On Fri, 3 Dec 1999, Doug Jones wrote:
> EvMick@aol.com wrote:
> > And does vacumn have a tempeture? Is the "fridgid depths of space'
> > real or a bogus holiwood meme?
> Partially true- in the absence of sunlight, an object radiates into the 3
> Kelvin universe. Near Earth, though, 1300 watts/m2 of sunlight is *real*
I'll extend this a little bit. You have 3 ways to cool something -- conduction, convection and radiation. In a vacumn, as in the case of a man in space, you have conduction through the suit leading to radiation into space. You have to balance this with the radiation he is receiving from space that may be absorbed by the suit. As Doug points out, white suits significantly reduce the radiation that gets absorbed.
Now, in the vicinity of our Solar System, even when you are out of the sun, you don't quite get down to the cosmic microwave background temperature. This is due to the infrared radiation being reflected back from the dust that remains from the original nebula from which the planets formed and the comet detritus. My impression is that this makes infrared observations in the planetary plane problematic. The lowest temperature you can radiate into would probably be the temperature of the dark side of the moon or Mercury. If Amara reads this she might be able to tell us the effective temperature when you are facing the planetary plane (say on most planets equators) vs. facing away from from the plane (say on the poles of most planets).
> > What is the tempeture of a vacumn? how come it keeps my coffee hot
> > in the thermos?
> A thermos has low thermal emissivity walls, so the radiative heat transfer
> is slow.
Its a little more complex. The inside of a thermos is reflective (that's why they look like a mirror on the inside), so radiation from the liquid gets reflected back into the interior. Then there is a vacuum between the internal and external thermos walls so that there is no heat conduction. The two ways you lose heat from a thermos is some radiation through the vacuum from the heated inner wall (from the energy that doesn't get reflected back inside) and a small amount of conduction through the those portions of the thermos where the inner and outer walls are joined together.
> > Would sweat in a vacumn cool you?
> Yes, this is a strength of the skinsuit concept, particularly for Mars.
True, but its questionable whether you can consider the Mars atmosphere a vacuum. Scientists talk about the "atmosphere" of Mercury and its pressure is much lower than Mars.
> > EvMick <------thinks cooling might be more of a problem in space than
> > heating..
Yep, spacecraft designers, and especially telescope designers have to put much more attention on keeping things cool than keeping them warm. For infrared telescopes they are doing designs that place a large separation between the telescope optics and the computers and communications because of all the heat they generate.