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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

by Udo 13y ago

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.

dalke 13y ago
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.
- Udo 13y ago
  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.
  - marcosdumay 13y ago
    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.
  - dalke 13y ago
    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?
- glimcat 13y ago
  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.
  - Pinckney 13y ago
    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
    - deleted 13y ago
      [deleted]
      - dalke 13y ago
        > 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.

by Udo 13y ago

dalke 13y ago

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.

Udo 13y ago
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.
- marcosdumay 13y ago
  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.
- dalke 13y ago
  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?
glimcat 13y ago
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.
- Pinckney 13y ago
  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
  - deleted 13y ago
    [deleted]
    - dalke 13y ago
      > 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.