Neptune Orbiter, Another proposed mission Options

[Rob Pinnegar]

This seems like a good place to start off the Uranus and Neptune forum: with the next ice-giants mission.

I will admit to not knowing a whole lot about the Neptune Orbiter With Probes (NOWP), other than the fact that it's in the planning stages, and a few other details I've gathered from Wikipedia and various other Internet sources. Anyone care to get this one going with a bit more information?

[Guest_Analyst_*]

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.

[antoniseb]

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.

[Stephen]

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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Stephen

[Greg Hullender]

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?)

[helvick]

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.