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#20 by Blackstar on 23 Jun, 2016 14:09
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As for experiments that are designed around presence of gravity - no need for the facility to be in zero g, unless that is required for some reason. Use a counterweight (e.g. spent third stage, or another hab) to spin it to generate artificial gravity (AG). Technically easy to do. Main thing would be how to dock - need a module at the hub of the spin for docking, docking port counterspun.
So, in order to do analysis that is regularly and easily done on Earth, you're proposing a large spacecraft with all new instruments plus artificial gravity, which hasn't actually been done yet either.
Do you have a cost estimate for this proposal? -
#21 by whitelancer64 on 23 Jun, 2016 14:29
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A flyby probe to capture some of the material from the jets at the tiger stripes would be great, and would be a relatively easy sample return mission
Can you explain what you mean by "easy"?
For starters, there's the issue of approach velocity--how do you capture the sample without destroying it with the high velocity impact with the sample collector?
Then there's the overall time required for such a mission--approximately 8+ years there and an equal or greater time back, for a total roundtrip of over 16 years.
Then there's the fact that if your Level 1 science goals define "success" as "return sample safely to Earth," and your spacecraft unsurprisingly dies in year 17 of its 18 year mission, it fails.
Then there's the lifetime cost of a mission that lasts 16+ years, which is not exactly cheap.
I didn't say it would just easy, I said it would be "relatively easy" compared to any other sample return mission, which would require landing on a body, excavating a sample, and sending that sample back to Earth.
You'd want to use aerogel or something similar to capture particles, this will be somewhat disruptive to the sample but the technique worked well on the Stardust mission. No new technology development required.
My comment already noted that the cost of the mission is one of the largest hurdles it would have to clear. -
#22 by whitelancer64 on 23 Jun, 2016 14:47
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A flyby probe to capture some of the material from the jets at the tiger stripes would be great, and would be a relatively easy sample return mission, but it is complicated by the challenges associated with getting a probe out to Saturn within a reasonable budget. RTGs would be much too expensive, so we're talking solar power. It would have to be a very simple probe and it would have to operate on a very small energy budget. There have been several proposals for a flyby sample mission like this or other exploration of Enceladus,
https://en.wikipedia.org/wiki/Enceladus#Proposed_mission_concepts
but none have been funded yet.
Actually with modern "labs on a chip" you can do a powerful in situ life finder mission for Enceladus. That would also let you study the plumes at different heights to sample different sizes of particles, and also watch for changes, e.g. if there are algae blooms or similar, or if you get better results at particular times in its orbit. All the mass that would be needed for the sample return could be used instead for extra instruments. With many instrument just a chip and perhaps half an amp of power, that's a lot of in situ study for the mass of a return capsule + fuel to get it back to Earth, and you get the results right away.
Also if there is life in the sample, then you don't know how to best preserve it for the journey back, until you know what it's like.
And then there's the issue of how you handle the sample return. I think it's best done above GEO in a telerobotic facility, given the rather high chance that there might be exobiology there not based on DNA, with almost no communication with Earth - if life is common in our galaxy, then there may well be exobiology on Enceladus. Impossible to assign a probability of that, but surely it's a few percent at least if life is common? Though almost zero if life is very rare in our galaxy. If returned to Earth's surface, then it's an immensely complex thing to sort out legally, Margaret Race looked into it, you wouldn't believe how many new laws would need to be passed and even quite simple international laws can take many years to pass - it might easily take over a decade to pass all the laws needed for a surface to Earth sample return while a return to a telerobotic facility above GEO can be done within our current legislation. Then return sterilized samples to Earth surface until you know a bit more about it.
I see that as possible, but in situ is just far easier to do first, and safest of all the life missions we can do both for Enceladus and for Earth. Almost no possibility of forward contamination and none at all of backward contamination.
Fine, but you're talking about a Flagship-class mission now, requiring an RTG, many instruments, and complex systems on the probe.
A simple sample return could easily be a Discovery-class mission. -
#23 by vjkane on 23 Jun, 2016 15:12
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Fine, but you're talking about a Flagship-class mission now, requiring an RTG, many instruments, and complex systems on the probe.
A simple sample return could easily be a Discovery-class mission.
Peter Tsou, who has led the proposals for an Enceladus sample return, stated in a meeting last year that a sample return is at least a New Frontiers-class mission. I don't know if that covers the costs of the sample return facility required on Earth (any life on Enceladus has to treated as dangerous until proven otherwise) or not.
The biggest downside to a sample return, as Blackstar pointed out, is that you need to wait 14 to 16 years to get your science. -
#24 by robertinventor on 23 Jun, 2016 16:01
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Because of the much more benign radiation environment, Enceladus is a lot easier place to work than Europa. In my mind, that would be the big "plus" of going there.
The big "minuses" are the much greater travel time, and the need for RTGs at Saturn's heliocentric distance.
A Cassini-like mission that was designed as an Enceladus orbiter with an advanced chemistry and mass spectroscopy suite for investigating the plumes, plus radar, would be pretty nice. And cost billions of dollars, not including a sure-to-be-SLS launch.
Actually no. The reason they had to put it on SLS for the Europa orbiter was because Congress mandated that it had to include a lander which made it impossible to launch it on anything smaller as a single mission. I think it's now changed to two missions so it is no longer forced to be an SLS flight.
The Enceladus orbiter, which is able to get into orbit around Enceladus equator with excursions to the poles, from the decadal review study in 2010 had a detailed costing of between 1.593 and 1.613 billion in FY2015 dollars. ( page 37 of this report). That was for a launch on an Atlas V 521 -
#25 by Torbjorn Larsson, OM on 23 Jun, 2016 16:20
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An Ocean lies a few kilometers beneath Saturn's moon Enceladus's icy surface
Summary:
With eruptions of ice and water vapor, and an ocean covered by an ice shell, Saturn's moon Enceladus is one of the most fascinating in the Solar System, especially as interpretations of data provided by the Cassini spacecraft have been contradictory until now. Astronomers recently proposed a new model that reconciles different data sets and shows that the ice shell at Enceladus's south pole may be only a few kilometers thick. This suggests that there is a strong heat source in the interior of Enceladus, an additional factor supporting the possible emergence of life in its ocean.
Excitingly, the quote leaves out the most salient points with the proposed reconciliation of data sets:
- The alkaline hydrothermal vent activity is asserted, since tidal interactions no longer suffice as heat source.
- The salinity of the ocean is lowered from highly (but extremophile survivable) alkaline to Earth normal.
- The ocean volume is increased to 40 % of the moon volume.
All of which increases the urgency to go there. Especially considering that any jet activity of Europa has been soundly rejected by the discovery of insufficient water concentration in its orbit. (Too low with 2 oom.)With news like this tell me again why we haven't got a mission planned already for Enceladus, as in many ways it looks an easier target than Europa other than being further out.
We have a 2nd one planned, the 1st got canned this year in competition with a (very late) return to Venus. And of course because certain Senate members know what Europe is, but has never heard of Enceladus, as witnessed in the hearing on Ice moon priorities. (Enceladus, Titan and Europa, likely in that order. Note that we eventually need to research Europa, since it is a case study for assessing life frequency of the largest potential habitats out there.)
Luckily the new proposal, Enceladus Life Finder, will refly the LIFE experiment that should be able to reject abiotic sources to the jet organics. As far as I understand Tsou has changed his 2014 LIFE sample return proposal at LPSC to a 2015 on board experiment suite. Anyway, it is ingenious, far from the clumsy Viking experiments where in retrospect one may wonder if they were optimists and never considered the possibility of confounds.
But instead of a 2021 launch, the web videos from the LPSC 2016 meeting had NASA declare that an approved mission would have a target launch date of 2031. :-/
[ https://en.wikipedia.org/wiki/Enceladus_Life_Finder ; http://www.hou.usra.edu/meetings/lpsc2014/pdf/2192.pdf ; http://www.hou.usra.edu/meetings/lpsc2015/pdf/1525.pdf ]
EDIT: Oh, for JPEGs sake! Changed to a hopefully more readable image attachment.
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#26 by jgoldader on 23 Jun, 2016 16:30
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The Enceladus orbiter, which is able to get into orbit around Enceladus equator with excursions to the poles, from the decadal review study in 2010 had a detailed costing of between 1.593 and 1.613 billion in FY2015 dollars. ( page 37 of this report). That was for a launch on an Atlas V 521
Many thanks for the study link, I hadn't seen that before. The basic scope of the mission is about what I'd like to see. Using moon flybys to reduce energy for Enceladus orbit insertion (=fuel weight savings) is awfully clever. One caveat is that there are no ASRGs (that project was cancelled, wasn't it?), so power requirements are going to be difficult to meet. The Atlas is a lot less expensive than SLS, which is good. -
#27 by robertinventor on 23 Jun, 2016 16:35
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Fine, but you're talking about a Flagship-class mission now, requiring an RTG, many instruments, and complex systems on the probe.
A simple sample return could easily be a Discovery-class mission.
Actually, no, can see why you might think so, but with modern miniaturization they can be small spacecraft.
You are talking about at most a few kilograms per instrument and of the order of a watt or two of power per instrument in the instrument suite. The Enceladus Life Finder is Discovery class.
As other example instruments we could send, Solid3 from 2012 using polyclonal antibodies to search for organics, a very versatile method, doesn't just detect life organics, was 7 kg and maximum power requirements 2.7 watts
Astrobionibbler, a tool developed for Mars - so not for Enceladus but shows what can be achieved through miniaturization, able to detect a single amino acid molecule in an entire sample, it weighed 3.0 kg.
That's just a couple to give an idea, they can get more lightweight and have less power consumption than that. And - instruments get much smaller as time goes on. The power requirements and mass requirements for in situ search for life will only get less as time goes on. Then think how much mass you need for the fuel to return a sample from Enceladus to Earth, then convert those to instruments you can use to study in situ and get results instantly. Plus with the in situ studies, you study the material exactly as just captured from the geyser moments earlier, with no possibility of it deteriorating or chemical changes happening during the return flight, or indeed if there is life there, you might get viable life in the sample maybe even alive. Maybe you can test it in a microbial fuel cell or Levine's chiral labelled release (those also are now tiny low mass and low power requirement instruments, don't think the labelled release of Viking). If there is viable life there - some kind of a spore or some such - then you have the challenge of keeping it viable during the long journey back, when it is used to surviving in a subsurface ocean. Maybe it can stay viable for the few hours it takes to get to the in situ orbiter - if so - can you keep it viable for the long journey back to Earth?
I think sample return is well worth doing, but not the first thing you do, I think you find out what is there first, and then design the sample return based around what you find. And it may be a very fast return mission if you use later technology.
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#28 by robertinventor on 23 Jun, 2016 16:48
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Many thanks for the study link, I hadn't seen that before. The basic scope of the mission is about what I'd like to see. Using moon flybys to reduce energy for Enceladus orbit insertion (=fuel weight savings) is awfully clever. One caveat is that there are no ASRGs (that project was cancelled, wasn't it?), so power requirements are going to be difficult to meet. The Atlas is a lot less expensive than SLS, which is good.
Yes it is, isn't it and you get to do close up flybys of those moons also. Rhea is intriguing with its possible very tenuous rings, they've only got an upper bound on their density not proved they are impossible. And there's a possibility that Dione and Tethys might have geysers too (2006). Not sure what happened about that idea, but at any rate Dione has some clean H2O ice suggesting it could have had recent tectonics. So they are interesting moons to study in their own right too.
Yes, it's an old study of course. You'd need more mass for RTGs. Or solar panels. Or the ESA's Americium based RTGs which have the advantage of a longer working life into centuries, if needing a bit more mass for the same power output. -
#29 by Torbjorn Larsson, OM on 23 Jun, 2016 16:51
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The Enceladus orbiter, which is able to get into orbit around Enceladus equator with excursions to the poles, from the decadal review study in 2010 had a detailed costing of between 1.593 and 1.613 billion in FY2015 dollars. ( page 37 of this report). That was for a launch on an Atlas V 521
caveat is that there are no ASRGs (that project was cancelled, wasn't it?), so power requirements are going to be difficult to meet.
The ELF proposal uses solar panels. As I remember it, it is believed to be a viable technology out to Saturn. -
#30 by Blackstar on 23 Jun, 2016 18:44
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Two previous comments:
"The Enceladus Life Finder is Discovery class."
"The ELF proposal uses solar panels. As I remember it, it is believed to be a viable technology out to Saturn."
I would caution against using proposals as examples that something is possible. At most they should be used as examples that something is proposable.
Remember that ELF was rejected as a Discovery class mission. Why was that? One possibility is that it was too expensive to make it as a Discovery class mission. Ergo, it does not prove that you can do this mission at that low level of funding. Another possibility is that it failed technically because of the solar panel proposal--ergo, it does not prove that you can do this mission with solar panels.
Now you certainly should be able to do some kind of Enceladus mission at New Frontiers levels of funding, but that mission might not be able to do sufficient science to be worthwhile. And while you might be able to use solar panels at Saturn distances, perhaps they create so many other restrictions on the mission that it is not worth doing. We'll see. There are lots of worthwhile targets, and some of them may offer much better return on investment than an Enceladus mission. -
#31 by Blackstar on 23 Jun, 2016 18:58
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Peter Tsou, who has led the proposals for an Enceladus sample return, stated in a meeting last year that a sample return is at least a New Frontiers-class mission. I don't know if that covers the costs of the sample return facility required on Earth (any life on Enceladus has to treated as dangerous until proven otherwise) or not.
The biggest downside to a sample return, as Blackstar pointed out, is that you need to wait 14 to 16 years to get your science.
Any NF proposal is going to have to include costs through Phase E (maybe even Phase F-closeout?). So the laboratory costs are going to be part of that. If you need a ground lab to do your science and the other guy does not, why should you be allowed to not include that in your costs? It's not fair to the other competitors to offload your missions costs.
And there is another thing that works against long duration missions in cost-capped programs like Discovery and New Frontiers--you have to include all those years of flight ops costs and the more years you have, the more costs. So let's assume (I forget the details) an 18-year roundtrip mission to Enceladus to return samples. You're going to spend nine years in coast out there and nine years back. Assume that you have to ramp up operations and ramp down in a few months either side of the encounter (which is going to cost more money). And then let's assume that buying very simply housekeeping time on the DSN for the coast period and paying your people to come in and do the basic checks is only $10 million a year. You are easily looking at over $200 million just for the operations, or 20+% of your budget. That duration is a tough thing to deal with. That's why very long duration missions probably can only work at the flagship class.
Now we're also talking about accounting here. If you're going to propose a sample return mission, then the mission is not over until the sample returns to Earth, which means that all those operations costs (18+ years) are included in the prime phase. But if you were to launch a mission to Enceladus that got there in nine years, it would probably include only two years in prime phase. That's 11 years figured into the mission cost. But then if it went into extended phase for another 8 years it would have the same total mission duration as the sample return mission, but the costs are accounted for much differently--it might cost more than sample return in the end, but be more affordable because you have multiple chances along the way to continue funding it or stopping. You don't have those options if you only get your science at the end of the flight. -
#32 by robertinventor on 23 Jun, 2016 19:28
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Now we're also talking about accounting here. If you're going to propose a sample return mission, then the mission is not over until the sample returns to Earth, which means that all those operations costs (18+ years) are included in the prime phase. But if you were to launch a mission to Enceladus that got there in nine years, it would probably include only two years in prime phase. That's 11 years figured into the mission cost. But then if it went into extended phase for another 8 years it would have the same total mission duration as the sample return mission, but the costs are accounted for much differently--it might cost more than sample return in the end, but be more affordable because you have multiple chances along the way to continue funding it or stopping. You don't have those options if you only get your science at the end of the flight.
There's another downside to sample return, at least the first one done from outside of Earth. I think the least cost and safest way to do it is to above GEO, which would reduce this cost a lot.
But if you go the traditional route and return it to the Earth you need to build a sample return receiving laboratory. And don't think Moon sample return laboratory - that's woefully inadequate - even at the standards of the 1960s the plan had no peer review as it was published on the day of launch of Apollo 11 and they had many breaches of protocol.
But modern sample return designs - they get estimated costs in the order of hundreds of millions of dollars. It has to contain particles that you can only see in an electron microscope, well beyond optical resolution limit due to newer ideas about the smallest possible size of an exobiology cell. Have to protect the samples from contamination by any Earth life even a few amino acids. When they figure that all out, then try to price it, you get maybe half a billion dollars for the receiving facility. That's including the costs I think of operating it. Because of the many breaches of protocol for Apollo they would require the staff to be familiar with its use and actually operating it - SEVERAL YEARS BEFORE THE LAUNCH OF THE SAMPLE RETURN MISSION. That's the recommendation for Mars anyway of the most recent sample return studies - maybe you could get an exemption of that for Enceladus? But at any rate certainly many years before hthe sample is returned.
And - there's masses of legislation to pass. Margaret Race looked at it. You'd be astonished, many domestic and international laws, needing to be passed - which were not needed for Apollo because the world nowadays is legally far more complex. After reading her paper,I think it could easily take well over a decade just passing all the laws even if everyone agrees and there are no objections, and surely longer if there are objections.
Returning to above GEO simplifies all that as no new legislation is needed, can be done within all the existing laws. Also, you don't have any concern about the staff not using the right protocols because it is all operated from Earth and there is nothing the staff could do by mistake or laziness that would lead to life from the facility escaping into the environment of Earth.
Still even a plan to return to a facility above GEO would add to the expense of the mission. If nothing like as much as to a surface facility. The orbital facility could just be a single spacecraft that receives the sample, and does preliminary studies. But then it's an open ended future after that. It would be like extended missions perhaps. So I think this idea that the mission plan is simply to return it to a spacecraft in a safe orbit above GEO would be the simplest one and lowest cost, but you get no science return until it is studied, so surely have to add a bit more on top of that for your minimal mission to study it for some time in its new location.
It's just far more complicated. And given that we have such capable ways of studying in situ, and it gives you fresh unchanged samples, and ability to follow up on results, sample at different heights, different times in the orbit etc, I think in situ has to be the way to go to start with. But sample return soon after if we find something interesting. At that point the funds would be easier to find, for the more expensive sample return, if you have evidence of exobiology in the samples studied in situ. -
#33 by Torbjorn Larsson, OM on 24 Jun, 2016 10:50
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Two previous comments:
"The Enceladus Life Finder is Discovery class."
"The ELF proposal uses solar panels. As I remember it, it is believed to be a viable technology out to Saturn."
I would caution against using proposals as examples that something is possible. At most they should be used as examples that something is proposable.
Oh, there are no guarantees of course. But science teams don't like to play with propositions and have them retracted on technicalities, it is a waste of time.And while you might be able to use solar panels at Saturn distances, perhaps they create so many other restrictions on the mission that it is not worth doing.
Now you are speculating. ELF will do a lot, based on known solar panel technology, so it seems like badly founded speculation to me.
EDIT: Changed likely rationale for science teams. I blame lack of coffee for not seeing the obvious. -
#34 by Torbjorn Larsson, OM on 24 Jun, 2016 11:21
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There are lots of worthwhile targets, and some of them may offer much better return on investment than an Enceladus mission.
Fine, let us talk strategy.
First, I note that the web available 228 AAS NASA presentation had Jim Greene describe that Enceladus and other ice moons are for free due to The Europa Senator folly, the Administration changed a legislative line. (I think, not being up on US legislative procedures.) Hence they could circumvent both TES's narrow focus and the Decadal Survey propositions who where not based on the latest science. (Europa bad, Enceladus and Titan good.) If NASA is really happy with accelerating outer system exploration while they concentrate on Mars is another thing, they made lemonade.
Second, the flyby/orbiter/lander/sample return concept is technologically viable throughout the system, and as the Voyagers show even sample return can be politically/economically viable. You can make a case for Enceladus as an ideal candidate for packaging a flyby/orbiter/"lander". Or even all 4, but likely the first 3 are enough for life detection. So, competitive ROI in comparison with Mars re astrobiology. But superior ROI compared to Europa as a case study of ice moons. Unfortunately Enceladus must eventually be complemented by an Europa mission to assess biopotential over generic ice moons, potentially the largest biosphere volume in the universe.
Third, Enceladus is an outstanding test of our best tested emergence theory, vent theory. As I noted in an earlier comment, the oceans looks to be neutral now (unless I am mistaken, have to read the paper). This satisfies the outstanding constraint on alkaline hydrothermal vents as ancestors for life, need the cell potential of alkaline inside and more acidic outside. You may sell others on not going to Enceladus ASAP, but few astrobiologists I think. The still dominant soup theory consensus is happy with Mars exploration. And since the missions are mainly astrobiological... -
#35 by Blackstar on 24 Jun, 2016 12:27
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There are lots of worthwhile targets, and some of them may offer much better return on investment than an Enceladus mission.
Fine, let us talk strategy.
First, I note that the web available 228 AAS NASA presentation had Jim Greene describe that Enceladus and other ice moons are for free due to The Europa Senator folly, the Administration changed a legislative line. (I think, not being up on US legislative procedures.) Hence they could circumvent both TES's narrow focus and the Decadal Survey propositions who where not based on the latest science. (Europa bad, Enceladus and Titan good.) If NASA is really happy with accelerating outer system exploration while they concentrate on Mars is another thing, they made lemonade.
Second, the flyby/orbiter/lander/sample return concept is technologically viable throughout the system, and as the Voyagers show even sample return can be politically/economically viable. You can make a case for Enceladus as an ideal candidate for packaging a flyby/orbiter/"lander". Or even all 4, but likely the first 3 are enough for life detection. So, competitive ROI in comparison with Mars re astrobiology. But superior ROI compared to Europa as a case study of ice moons. Unfortunately Enceladus must eventually be complemented by an Europa mission to assess biopotential over generic ice moons, potentially the largest biosphere volume in the universe.
Third, Enceladus is an outstanding test of our best tested emergence theory, vent theory. As I noted in an earlier comment, the oceans looks to be neutral now (unless I am mistaken, have to read the paper). This satisfies the outstanding constraint on alkaline hydrothermal vents as ancestors for life, need the cell potential of alkaline inside and more acidic outside. You may sell others on not going to Enceladus ASAP, but few astrobiologists I think. The still dominant soup theory consensus is happy with Mars exploration. And since the missions are mainly astrobiological...
I've read this twice and still don't understand it. I don't think you're making the case that you think you are.
NASA has policies and procedures for picking planetary mission targets. There are lots of other possible missions besides Enceladus. Any Enceladus mission proposal is going to have to prove that it is better than the other options. -
#36 by Blackstar on 24 Jun, 2016 12:35
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Oh, there are no guarantees of course. But science teams don't like to play with propositions and have them retracted on technicalities, it is a waste of time.
But that's what a review process is for--to weed out things based upon various criteria. I happen to know several of the Discovery mission proposers (not talking about ELF here). I was talking to one of the guys whose proposal was rejected. He thought his science return was great. The review team rejected it on basis of science return. That kind of stuff happens. What also happens is that a proposer thinks that they have a great team and then the reviewers say "You don't have enough experts about X on your team" and the proposal gets rejected. And sometimes teams think that they have a great technical proposal and but the review team identifies things that they miss.
Just because a mission has been proposed does not mean that it is perfect. It can have flaws. Happens all the time.And while you might be able to use solar panels at Saturn distances, perhaps they create so many other restrictions on the mission that it is not worth doing.
Now you are speculating. ELF will do a lot, based on known solar panel technology, so it seems like badly founded speculation to me.
But this kind of stuff happens all the time. A mission proposal says "Our spacecraft will do X" and then the review team finds something that they missed or forgot about, or did not understand. One issue that I've seen raised for solar panels at Saturn distances is that they are so big that they introduce other problems. They can jitter, they can get in the way of instruments' fields of view. They can overwhelm the control system. They may make it hard to slew the spacecraft to point the instruments in time. This is a systems engineering challenge, not simply a case of working out the power supply vs. the power demand--the devil is in all the other details, and the devil can reject a proposal. -
#37 by baldusi on 24 Jun, 2016 13:14
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What's the solar energy at Saturn distances? 15W/m2? I know it is 9.5AU. If light falls to the distance squared, that's 90 times less energy than the 1366W/m2 in the Earth.
Say that they can get 25% efficient cells, they will get (round up) 4W/m2. And that's perpendicular to the sun.
So, to get New Horizons electric supply (300W), they would need 75m2 of solar cells. Let's say 100m2 after margin. That's double the commercial GSO comm bird. And that's assuming 25% efficiency. At 15% you would need more than 165m2. That's a bit less than one strip of ISS solar panel, to put it in perspective.
Seems doable, without much constraints. But it would be expensive and put a lot of mass and structural limitations. And we aren't heating anything there. -
#38 by Robotbeat on 24 Jun, 2016 16:59
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The important figure of merit for an orbital spacecraft is specific power.
MMRTG has about 2.2-2.5W/kg.
SoA solar at Earth has 150W/kg. Divide by 90 (yes, I am aware of other effects from low light levels), and you have 1.7W/kg, so nearly the same but a LOT cheaper. Additionally, ROSA arrays currently under development can approach 1000W/kg at 1AU, so about 11W/kg at Saturn, FAR exceeding MMRTG's performance for a given mass.
If you're in orbit, then solar power is fantastic. Keep the RTGs for surface missions and missions well beyond Saturn. -
#39 by baldusi on 24 Jun, 2016 17:27
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The important figure of merit for an orbital spacecraft is specific power.
You still haven't heated anything. GPHS-RTG have something like 4kW of thermal output and 300W of electrical power. And those had a W/kg more like 5.2~5.4. But yes, MMRTG have more like 2.3 or so. And sill give 2000W of thermal power each (~100W electrical).
MMRTG has about 2.2-2.5W/kg.
SoA solar at Earth has 150W/kg. Divide by 90 (yes, I am aware of other effects from low light levels), and you have 1.7W/kg, so nearly the same but a LOT cheaper. Additionally, ROSA arrays currently under development can approach 1000W/kg at 1AU, so about 11W/kg at Saturn, FAR exceeding MMRTG's performance for a given mass.
If you're in orbit, then solar power is fantastic. Keep the RTGs for surface missions and missions well beyond Saturn.
So, if the craft needs 300W of electrical plus 900W of thermal (I'm guessings, insight would be appreciated), then you have to correct by that difference. Thus you would have to divide by 4 each of your figures of merit. And that's not all. You have higher moment of inertia. That means bigger reaction wheels, which need more power. And you need stiffer arms on the solar panels. And you need to do double axis joints that can react to the slew requirements of cameras without failures after 9 years of travel.
And you need batteries for orbital occultations. And you have less radiation tolerance. It is not, at all, a magic solution. Juno was quite constrained by going solar and basically can't take pictures.
But more importantly, we are talking about a mission that has to fly through a gas plume that might contain ice particles, at orbital speeds. You don't want 150mē or front area for doing that.
I'm not saying that it is impossible. I'm just stating that just saying it's cheaper and technically easy is more an act of faith than an actual engineering statement.