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Solar Panels at Saturn

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Since Juno, a Jupiter orbiter (has not arrived yet) is the first spacecraft to use solar panels at Jupiter thanks to recent advances in photovoltaic technology, I've been wondering if in a couple of years after more advances in solar technology if solar power would be possible at Saturn? This would make a Saturn orbiter (Cassini's successor) if paired with electric propulsion (Ion engines, Hall thrusters, VASMIR, etc.) able to enter and leave orbit around moons, go to moons at will instead of using the "pinball" slingshots from one moon to the next, and instruments won't have to be shut off because of the power decay from RTGs.

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Depends what kind of mission you're talking about-we're already confident we can do small missions with low-power payloads at Saturn, hence the encedalus life finder discovery proposal, but SEP is a different matter entirely.

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Out at Saturn, Nuclear reactors might be more viable than solar, since the energy solar panels can produce out at Saturn is a LOT less than what's possible even at Jupiter.

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RTG all the way, or if the budget is enoght and you need energy, a nuclear reactor then.

Solar panels are great, but each technology has their place, and beyond jupiter are kinda pointless.

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A simple calculation based on comparison of orbital radius (Saturn is farther than Earth from Sun 10x) will give you a fact that sun's flux at Saturn is 100x fainter than Earth, or approx. 137 W/m^2. Even so, not all of this can be utilized by any PV, because of wavelengths. Rosetta have 64 m^2 panels that generate ~325 W at 5 AU - dragging it to 10 AU would yield only 1/4 of that (~ 80 W) if the dimensions are the same. Now I'm not sure, Rosetta weighs ~3000 kg already and NH needs 228 W at Pluto and weighs only ~500 kg. It's not hard to tell that a mission to Saturn needs to carry a fair amount of fuel... We don't have the launcher I guess ?

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Even with perfectly efficient solar panels Saturn is problematic because of the inverse square law of radiation.

I'm not sure who the author of this image is but it captures the essence of the problem nicely. Cassini with solar panels instead of RTGs (note that Cassini only drew 700W or so of power, far below the practical requirements of any useful electric propulsion of scale):

solar-cassini.png

Edited by architeuthis

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That's...discouraging, to say the least :( I guess it can be improved when we develop lighter materials and more efficient photoelectric elements. But it will still be an engineering challenge to fit big enough solar arrays on necessarily small and lightweight probe.

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los-alamos-reactor.png

A small reactor is a better solution that solar that far out. If solar panels have to be THAT big to produce enough power, it's the obvious choice. Los Alamos didn't build that thing just for it to rust in a warehouse :P

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Juno's average solar power production while at Jupiter: 450 W (a performance gaming PC's power consumption)

Juno's theoretical solar power production while at Saturn: barely over 100 W

Dawn's NSTAR ion drive operational power requirements: 2,300 W for 0.000009 kN @ 3100s Isp (because KSP players are most familiar with thrust measured in kN)

VASIMR drive operational power requirements: 200,000 W for 0.0025 kN @ 5000s Isp

Ummm, yeah... no :P

Solar electric propulsion is currently impractical if you go much past Mars. Dawn in the asteroid belt is already struggling and making do only because its ion thrusters are undersized for a spacecraft of its weight. That's the reason why it takes the probe one to two months to lower its orbit around Ceres by just a few thousand kilometers. Perhaps future technology will make it truly viable in the asteroid belt, but until it becomes viable at Jupiter you have to make orders of magnitude improvements. And Saturn is so much further still.

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So wait, if it is like that, would the inverse also be true? Can we build a probe with really tiny solar panels and send it nearer to the sun and still get adequate power? Or is there a practical lower limit somewhere?

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Juno's average solar power production while at Jupiter: 450 W (a performance gaming PC's power consumption)

Juno's theoretical solar power production while at Saturn: barely over 100 W

Dawn's NSTAR ion drive operational power requirements: 2,300 W for 0.000009 kN @ 3100s Isp (because KSP players are most familiar with thrust measured in kN)

VASIMR drive operational power requirements: 200,000 W for 0.0025 kN @ 5000s Isp

This bears quoting for emphasis - there's a lot of talk about electric propulsion and especially about VASIMR... but few people really seem to grasp just how much power they require to be any kind of useful. And power supplies are heavy. Electric propulsion isn't a way around the brutality of the rocket equation (despite their seductively huge ISP), and in some ways it's worse than chemical propulsion because you don't get the performance boost that chemically propelled vehicles do as they burn off their fuel supply.

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That would definatly work, ESA is using that principle to move it's next Mercury mission, BepiColoumbo, into orbit, along with a JAXA magnetospheric probe. The issue is solar panels lose effectiveness as the heat up. This can be countered using reflectors on the panels (MESSENGER used this principle).

EDIT, this is in reference to RainDreamer's question

Edited by MinimumSky5

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That would definatly work, ESA is using that principle to move it's next Mercury mission, BepiColoumbo, into orbit, along with a JAXA magnetospheric probe. The issue is solar panels lose effectiveness as the heat up. This can be countered using reflectors on the panels (MESSENGER used this principle).

EDIT, this is in reference to RainDreamer's question

The only issue is that you also need power close to earth however here you don't need to run the science modules.

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That would definatly work, ESA is using that principle to move it's next Mercury mission, BepiColoumbo, into orbit, along with a JAXA magnetospheric probe. The issue is solar panels lose effectiveness as the heat up. This can be countered using reflectors on the panels (MESSENGER used this principle).

EDIT, this is in reference to RainDreamer's question

PV cells lose efficiency due heat, so if you radiate that heat away, no matter how close are you from the sun, they will capture the same % of energy.

That is why some solar panels design, use PV cells with higher efficiency (much more expensive) but concentrating solar light (using lens or parabolics) in the small cell, and they use heat pipe or different ways to radiate the heat in passive way.

First-Solar-and-multicrystalline-DC-power-output-vs-temperature.png

This is not a global tech graphic because I dint find any for this year, this is between (first solar) tech vs SI, but it work as example of efficiency vs temperature.

Edited by AngelLestat

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So wait, if it is like that, would the inverse also be true? Can we build a probe with really tiny solar panels and send it nearer to the sun and still get adequate power? Or is there a practical lower limit somewhere?

Look at the design of Solar Probe Plus. It's meant to go within 0.044 AU of the sun:

http://solarprobe.jhuapl.edu/mission/docs/SolarProbeME.pdf

7ULeDB5.png

At its closest approach, only 34 cm2 of solar cells will be exposed to the sun, with 2 m2 of radiators rejecting heat -- a ratio of 600:1. So it doesn't look like you can save mass this way: you're limited fundamentally by the temperature limit of the solar cells (about 100° C here), and the radiator area needed to keep the equilibrium temperature that cold.

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That is right, but those radiators are not only needed to reject the heat from the small solar panels, they are also to reject the heat received over all the sat surface.

So is not a good example to compare the direct relation between (PV area---> radiator area needed), and I remember you that RTG or nuclear power will also need radiators which size also depends on how close are you to the sun.

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It really should be mentioned that the inverse square law also applies to transmitting data back to Earth. So, not only do you get far less power from your solar panels around Saturn, but you also need increasingly more power to get the data back to Earth. Robert Zubrin wrote an excellent section about this subject in "Entering Space", the whole book is well worth reading.

Edited by Finox

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There is also a hard limit for PV cells in that the PV cells have a certain operating temperature, as do the cables that connect them and the electronics that regulate power. Freeze below that, and the probe is toast, no matter how big the panel is or how little its instruments draw. Also note that the cells that power you on the outer system are also receiving solar radiation when you leave Earth... how hot can they get, and how much power can the transformers handle without melting? You are asking both extremes out of the same equipment during the flight.

That is right, but those radiators are not only needed to reject the heat from the small solar panels, they are also to reject the heat received over all the sat surface.

So is not a good example to compare the direct relation between (PV area---> radiator area needed), and I remember you that RTG or nuclear power will also need radiators which size also depends on how close are you to the sun.

Not really, the wattage out of a reactor is constant, so the only variable would be the solar radiation on the ship itself, which mainly depends on its cross section (and the radiators would be edge on to the sun, obviously). A solar system's heat output goes up as it receives more power, and down as it receives less, just the same as its power production varies. So a less dramatic variance for nuclear powered system, especially when the power considered is high with respect to the mass of the spacecraft.

Me, I hope we see some space rector finally flying one of these decades. Once we get over our nuclear phobia and have some reactor operating safely for a few years, people will realize what a huge waste of time electric propulsion is in general, and we can get on with the thermal rockets that can truly open up the solar system to, you know, humans. VASIMIR and the like are a horrible diversion that doesn't stand up to the slightest scrutiny: that type of technology will never push humans, on account of humans not wanting to fry themselves spending weeks passing through the radiation belts. And they would never, ever, make a trip to Mars shorter, if anything they would make it longer. That particular bit always irks me when I read it.

Rune. Ion systems are for unmanned probes, and even then only sometimes.

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