4 ms·
With distances that big, is it even necessary to slow down much? The depth of focus is probably a couple dozen AU? Even if it takes the probe a century to get t
by dotnet00 1y ago
With distances that big, is it even necessary to slow down much? The depth of focus is probably a couple dozen AU? Even if it takes the probe a century to get there, if you can squeeze a decade or two of observation out of it without slowing down, there's no reason to bother and instead send a new upgraded telescope every decade or so.
As far as power requirements go, assuming a doubled power demand from Hubble might be a bit excessive. A telescope that far out would have to be nuclear powered, so thermal regulation is 'free'/passive and RCS load is reduced (don't have to constantly adjust to point away from the Earth), which I expect are the biggest power draws on Hubble.
If we assume a 150 year lifetime, with a 3kW draw by EOL and current RTG tech... RTGs have ~6% efficiency, so for 3kW electricity, you need 50kW in heat. RTG electricity output drops ~2% per year, so after 150 years, you have 5% of the initial electrical output, and you get ~0.57W/g of Pu-238. Meaning, you need ~600kg of it to power the telescope this way [https://www.mathscinotes.com/2012/01/nuclear-battery-math/ https://www.mathscinotes.com/2012/01/nuclear-battery-math/].
That's not a politically feasible amount, but it's not technically impossible with current/near future tech whose development could be spurred on by serious interest in this kind of mission.
'Proper' fission reactors can also do the job, you get higher efficiency and don't have to run the reactors for the entire 150 years besides accounting for decay (e.g. an RTG that needs to provide enough power to keep some clocks running, the electronics and batteries warm, and trigger whatever mechanism would start up the reactor). Probably less than 100kg of Pu-238 just by better reactor efficiency.
- ycui1986 1y agoi don't think modern semiconductor device will last more than 100 years, even without all the radiation. making something last more than a few decades is very hard.
- le-mark 1y agoDoes encasing electronics in lead help against high energy cosmic rays? With cheap kg to orbit one could assume the mass budget would be large.
- chuckadams 1y ago> Does encasing electronics in lead help against high energy cosmic rays? Makes them worse unless and until you make the shielding several times thicker than anything you'd be able to launch from the ground. Watched one science program that demonstrated it beautifully where the interviewee stuck several balloons on a board and shot at it with a high-powered rifle, popping just one. Then he stuck a metal plate in front of the balloons and shot it, and the resulting shrapnel popped all the balloons behind it. That's a cosmic ray hitting shielding. That's also an unsolved problem with any Mars trip. Electronics can be built redundantly to recover from cosmic ray hits. Humans not so much.
- alex_young 1y agoConsidering that the longest continually operating computer is in Voyager 2 and has been running for nearly 50 years I would be surprised if this was actually a problem. https://www.guinnessworldrecords.com/world-records/635980-longest-period-of-continual-operation-for-a-computer https://www.guinnessworldrecords.com/world-records/635980-lo...
- os2warpman 1y agoI agree with you. It is indeed spherical frictionless cow-ly possible if we spend a trillion dollars to increase ORNL's annual Pu production capacity so that it doesn't take 200 years to make 600kg of Pu-238. When someone demonstrates a complex device (let's set aside power generation how about a valve? Or a capacitor?) that can last a century in space I'll agree that it is actually possible. That's what "current level of technology" means. The lego bricks exist, now, today, preferably in stock ready for immediate shipment on Digikey, and can be snapped into place.
- dotnet00 1y agoI did say it's politically infeasible. Producing 600kg of Pu-238 is entirely technically feasible with current level of technology, as it has already been done when the US and USSR built their stockpiles. Current level of technology means things we are capable of achieving right now, it does not mean that the pieces literally exist ready to use, unless you genuinely believe that the thousands of satellites orbitting our planet are not part of the current level of technology because they're purpose built without using off-the-shelf parts.
- deleted 1y ago[deleted]
- Dylan16807 1y ago> It is indeed spherical frictionless cow-ly possible if we spend a trillion dollars to increase ORNL's annual Pu production capacity so that it doesn't take 200 years to make 600kg of Pu-238. Oh come on, we used to make so much more of it. I see estimates that it costs 4 million dollars per pound, plus some scaling costs? A trillion dollars is not even close to "spherical frictionless cow" when the benchmark is "humanity's current technological capabilities", and a few billion is basically nothing at that scale. > When someone demonstrates a complex device (let's set aside power generation how about a valve? Or a capacitor?) that can last a century in space I'll agree that it is actually possible. Is a bunch of stuff lasting 50 years not good evidence? What is your threshold for "demonstrate", do we have to wait 200 years before you can be convinced?
- griffzhowl 1y agoWouldn't there be a problem putting 600kg (or even 100kg) of Pu-238 together, because of supercriticality? I couldn't think of a plausible design, but I know next to nothing about this area. Basically I've heard that if you put a lot of this stuff together it'll make a big explosion
- ben_w 1y agoCriticality isn't hard to avoid, just split it between e.g. 344 units arranged in a 7x7x7 cube with 10cm gaps each way. Or more, I picked that separation and mass division based on guessing.
- griffzhowl 1y agoYeah, I thought about doing something like that, but that would make many parallel power-generating units that would only last as long as one unit wouldn't it? Maybe the individual units could be subdivided further and the subunits could be brought together only when a previous unit runs out of power. I don't know enough about how it would actually work
- ben_w 1y agoI don't think so. The radioisotope itself is an exponential decay, and only goes faster when critical, not when subdivided; the part I'm not sure about is the thermocouple and why that decays.
- griffzhowl 1y agoI see. So the maximum time these units could provide power is the time it would take a subcritical mass to decay to the point that it's no longer useful. So the idea is unworkable for powering very long journeys
- ben_w 1y agoVery long journies, eventually everything fails. I suspect that we don't have enough practical experience to be confident of any space mission lasting 100 years even when the power supply is fine. But also, there are other radioisotopes besides the one currently used. The ~90 year half life of the current normal radioisotope is great for current missions, not the only option.