Re: Detection sources

Oerjan Ohlson wrote:
> 
> Thomas.Barclay wrote:
> 
> >PS - I think I have to question Oerjan on his comments on the
> >radioation of heat/IR. Though I agree with the physics he is
> suggesting >(that energy is a wave-particle duality and one can emit
> without having >air to conduct the heat), I can only conclude this
> emission is very >inefficient because the Space Shuttle engineers had
> to put quite a bit of >thought into how to dispose of waste heat.
> 
> Conduction and convection are much more efficient mechanisms for
> transporting heat than radiation is, certainly. However, radiation is
> what's most readily available to spaceships :-/

	Also, you have to consider the relative temperature of the
surroundings. The average temperature of the Earth is 16 degrees, which
means that radiation won't do that much for you. Radiation is also of
course proportional to surface area (which may explain part of the
shuttle's problem. Another part of it may be re-entry - the shuttle was
specifically designed to be well-INSULATED so that heat can't get in
during re-entry, and this might have the effect of making it hard to get
heat OUT when in orbit)

> >So obviously the radiation of IR/heat energy isn't sufficient without
> such >additional methods - at least as it pertains to keeping the
> shuttle a nice, >habitable vehicle.
> 
> The problem here is that the shuttle is out in space for a week at a
> time or so. FT warships tend to stay out for much longer periods - how
> much "bunker mass" would they need just to stay cool? How have the
> various space stations handled the excess heat problem?

	Mostly through heat radiation. The difficulty here is all the
different
plans for suppressing heat radiation. If just keeping your crew from
boiling requires you to dump a LOT of heat, it's much harder to suppress
your emmission.

> >Now, whether this means the radiation is insufficient for detection
> >(since something IS radiated despite the vacuum outside) remains a
> >topic for debate.
> 
> Any IR sensor (sight, goggles, binoculars, whatever) detect IR
> radiation, rather than convection or conduction. (OK, they can see
heat
> being convected away from a warm body - but they see the IR radiation
> being radiated from the warm air or fluid doing the convection... and
> as for heat *conduction*, well - if the sensor has to physically touch
> its target in order to detect it, it probably isn't that much use in
> deep space <g>)

	Definitely. The other (semi)-missing factor here is relative
temperature. On the Earth, there isn't THAT much of a temperature
differential, so you need to have a pretty high differential to detect
anything. In space, though, things are different. The background is MUCH
cooler (a random patch of sky is probably going to be within shouting
distance of absolute zero), and a ship hull just CAN'T be kept at that
temperature very well (or, at least, I wouldn't really want to try to
design a ship where the outer hull is more than 250 degrees centigrade
cooler than the inner hull). Current technology is well able to detect a
few degrees divergence from background (not via earth-based telescopes,
of course), and I see no reason to suppose that future technology will
be any LESS capable.

-Brian Quirt