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Justanotherrandomguy
2007-Oct-14, 10:03 PM
Hi,

New to this board.

Was reading recently about a Jupiter Orbiting spacecraft called Juno that might be launched by NASA in the future. What is special about this spacecraft is despite being in one of the outer planets orbit (albeit the nearest) solar panel technology has improved to the extent that it is sufficient to power a craft that far away from the Sun.

This made me wonder I realise that RTG's are very scarce and in short supply (can someone explain why) but can a solar powered craft effectively continue orbiting in power indefinately as it has an indefinate source of power? I suspect not, but can someone explain why this would be?

Thanks

Romanus
2007-Oct-15, 03:56 AM
Solar powered probes can be highly durable; Mars Global Surveyor survived for a decade, and Pioneer 6 is still operational 42 years after its launch. These are just the ones I can think of off the top of my head.

Though I admittedly don't know much about panels in particular, IIRC they tend to degrade over time due to radiation exposure--a problem for any space probe.

Larry Jacks
2007-Oct-15, 01:18 PM
RTGs are scarce and in short supply because they're expensive. In order to be safe even in the event of a launch failure, RTGs have to be build very rugged and tested a great deal. They're still the only viable solution for very deep space missions but there simply aren't very many of those, so there aren't very many RTGs.

Newer generation solar panels featuring materials like GaAs are more efficient than silicon panels and more radiation resistant, too. If they can build panels large enough to meet the electrical needs of a probe at Jupiter and rugged enough to stand up to the radioactive environment then you could potentially have very long lasting vehicles. In that case, other factors will ultimately limit how long the probe can function. These other factors include propellant supply (used for maneuvering and to unload stored momentum in reaction control wheels), the ability of the systems to stand up to long term radiation exposure, failure rates and redundancy of essential systems, and even the program budget.

Justanotherrandomguy
2007-Oct-15, 10:26 PM
Thanks for the info. I always thought that with an unlimited power supply they should try and make more use of spacecraft with solar panels. Given that it is so expensive to create RTG's I think solar panel technology is really encouraging if we get it as far as Jupiter without using RTG's surely it must cheapen the flight without having to go through all that expensive tests that would normally be required, therefore making more trips to Jupiter possible at a lower price. If we could get the technology to work as far as Saturn then we have a vast area of space that we can travel without RTG's not just the planets but also the moons.

Was there a time when we were spaceworthy but couldn't use solar panels to power Mars based spacecraft?

JonClarke
2007-Oct-15, 10:33 PM
Thanks for the info. I always thought that with an unlimited power supply they should try and make more use of spacecraft with solar panels. Given that it is so expensive to create RTG's I think solar panel technology is really encouraging if we get it as far as Jupiter without using RTG's surely it must cheapen the flight without having to go through all that expensive tests that would normally be required, therefore making more trips to Jupiter possible at a lower price. If we could get the technology to work as far as Saturn then we have a vast area of space that we can travel without RTG's not just the planets but also the moons.

Was there a time when we were spaceworthy but couldn't use solar panels to power Mars based spacecraft?

Solar technology was not viable for the Mars surface when the Vikings designed in the early 70's.

Saturn is a very different kettle of fish. Solar irradience is about 1% of earths, and I don;t see how any currently feasible solar technology could power a spacecraft at those sort of distances.

On the RTG issue, another bottleneck there are only a limited number of facilities that can extract the required Pu isotope from spend reactor fuel. It's not the normal Pu239 normally extracted and recycled. The process itself is also complex and expensive.

Jon

Justanotherrandomguy
2007-Oct-15, 10:51 PM
So do you think we could see more Jupiter Missions with almost the regularity we see Mars Missions? I mean surely that is quite a large obstacle out of the way for that, albeit with several others there.

You are right about Saturn, but considering that we didn't have the technology at Jupiter, it is not unthinkable that technology might improve enough that this will become a viable option in the future.

OTOH we are going to be using RTG's for the Mars Science Laboratory.

neilzero
2013-Mar-18, 04:34 PM
Hi,

New to this board.

Was reading recently about a Jupiter Orbiting spacecraft called Juno that might be launched by NASA in the future. What is special about this spacecraft is despite being in one of the outer planets orbit (albeit the nearest) solar panel technology has improved to the extent that it is sufficient to power a craft that far away from the Sun.

This made me wonder I realise that RTG's are very scarce and in short supply (can someone explain why) but can a solar powered craft effectively continue orbiting in power indefinately as it has an indefinate source of power? I suspect not, but can someone explain why this would be?

Thanks


The RTG uses an isotope of plutonium that is a biproduct of making plutonium for bombs. We have recently made very little plutonium so the stock pile of the isotope for RTGs is diminishing. This isotope has a half life of about 12 years, so the stock pile is also decreasing. Likely there are other options, but allowable human exposure to radiation has decreased in recent decades, making costs much higher. Possiblly the French have a different isotope that they are extracting as part of their nuclear waste recycling program that is suitable for a new design of RTG.
The short half life means the RTG is producing very little electricity after 30 or 40 years. Solar panels orbiting Venus also deteriate to near useless after 30 or 40 years, but deteriate much slower in Mars orbit due to reduced X rays, gamma rays and solar wind farther from the sun. The intense radiation belts near Jupiter may drastically reduce the life of the solar panels, so I hope they have a back up for Juno. Nearly everything deteriates with enough time elapsed. Neil