Powering Jupiter Probes

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LURCH

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As most of you will recall, there was considerable public outcry when the Cassini probe was launched with its nuclear power supply. However, no alternatives to nuclear decay were offered as a power source. The main thinking seemed to be that the probe should just go without the power it provides.

This got me to wondering about a power source which seems rather obvious to me. I know that when we send a probe to Jupiter, we have to shield it pretty heavily against the fierce radiation around that planet (not sure if the same is true for Saturn). It seems to me that shielding a probe against radiation, and then supplying it with an internal source of radiation to power its electrical systems is ironic, to say the least. Doesn't it seem like there ought to be some way to use the copious amounts of raditation coming from the planet for a power source?
 
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neutroncount

How is that? Power generated by nuclear decay uses the heat off of that decay and is converted into electricity by thermocouples. For planets past the asteroid belt, there isn't enough light from the sun to power probes with just solar panels. The panels would have to be enormous! There isn't enough energy in the radiation coming off of Jupiter or Saturn to power a probe that small. Remember that detectors need to convert the impacts into an electrical signal, but that signal is extremely small and even with millions of impacts, doesn't add up to much in the way of voltage for a power hungry probe. And don't forget the trip out there. What is going to power the probe on it's way out to the outer planets? Those solar panels will only take it so far. So in retrospect, nuclear power seems like a highly viable choice.
 

Phobos

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There isn't enough energy in the radiation coming off of Jupiter or Saturn to power a probe that small.
Can we explore this further? What are the power requirements of a probe once it's in orbit around Jupiter/Saturn? How much energy can we draw from a planet's radiation? Are there other options such as using a lengthy tether to draw energy from the planet's magnetic field as the probe whips through it?

Granted, getting there is another matter. How much energy is required for maneuvering en route? The Galileo and Cassini probes used gravitational slingshots to increase their velocity. Ion propulsion, solar sails, etc. are alternatives, albeit still new techs, for getting there. Offhand, I'm not sure how useful those are for maneuvering.
 

enigma

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Originally posted by Phobos
Can we explore this further? What are the power requirements of a probe once it's in orbit around Jupiter/Saturn?
From this saturn.jpl.nasa.gov/cassini/msnstatus/quickfacts2000.shtml[/URL] It says that it provided 875 watts at launch and 700 watts at mission end. Unfortunately, the page isn't coming up for me, so I got the information by googling for "Available Power Cassini Orbiter RTG", and looking at the cache.

[quote]
How much energy can we draw from a planet's radiation? Are there other options such as using a lengthy tether to draw energy from the planet's magnetic field as the probe whips through it?
[/quote]

Drawing energy from radiation is unfeasable AFAIK, because you can't predict where the particles will hit. Even if you could draw power from it, it would do more damage than the electronics could survive. The tethers are an option, but they are really big, and really heavy. Also, after sitting in the cold for a few years, things have a tendancy to break. If a tether is your only option for power production, you can't build in any redundancy without really upping the weight of the vehicle. They also aren't technologically mature enough to risk sending on an exploration mission IMO. We've only tried using them once, and they melted their attachment to the test base.

[quote]
Granted, getting there is another matter. How much energy is required for maneuvering en route?
[/quote]

This [PLAIN]http://en2.wikipedia.org/wiki/Gravitational_slingshot [Broken] says that the Cassini needed 2km/sec of delta V. It's a decent amount, but certainly attainable. With an Isp of 350, the mass of fuel needs to be about 80% of the mass of the probe for that magnitude burn.

The Galileo and Cassini probes used gravitational slingshots to increase their velocity. Ion propulsion, solar sails, etc. are alternatives, albeit still new techs, for getting there. Offhand, I'm not sure how useful those are for maneuvering. [/B]
Solar sails are not enough. You need to slow down once you get there to go into orbit, and sails only accelerate one direction. Ion propulsion would decrease the fuel required, but you'd still want to use the "free" energy provided by slingshotting. The previous site said Cassini would have needed in excess of 15km/sec delta V to go there directly. That works out to 78 times the mass of the ship in fuel for a bi-propellant or around 4 for an ion engine.
 
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LURCH

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Wasn't thinking about propulsion, that's a whole different kind fo power. I'm just talking about power for the instruments and other electrical systems. What I don't get is the idea that the planet emits enough radiation to fry the electronics, but not enough to power them? As for the mechanism, I wasn't thinking of sails or tethers, but rather the shielding used to protect the probe. It is specifically designed and placed to absorb radiation from the planet. Can't that energy be converted to electrical power? Or perhaps the method of conversion would be heavier than the power source currently used.
 

neutroncount

Originally posted by Phobos
Can we explore this further? What are the power requirements of a probe once it's in orbit around Jupiter/Saturn? How much energy can we draw from a planet's radiation? Are there other options such as using a lengthy tether to draw energy from the planet's magnetic field as the probe whips through it?
Well sure there are ways around it. I guess I misunderstood and thought he was talking about Galileo and Cassini probes specifically. But I'd think you'd have to have a pretty large "detector" setup to absorb enough radiation (emr, neutrons, nuclei) coming off of Jupiter or Saturn. The tether idea is a good idea, but it would have to be a pretty long and tough tether.
 

neutroncount

Originally posted by LURCH
Wasn't thinking about propulsion, that's a whole different kind fo power. I'm just talking about power for the instruments and other electrical systems. What I don't get is the idea that the planet emits enough radiation to fry the electronics, but not enough to power them? As for the mechanism, I wasn't thinking of sails or tethers, but rather the shielding used to protect the probe. It is specifically designed and placed to absorb radiation from the planet. Can't that energy be converted to electrical power? Or perhaps the method of conversion would be heavier than the power source currently used.
The radiation is random and sporatic at best. It's not is a full bath of radiation. And the way that it fries electronics, at least for nuclei and neutrons, is that it pits electonics as it passes through them. It happens regardless of shielding (unless you have enough shielding), and probes seem ok around the planets showing to me at least that a catastrophic hit to the electronics is rare and that the highest energetic particles couldn't provide computers and instrumentation with 875 to 700 Watts of power. Even at lower power, I just don't feel that the interactions would be enough.
 

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