This reminds me of a SETI idea: looking for infrared signatures that could match the thermal waste output of interstellar ships.
This comes with an interesting model that has far-reaching implications, such as "should we have detected these a long time ago?" Of course the method comes with all the usual Fermi caveats, namely that space is big and the statistical likelihood of the effect happening nearby enough for us to see it.
A "thermal waste signature" is black-body radiation. We have infrared telescope which can pick up red dwarf and brown dwarf stars, even down to a surface temperature of 300K.
The primary difference would be the lack of spectral bands. That would be pretty easy to pick out, and so I don't think there's an advantage to having a SETI search with this as the primary goal.
The limiting factor there is radiative temperature x radiative area. For something the size of a small star, sure, you can pick up 300K.
For something the size of a plausible ship, the surface area is astronomically smaller, while the temperature of most of its surface would likely be kept at or lower than the range which is compatible with macro-scale organisms and biochemical processes. A ship would also be trying to manage its heat, probably using an insulating exterior.
Meanwhile it would be pointing its thruster - the only part meaningfully above 300K - away from you for at least half the trip if directly oncoming, with angular efficiency coefficients if heading somewhere else. Recall that the ship itself is insulated & temperature-regulated, and therefore shields the emissions of the thruster. The thruster would also likely be focused as much as possible, so we could expect thermal radiation at steeper angles to drop off sharply.
Even something like a nuclear pulse rocket would be hard to spot until it was within a very small neighborhood, and then it would be up to whether you're actually observing that part of the sky during the time when it might be visible.
Ships are very hard to detect until they're almost on top of you.
Any engine worth using will produce large amounts of waste heat, which the ship will need to reject somehow. The most plausible option seems like large radiators; efficiency is as the fourth power of temperature, so it is very desirable to run radiators as hot as possible. 600-2000K are fairly plausible values.
Thermal insulation isn't necessary or helpful, except as it separates crew spaces from hot spaces and gives the radiators time to reject heat.
Also, the exhaust plume will be very hot and very bright. Even if the ship doesn't radiate, the exhaust will
> In order for a radiator to do its job, it has to be immersed in some sort of working medium ... You can sort of make a "radiative transfer" radiator, but they don't work quite the same.
You should re-read Pinckney's essay on the assumption that it specifically concerns the latter type of radiator. Consider that "efficiency is as the fourth power of temperature" is a reference to the Stefan–Boltzmann law for black-body radiative heat transfer.
Consider too that the linked-to essay, in the part about radiative cooling and with worked-out details about Stefan-Boltzmann, refers to a working temperature of 2200 K, where the T-to-the-fourth really kicks in.
I'm sorry, I should have been clearer. When I talk about signature, I don't only mean the radiation itself but also position data and other markers that would help isolate the signal from astronomical sources, and - as you said - spectral information.
> That would be pretty easy to pick out, and so I don't think there's an advantage to having a SETI search
So, because you say it's an easy search that's an argument why we shouldn't do it? It's not at all clear that we should have discovered this already by accident, if that's your point. I say it's incredibly hard, and also that it's worthwhile.
You left out the rest of the quote "with this as the primary goal."
Any search which can pick out a small hot interstellar spaceship will be very effective at searching for brown dwarfs and other low temperature objects in space. ("Low" means "not a fusing star".)
I doubt there's anything you can do to optimize a SETI search which makes the identification of brown/red dwarfs more difficult. In fact, since you don't know what the background low-temperature IR sources are, you would need the brown dwarf survey to help characterize what's normal vs. a possible spaceship.
There's already funding for dwarf studies, and it's an area of astronomy which is relatively new and unexplored. (The first brown dwarf was not discovered until 1995, for example.) So we know that's a proposal that many can get behind.
Identification of possible non-stellar objects falls out necessarily and almost trivially (compared to the survey itself) from the data.
Position and velocity data is also important for brown dwarf observations, so you'll get that anyway.
So you can tell me - why should we prioritize a very low probability SETI search when there's a high probability that a brown dwarf survey will be scientifically useful, and end up with useful SETI information?
It does not impose any extra constraint on the design of a telescope, and very little help telling where to point it, or how to analize the data.
Our current telescopes probably aren't good enough to catch a small hot object by now, but there is nothing to gain from explicitly including that as a goal.
Q: Why did you choose fusion based propulsion for Project Icarus?
A: Well, we thought about it and we realized that the only appropriate power source for an imaginary starship was a likewise imaginary one. And while there are certainly many options in the field of imaginary power generation, we rejected some of the main contenders for a variety of reasons.
"Maxwell's Demons" would have alienated both the new-agey spiritual sci-fi crowd and also the fundamentalists because it sounds medieval and just evil at the same time.
"Dilithium crystals" was taken. The zero-point people are bunch of wack jobs and we certainly didn't want to be tarred with that brush. "Fairy wings" just sounds too wussy for our tastes.
So, you see, there aren't all that many choices of power left for an imaginary starship. Fusion sounds just about right, especially if you say is slowly, with gravitas, like "fuuuuuuusion".
Tangent: fusion will require carrying...how much hydrogen? I once calculated that trying to gather it along the way (interstellar hydrogen) is pointless, as a 1m^2 swath from here to Alpha Centauri would net 0.01g of the stuff.
ETA: Yup, I was considering the Ramjet design. Scale of the scoop overwhelmed itself trying to acquire enough to function. I was just startled at how little interstellar hydrogen there is; I knew it wouldn't be much, but 0.01g in ten million cubic kilometers was even less than I'd assumed.
But this isn't meant to dispute your calculations. The linked article comes to the same conclusion: it's not feasible, too little interstellar hydrogen.
Most notional implementations of the Bussard Ramjet depend upon some sort of heretofore undiscovered force field rather than a physical structure to collect hydrogen, because size matters.
Naturally, a very large force field ramjet will deplete interstellar hydrogen along the main routes between stars, leading to a situation known as "peak hydrogen", and possibly prompting the invasion of developing hydrogen-rich planets on some sort of flimsy pretext.
Most scientists working on imaginary spacecraft projects therefore eschew the Bussard, correctly anticipating the moment when, shortly after the leader gives a speech on the deck of a starship in front of a huge "Hydrogen Accomplished" banner, the price of hydrogen rises to over $5 a gallon and starships are all left to rust in the front yard.
"the waste power from the fusion reaction can be 15,000 GW"
That reminds me on a project I worked on where we were getting the operations of the Hunterston B AGR explained to us and I asked an incredibly stupid question - given the 1,500 MW thermal output of each reactor and ~700 MW electrical output, where does the rest go. The answer is, of course, "to heat the Clyde".
15,000 GW is a lot of waste power!
[Edit - the power figures are per reactor - with each AGR station having two].
If there's a lot of it and you're not making it go elsewhere, assume that the system and it its environs are going to get a bit toasty.
Sometimes you also get things like radiation and vibration. These mostly show up in meaningful quantities in predictable places - nuclear systems, mechanical systems, high-power A/C or RF systems, high-power transducers.
You'll usually do okay if you guess that the system uses energy at around ~60% efficiency, which is enough to give you some idea of the heat to expect if you know the input power.
Or more generally, assume all input power goes to heat and plan to handle operation at that limit - while being aware that your expectations of power consumed by the system may well be low.
But they tend to have low efficiency, thermally-dependent efficiency, and the need for a temperature differential. They operate in response to a temperature gradient, not just heat.
Using Wikipedia for a quick-and-dirty figure, you can expect efficiencies to peak around 5-8%. And you can't just stack infinite thermocouples until system efficiency reaches 99.99% because of the thermal gradient issue.
Possible, but using the heat directly for industrial or heating purposes is probably the best bet - it's just awkward cause transporting heat over large distances is significantly more expensive than electricity, which is usually why you want some sort of co-generation.
Ultimately, you don't even need thermocouples, you just need more heat engines with ever decreasing working fluid temperatures, and then it all comes down to your capital costs.
You can even make several layers of that, with thermal machines between the layers. As the number of layers increase, the efficiency of each one reduces, but the total work goes always approaching a constant assintote.
For anyone wanting to play with fictional starships and radiators, I'd like to suggest Kerbal Space Program with the Interstellar Mod. It adds fission, fusion and antimatter reactors with all their drawbacks(), generators (even gives the carnot efficiency), and of course radiators, with the generator efficiency depending on how hot the reactor is vs how cold the radiators are.
() fission produces waste subproducts, have to be shutdown for maintenance and cool down, fusion requires an electrical generator to sustain the fusion (also fuel, have to centrifuge deuterium etc) and of course antimatter have to be collected and stored in magnetic containment devices which require power to maintain. etc.
If you want to build a functioning starship along the lines of Icarus by the year 2100, there is a lot of basic science that has to be done. It is very difficult to predict the course of basic science, because you don't know what you don't know, and it is therefore hard to anticipate how far away the frontiers might be.
The main problem with Icarus is that it aims to create a ship that can do the trip on the scale of a human lifetime. With space travel, the amount of energy required, and therefore the difficulty of the project, is related almost exclusively to how fast you want to get there, the delta-v, if you will.
Therefore, it is unreasonable for Icarus to set a timetable.
If you really want to set a timetable, there is an easier way: stop worrying about how long it takes. Concentrate all of your effort into curing death, and then build a starship with a solar-sail, or some other sort of environmentally-friendly, but slow, technology.
The advantages to this approach are many: easier to get funding, direct economic payoff, no difficulty convincing people that your effort is worthwhile, and, oh yes, you get to cure death.
Then, while you are aboard your starship, you can spend the hundreds of years (say) that it takes you to get to Alpha Centauri further advancing science. If you are adequately equipped, as a nuclear-pulse propulsion vessel would likely be, you can invent better propulsion technology as you go along.
In a sense, the problem of human senescence and its attendant illnesses and the problem of interstellar travel are one and the same. If someone asked me to solve the latter as quickly as possible, I would begin by solving the former.
I'm pretty sure any sort of practical space based radiator system would not use circulating coolant but would probably use heat pipes. No moving parts and for large systems you generally use many small pipes in parallel. Combine these and you get an extremely reliable system that fails gracefully. Which sounds like a good idea for any sort of nuclear reactor safety system.
The Shuttle used a combination of heat pipes and circulating freon (and dumped the waste heat during ascent into the cargo bay, poor cargo!), but also did not have a nuclear reactor on board.
To focus and direct it, you would have to interact with it with real physical things, which gets you right back to square one - now you have this hot thing and you don't want it hot.
Well yeah, but let's say you radiate out as planned, but near the front of the spacecraft you expand a very large reflective dish, like a lightsail, say a km across. Now you're slightly biasing the direction the heat is radiating and the spacecraft will have to move in the opposite direction. Small changes in angle of the dish will move the vector of the reflected radiant heat.
In other words, you could use the main engines to get to Alpha Centauri, then use the radiant heat for the precision maneuvers around the system.
Certainly -- our sun will eventually radiate away all its useful energy and that energy, although not lost, will assume a form that can no longer be exploited. This shows the truth of the expression "entropy increases over time". Reference:
http://static3.wikia.nocookie.net/__cb20100127043621/jamesca...