival timing would be everything. Getting out there again after entry into orbit even when Nereid is near periapsis (esp. if Triton is the prime focus, which is probable) seems unlikely in the extreme, esp. because the relative velocity would probably be pretty bad (Kepler strikes again!)...image smearing would be a significant concern, for example. Besides, Proteus is larger, probably more interesting, and far more accessible; I'd expect Nereid to be a lot like Phoebe.


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JRehling
post Feb 12 2007, 01:42 AM
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It's odd that people brought the topic of Nereid up this week -- I was just thinking about the same issues.

Here's the key fact about Nereid: It rotates. You don't need to have multiple flybys to see most of its surface.

If geometry permitted, the ideal situation would be to find Nereid near its apoapsis while the craft were still en route to Neptune. It could make a leisurely approach while Nereid also approached Neptune, and could make close observations over a span of 12 hours. That alone would reveal most of Nereid's surface. Then, why fly by again?

With a periapsis four times Triton's, there's no need to have the Neptune orbiter ever venture out so far again. The other "major" satellites are all well inside of Triton's orbit. Unless the energy to keep the orbit in so tight cannot be spared, it should head inside and never venture out again. Most likely, the non-Triton satellites are minor priorities themselves. Gettings lots of looks at Triton (active over time), Neptune (active over time), and the rings (active over time) would comprise 85% of the mission.
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nprev
post Feb 12 2007, 04:13 AM
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Good point, JR. In fact, Nereid spins pretty fast: 11.5 hrs per this paper, so if a low relative velocity inbound encounter could be arranged, you'd get a very good global view of it during one visit.


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tasp
post Feb 12 2007, 06:24 AM
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OK, I am not smart enough to visualize this in my head, but if Triton's orbital plane is tilted 157 degrees, and Nereid is tilted 27, then aren't the orbital planes only tilted from each other by 5 degrees?



I must be missing something . . . .
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mchan
post Feb 12 2007, 08:05 AM
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Possibly that the node crossings of the two orbits are offset from each other. E.g., imagine if Saturn's A rings were inclined 20 degress and B rings were inclined 160 degress. The A and B rings would be coplanar if the node crossings were the same. (If going by convention of using the ascending node, then node crossings are offset by 180 degrees.) But if the node crossings were offset, you would get the B rings twisted out of the plane of the A rings. The two rings will still come close at two points, but this is because their orbits are circular. With two elliptical orbits, particularly if they do not have resonant periods, objects in the two orbits could potentially never get close to each other.
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