[TML] Astronaut prepares to risk life for spacewalk

[Timothy Little]

On Sat, Nov 03, 2007 at 06:01:42PM -0700, shadow at shadowgard.com wrote:
> Radiation gains/loses are *slow* unles the object is very dark
> colored. And not that well polished.

Not terribly slow.  Almost all painted surfaces, even those that are
visually brilliant white, have high infrared emissivity - on the order
of 90%.  That means a heat loss on the order of about 500 W/m^2 for
near "room" temperature.  If the visual reflectance is high, they can
cool to dangerously low temperatures (e.g. -100 C) even in direct
sunlight.

Conversely, most metals and some other substances have moderately high
emissivity in the visible spectrum, but lower in IR.  For example,
mirror-polished stainless steel has a visible absorption of about
40-50%, but infrared emissivity about 5-8%.  In direct sunlight it
would have a net heat gain of about 500 W/m^2 up until the temperature
went to about 250 C.


> Those huge temp differences you are always hearing about are
> *equilibrium* temperatures. And it takes a significant amount of
> time to reach equilibrium.

Yes, on the order of many hours to even days for objects of
significant thickness such as a person.  The thinner or less dense an
object is, the faster it will change temperature.  A sheet of 2mm
stainless steel in sunlight, for example, could approach its
equilibrium 250 C within about an hour.

More complex objects would have internal heat flows making them more
difficult to model, but the same general principles would apply.  For
one common type - an object with thin casing attached at only a few
points to other components inside - the casing may approach a
transitional equilibrium temperature much sooner than the interior.


- Tim