RTG or solar or other? What are the practical limits of how long we can expect we could make it last. I’m talking about power supply only. If we needed one to last 400-500 years is that remotely possible?
Also regarding solar, if we are sending a probe to another star, could we expect to send it ‘dead’ to wake up when enough starlight hits the solar panels. (again looking at the 400-500 year range)
if you are looking at 400-500 years, RTG is still your best bet. You just need to design one around an isotope with a half-life in the needed range, and that can provide the necessary power.
Voyager 1 is going to take 40,000 years to reach the nearest star that is on it’s trajectory, while its sister is going to take 300,000 years to do so. Looking at that time frame, our civilization will either be gone or it will have evolved to the point where we can make the trip ourselves and pick up the probe along the way. So, I’m wondering how you are going to get a probe up to a speed that is capable of reaching another star system in 500 years. I suggest building the vehicle in space where weight is irrelevant. We can have a large supply of nuclear material that will accelerate the vehicle for an extended period of time until it reaches a speed that will accommodate your time parameters.
I like your idea of a “wake up” solar powered probe. Since 99+% of the trip will be in the depths of virtually empty space, there is no need for it to do anything so it can remain inert.
We can certainly make nuclear power supplies that could theoretically last long enough. But we have no idea, practically, if any given technology would last that long, because we’ve never had the opportunity to test anything for tens of thousands of years.
An RTG would be tempting, since it’s a very simple device with no moving parts. Something like that would be our best bet for lasting a very long time. But a half-life long enough presents a problem, because anything with a half-life that long won’t produce very much energy. Maybe you could use an RTG to provide a trickle of power to keep the probe in a standby mode, and then to slowly charge up a capacitor (another no-moving-parts technology) once it nears its destination, and then run the serious instruments off of the capacitor.
Radiation induced degradation of the RTG thermocouples and components is an issue that needs to be addressed, as well as the half life of the fuel. Current RTGs suffer from radiation induced damage. Something that can be ameliorated by design, but it adds another headache and likely reduces efficiency. Making anything that will operate maintenance free for half a millennia is going to be hard, but one that is cheerfully irradiating itself is a whole new difficulty. RTGs are at least free of moving parts, which is good start.
The Plutonium-238 currently used has a half life of about 88 years. 500 years is a long stretch, nearly six half lines or about 60 times reduction in available energy at the end. Going to want a different isotope. Whether there is one, and one we can obtain in useful quantities is another matter.
Worse, 500 years is something that is likely to stretch our understanding of materials. Electronic systems have a terrible habit of not playing well with long periods of time. Stuff migrates, things diffuse into places that are not wanted, or diffuse out from where they are wanted. I would be very surprised if any of our current semiconductor processes could survive these sorts of periods.
That’s not my job, but it seems like it’s a engineering issue, not a possibility issue:
And
You can only spend half the available energy supply for propulsion on acceleration. The other half will be spent on deceleration to slow down the probe as it approaches its destination. At the halfway point, you’ll have to change from acceleration to deceleration.
Not needed for a flyby.
I believe you can use less than half your supply to decelerate, you don’t need to go to a complete stop for starters, and you can use the gravity of you destination star to help.
That assumes that the gravitation of your destination star is stronger than that of your star of departure. Your probe has at least escape velocity - if it had less than that, it could not have reached interstellar space. So if the escape velocity of your destination star is at least the same as that of our Sun, you’ll need to slow down, otherwise the probe will zoom past it.
As long as it zooms at a speed that allows it to fulfill its mission that’s acceptable.
If the system is multi-star you can play some system pinball. And many systems are multi. In a 3 body system I believe an unstable capture is possible, but practically I suspect not at the speeds we are talking about here.
Many have commented about RTG, is there any hope for the solar option? Even if the panels were covered with something that would ablate in the light of the other star upon arrival and activating them?
Gravity is time-symmetric, so any orbital dynamics tricks you can use to slow down at your destination, you can also use to get up to speed at your launch. That doesn’t amount to much, though: Even with an ideal arrangement of orbital objects, the highest interstellar speed you could get (or shed) that way would be around sqrt(3) times the escape speed at the star’s surface. For a launch from the Solar System, that’s still a travel time of over a thousand years to alpha Centauri.
I understand. I just checked and, if one could reach just 1% of the speed of light, one could reach the Alpha Centauri system in 440 years. That’s pretty good and much better than I expected. The bad news is that we have not come remotely close to that speed, which is @ 7,000,000 MPH. Something called the Parker Solar Probe is the fastest at 330,000 MPH. Way short.
I think the ‘cold storage’ probe might have merit. Because if you want to maintain active circuitry you need to keep everything warm, and heat means damage over time.
Perhaps a multi-layer probe. An external RTG powered probe section handles the departure, manoevering, communications, etc. A second, inert probe is carried along. When the RTG runs down, its last act is to separate the inert probe, which then flies through the interstellar medium for hundreds of years.
When it approaches a star system, some detection mechanism triggers thr probe to wake up. Maybe the heat from the star activates a chemical which eats through a link that was holding solar panels shut and they pop open, powering the probe while it is near the star.
Of course, you only need 50-50 fuel to start and stop if the fuel amount is a minimal part of the probe. Otherwise it’s a caculus formula, the fuel to depart needs to accelerate the fuel used to slow down, but the deceleration fuel is working with a much lighter craft. It’s still going to be a lot. I assume the most efficient would be an ionic drive which, again, needs a decent power source.
The standard cheap light power source for science fiction writers is of course, the Mr. Fusion (as seen in Back to the Future or failing that, an antimatter drive.)
Another possibility is intermittent life, the power source charges a capacitor and every so often, it reaches a level where the probe wakes up, checks itself out, maybe phone home a status report, then peters out for lack of power.
The limits on propulsion aren’t just energy but reaction mass needed to affect a change in momentum in order to develop thrust. As an example, to accelerate a spacecraft up to a speed of 0.01c relative to its starting reference frame (sufficient to get a spacecraft from our system to Alpha Centauri in under five centuries) using a theoretical fusion rocket with a specific impulse Isp ~ 20,000 seconds (a reasonable estimate for a high performing fusion ‘torch’ rocket) would require a mass ratio (the final ‘dry’ mass to initial mass containing the propellant) of almost 4.4x106; that is, for every kilogram of payload you would need over four million kilograms of propellant. Even if you assume much higher specific impulse, say, Isp ~ 50,000 seconds, the mass ratio would be about 453. That might not sound too bad except that is just the propellant needed to accelerate the spacecraft up to speed; unless you want it flying through the target system at ~1% of the speed of light (such a craft would fly through our system in about 800 hours, likely never getting close enough to a planet or moon system to make detailed observations) across the diameter of Neptune’s orbit) then you actually need to carry enough propellant to decelerate, which would be the square of that mass ratio, ~205,000. So even notwithstanding the required energy to transit space to even our nearest stellar neighbor, it just isn’t feasible to carry sufficient propellant to do so, nor are you going to be able to do any ‘chaotic capture’ shenanigans at those speeds.
There is another issue that is rarely considered; any high power throughput process is going to generate an enormous amount of waste heat even if it is really efficient; a system capable of generate continuous thrust to get a spacecraft up to 0.01c will produce an egregious amount of heat that has to be rejected, requiring massive outward-facing radiators, which means more weight, which means more propellant. When you start adding up everything needed to send even a small ‘lightweight’ probe across interstellar space in anything like a reasonable time period, your spacecraft stops looking like something out of science fiction and starts looking more like a medium-sized moon.
As for power sources, the problems with RTGs have already been well addressed, and while you can keep a nuclear fission pile tickling over for a long time at minimal rates of consumption (or shut it down almost completely when not needed), just the natural decay processes will make fuel elements unstable over those timespans. The only ‘conventional’ source of power, aside from solar (which is obviously not workable if you are much further away from the Sun than Mars, and even worse for K or M type stars) that would be useful over that timespan would be nuclear fusion. The issues with having both moving mechanical assemblies and electronics (especially in the high energy ionizing radiation environment of interstellar space) remaining functional without maintenance or repair for a duration of many centuries has been touched on, and I’ll just note that even advances in materials and radiation hardening aren’t likely to change that; you would need a ‘self-healing’ system that can maintain itself indefinitely.
We’re going to be ‘exploring’ other star systems via remote observation for the foreseeable future. Which is okay, because our abilities to do this are increasing by leaps and bounds (including infrared, ultraviolent, X-ray, and most recently detection of gravitational waves), and space-based interferometer arrays would improve current capabilities by several orders of magnitude yet. It would make far more sense to invest in such systems which can image and examine many star systems thousands of lightyears distant rather than to pour enormous resources into one ‘Hail Mary’ effort to visit a nearby system that isn’t even particularly interesting in astrophysical terms, nor is likely to harbor environments suitable for the evolution of extraterrestrial biology.
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Practically vs theoretically?
U235 has a half life of 708 million years, so a fission reactor could still have fuel very far into the future.
But of course that requires moving parts (with current technology, at least), and I don’t know how we might design something that would work for that length of time.
“No machine should have any moving parts”: as Arthur Clarke put it.
Then there are the control systems: as others have noted: semiconductor materials do seem to degrade with time. Maybe back to (maybe microscopic) vacuum tubes?
235U does have a very long half-life (although enriched 235U will degrade faster ) but neutron impingement on a solid state moderator such as graphite (which is necessary for obvious reasons) creates Wigner defects that require periodic annealing, and the fuel elements need to be managed and replaced as they are ‘depleted’.
It isn’t just that semiconductors degrade normally, but the very high energy ionizing cosmic radiation would make any microelectronics unreliable over such a long timeframe even with high levels of redundancy.
Thermionic systems would be more robust to even high levels of radiation but would be very limited in computing capability, would consume an enormous amount of power, and vacuum tube computers of any useful size had uptimes measured in hours and were very sensitive to dynamics (shock, vibration). They would be completely unsuitable for this kind of application.
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