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A-L-E-X
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16 Sep 2017 02:00

Mosfet wrote:
A-L-E-X wrote:
Source of the post That's why I was so curious about why they went to the trouble of ensuring no contamination to the moons- do they think there is life there?

https://en.wikipedia.org/wiki/Interplanetary_contamination

A-L-E-X wrote:
Source of the post had Cassini been launched in this era instead of 20 years ago, would they have used solar sails instead of Plutonium?


According to this, probably no:
NASA's current RTG design, known as the Multi-Mission Radioisotope Thermoelectric Generator, requires 10.6 lbs. (4.8 kg) of Pu-238. So, currently, the U.S. has enough Pu-238 to power just three or four more deep-space missions.


Wow, I wonder if they'll switch in the future since Pu is so rare.
The issue of interplanetary contamination is a pretty big, I thought maybe they gave priority to the moons over Saturn itself because they thought there might be life on one of them.  The same topic comes up with regards to Mars, especially with a possible manned mission coming up in the 2020s.
 
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16 Sep 2017 07:43

A-L-E-X wrote:
Source of the post Still, there's the ethics of altering a planet from what it was.

Sounds like a silly human concept to me.  I wouldn't have a problem if humanity moved out into the solar system and destroyed all the planets to build a Dyson swarm. 
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16 Sep 2017 10:51

A-L-E-X wrote:
Source of the post Not really happy about the plutonium contamination- plutonium is the longest lasting toxic substance we know of.  I guess that's better than bringing it back here though

A-L-E-X, in order to assertain this you need to get in depth with the orders of magnitude involved.

How much plutonium we have thrown into Saturn?
The Cassini spacecraft had 3 GPHS-RTGs (General-Purpose Heat Source - Radioisotope Thermoelectric Generators), each containing 7.8 kg of Pu-238 inside. Cassini also has 82 small RHUs (Radioisotope Heater Units), each containing no more than 10 grams of Pu-238. This means that overall Cassini had 24.22 kg of plutonium inside.
But this is tricky, because the fact that plutonium is used as a radioactive heat source means that it gets transformed in energy and decay residuals. So considering thatPu-238 has a half-life of 87.74 years, the exponential decay law says that of the 24.22 kg present at the start of Cassini's voyage only 20.7 kg of plutonium were still there at the moment of entry in Saturn's atmosphere. The other 3.5 kg of plutonium where converted primarily in Uranium (a natural ocurring element) and also in the heat that powered the mission for two decades.

How long the plutonium will last in the Saturnian atmosphere?
By the law of exponential decay those 20.7 kg of plutonium should have decay in 20.68 kg of uranium in no more than 880 years

How the decayed uranium would affect the enviroment?
No effect at all. Consider that there are8 ppb (parts per billion) of uranium in meteorites. Considering that asteroids have usually densities in the range from 2 g/cm3 to 5 g/cm3 this means that an asteroid between 100 and 135 meters in size would have the same amount of Uranium that Cassini left on Saturn.An asteroid of that size is expected to impact Earth between each 5000 and 11000 years. This means that Saturnian atmosphere has received this Uranium dosis at least 400.000 times since it was formed. I don't see saturn really affected by this naturally ocurring phenomena.

The uranium in Saturn is in fact a lot more. 1) asteroids impact more usually Saturn than Earth (from where the impact frequencies where considered), this is because Saturn is bigger and easier to get hitted and also has more gravitational atraction than Earth, making it a easier target (so probably there have been millions of asteroids like those I've described dropping uranium at Saturn), 2) the rate of impacts is dependant on time (I calculated it with current rates but those are the lowest in the entire history of the solar system), if we consider all the uranium left by the late heavy bombardment we would be outstanded, 3) Uranium was in the first place on Saturn when it formed (even tons of plutonium has to be there until this day), consider that the internal heat of Earth is partially due to radioactive elements sitll throwing heat, Saturn has to have greater amounts of those just by sheer mass.

How would plutonium dilute in the Saturnian atmosphere?

Let's care about the plutonium now and not the uranium in the future. Those 20.7 kg of plutonium are been mixed with all the gasses in Saturn and diluting in the atmosphere this week. Let's assume that this plutonium dosen't mix with the liquid hydrogen mantle and the core of Saturn but just with the 1000 km gaseus layer (something that hasn't to be remotly true at all but just to be optimistic let's take that). I estimated the volume of the Saturnian atmosphere as been 42 septillion liters (4.2x1022 m3). If you sparse plutonium in that volume you get that the increase in plutonium in the atmoshpere would be just of 50 septillionth of a gramm per cubic centimeter. This ammount is so absurdly tiny that we exceed the realms of homeophathic dilutions. The molar mass of Pu-238 is 238.05 g/mol so in each cubic centimeter you would find 0.001 plutonium atoms!!!!
If you believe the Cassini mission has contaminated Saturn then you must belive in homeopathy in the end.


What is the radiation harm to the enviroment?
Well once again with the exponential decay law and considering that the decay heat of Pu-238 is 560 W/kg, those 20.7 kg of plutonium released by Cassini are going to release 4.6x1013 Joules of energy at Saturn for over a milenia of time. Is that much? Well consider the fact that the energy released by an hurricane here on Earth in just one second is more than 10 times bigger. Saturn has bigger hurricanes than that, that last for years (not one second), and they don't destroy the enviroment at all. Ten times more energy still is the Chelyabinsk meteor impact (a rock the size of a house), an impact like that occurs at Saturn probably each year (and don't forget we are releasing this energy into saturn not in a mere second like the meteor strike but over 1000 years!!). No harm at all, we could send all the plutonium on Earth and nothing would happened at Saturn.
 
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16 Sep 2017 11:10

FastFourierTransform, great analysis!  The Plutonium indeed gets diluted to ridiculously small concentrations very quickly.  Even if much of it survives the atmospheric entry, it will sink into the lower layers and be vaporized and mixed into the core anyway. :)
 
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16 Sep 2017 12:11

I feel so much better now!
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16 Sep 2017 12:56

A-L-E-X wrote:
Source of the post Also, had Cassini been launched in this era instead of 20 years ago, would they have used solar sails instead of Plutonium?

When you say "solar sails" do you mean as in using the solar wind for propulsion? Or do you mean solar panels to generate electricity? Because — either way, actually — there is not enough sunlight at Saturn to make either of them feasible with our current technology.
 
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16 Sep 2017 13:23

Watsisname wrote:
FastFourierTransform, great analysis!  The Plutonium indeed gets diluted to ridiculously small concentrations very quickly.  Even if much of it survives the atmospheric entry, it will sink into the lower layers and be vaporized and mixed into the core anyway. :)

If aliens at our tech level happened to be observing Saturn or a Saturn/Sol transit, from their system, would they be able to notice any addition of Pu-238 or its decayed descendants to Saturn's atmosphere at all, once the light reaches their system?
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16 Sep 2017 14:43

PlutonianEmpire, that's a definitive no if they are anywhere around our tech level.
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A-L-E-X
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17 Sep 2017 15:45

Watsisname wrote:
FastFourierTransform, great analysis!  The Plutonium indeed gets diluted to ridiculously small concentrations very quickly.  Even if much of it survives the atmospheric entry, it will sink into the lower layers and be vaporized and mixed into the core anyway. :)

I appreciate what FFT said and of course on a planetary scale it isn't much- my main concern is why we're still using plutonium as opposed to newer, cleaner technology like solar sails.  It's why I originally asked if plutonium would continue to be used even though technology has evolved so much in the past 20 years.  I wouldn't use a 20 year old computer, after all, and I think our rate of technological evolution in the space program should evolve just as quickly as what the tech sector has been doing.

I use the same arguments for moving away from fossil fuel "technology" and why it's why I switched to solar power.  I'd like to have a solar powered car one day too.
 
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17 Sep 2017 15:47

DoctorOfSpace wrote:
PlutonianEmpire, that's a definitive no if they are anywhere around our tech level.

Doc, it's my strong feeling that any sentient aliens would stay far far away from any planetary system where fossil fuels or fission was still being used.  Now solar power or fusion is another matter ;-)
 
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17 Sep 2017 15:50

Mr. Abner wrote:
A-L-E-X wrote:
Source of the post Also, had Cassini been launched in this era instead of 20 years ago, would they have used solar sails instead of Plutonium?

When you say "solar sails" do you mean as in using the solar wind for propulsion? Or do you mean solar panels to generate electricity? Because — either way, actually — there is not enough sunlight at Saturn to make either of them feasible with our current technology.

Hmmmm I was wondering about that, because solar sails were listed as one of the ways to get to the outer solar system and Oort comet cloud. On the following page it was even mentioned as a way to reach the nearest stars.   Would it not work?
https://en.wikipedia.org/wiki/Solar_sail#Applications

I found this really interesting design

https://en.wikipedia.org/wiki/Starwisp
 
A-L-E-X
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17 Sep 2017 15:56

DoctorOfSpace wrote:
A-L-E-X wrote:
Source of the post Still, there's the ethics of altering a planet from what it was.

Sounds like a silly human concept to me.  I wouldn't have a problem if humanity moved out into the solar system and destroyed all the planets to build a Dyson swarm. 

I actually like the idea of building our own "artificial" planets complete with their own "artificial" gravity and "artificial" atmospheres and sailing to the stars on them across many generations.  We could even simulate sunlight and control the climate.  Without ever needing any exotic power sources, we could explore the whole galaxy across a myriad generations!  We would have our own hybrid of a generation ship and rogue planet ;-)
 
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17 Sep 2017 17:40

The US has been siphoning off Russia's supply of Plutonium since 2010. We have only around 35kg that's usable for 3 or 4 future space craft. But production might start up again in the near future.
https://www.space.com/36217-plutonium-2 ... ction.html

The only real dangers of using Plutonium is during take off. http://www.encyclopedia.com/science/sci ... ustifiable
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17 Sep 2017 19:32

Ok A-L-E-X, I'm sorry to do this, but I did give you a strict warning, and after both Doc and I had given several gentle warnings.  4 posts in the same thread in rapid succession is not acceptable; I'm giving you 4 days off for it.  You must figure out a way to avoid doing multi-posts. You can respond to multiple people in the same post.

Back on the topic, I think there might be some confusion between power source and propulsion.  The plutonium, in the form of an RTG, supplies power for the instruments, but not propulsion.  For propulsion Cassini used hydrazine monopropellant for attitude control and fine course adjustment, and bipropellant for main thrust.  (And if we want to talk about hazardous materials in spaceflight, hydrazine is seriously toxic and reactive.)  

As others noted, solar power isn't very efficient that far out from the Sun -- the size of solar panels required would dramatically increase the mass and thus fuel required, and that spirals the cost.  

We could use solar sails for propulsion, but there are a few hurdles.  The first is that it is fairly slow.  Mission designers would like to get there in their lifetimes.  Also, the longer it takes to get there, the more the power provided by the RTG will decay, and the more the spacecraft's electronic components will degrade from radiation and cosmic ray strikes.

A more serious problem with solar sails is you cannot slow down with them unless you have some light-based braking system already in place at the destination.  So to get Cassini into orbit around Saturn we'd have to combine it with some other propulsion method anyway.

One of the cleanest propulsion methods we know of is the ion drive (particularly with xenon), which has been used on the Dawn mission to the asteroid belt and several others in Earth orbit.  But the problem there is that it is still quite slow, and is better suited for course corrections or for orbital changes far from the object it is orbiting, than for boosting a craft onto an interplanetary trajectory to begin with.
 
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17 Sep 2017 22:30

Should this discussion be moved to a more suitable thread? I'd like to join, but I think the discussion is drifting away from the news topic.
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