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_AlexBlackwell_*]

I apologize for reviving a dead thread; however, Frank Morring Jr., reporting from the Farnborough 2006 Air Show, has an interesting article ("Improving Solar Cell Efficiency Enables NASA's Solar-Powered Jupiter Probe") in the July 17, 2006, issue of Aviation Week & Space Technology.

While the bulk of the article relates to the Juno New Frontiers-class mission, there is an interesting passage relating to a Neptune orbiter concept:

While Juno will be pushing the state of the art for deep-space use of solar power, scientists have analyzed the use of sunlight to power spacecraft at planets even more distant from the Sun. NASA has studied a Neptune orbiter that would use inflatable structures to deploy the ultra-large solar arrays that would be needed there and concentrators to focus the dim sunlight on the collectors (AW&ST Dec. 13, 2004, p. 56).

[Paul] Stella [principal engineer for space power systems at JPL] is skeptical. Even without considering the inflatable structures that would be needed to hold the solar concentrators and underlying cells, there would be difficult problems to overcome with the power-generating system itself. NASA flew a solar concentrator based on Fresnel lenses on its Deep Space-1 technology testbed, which he says "worked very well." But array pointing is more critical with concentrators than with conventional planar solar arrays, and the concentrator aperture will be "about the same size" as a planar array, even if the number of cells beneath it is reduced. Ultimately, it becomes a question of whether an array large enough can be built and delivered.

"Those are pretty advanced studies," Stella says. "A solar cell at Neptune will probably generate some power, but . . . the amount of power is going to be dropped down by literally hundreds, close to a thousand, and that makes for a very big solar array. The question is, if you need 30 watts at Neptune, which is a small amount, how many thousands and tens of thousands of watts do you have to build on Earth? Our biggest communications satellites are probably on the order of 20 kw., and that's lot of solar array, very complex, and again we might need even more than that. We're getting into uncharted territory."

Engineers at JPL also are working on boosting solar-cell efficiency by increasing the number of junctions, with promising results. But there is a limit to how much sunlight can be converted to electricity.

"I would think that numbers approaching 50% are probably at least analytically feasible," says Stella. "We've done some work here. We looked at going from three junction cells to four to five to six to seven. And there's a point of diminishing returns . . . . I think we ended up probably in the order of 50%, and that doesn't mean you're going to get there."

[JRehling]

I would think that at some point the weight of the solar panels would be prohibitive.

I guess another concept for a Neptune orbiter would be to have solar panels spend 90%+ of a highly elliptical orbit charging batteries, with periapsis spent using that battery power to run instruments and transmit data home. In principle, there's almost no limit to how high the apoapsis is -- and such an orbit would also require less propellant. If the Neptune encounters are far apart in time, that would increase operations costs, but there'd be a lot of savings in engineering.

[Chmee]

I would think that at some point the weight of the solar panels would be prohibitive.

I guess another concept for a Neptune orbiter would be to have solar panels spend 90%+ of a highly elliptical orbit charging batteries, with periapsis spent using that battery power to run instruments and transmit data home. In principle, there's almost no limit to how high the apoapsis is -- and such an orbit would also require less propellant. If the Neptune encounters are far apart in time, that would increase operations costs, but there'd be a lot of savings in engineering.

By why even consider a solar array when RTG's are a proven, safe, and stable power source for deep space missions? It seems such large, heavy, and complex solar arrays are solution for a problem that does not exist.