[meteorite-list] MARS off topic==>on topic

From: MexicoDoug <MexicoDoug_at_meteoritecentral.com>
Date: Mon, 16 Apr 2007 14:45:34 -0500
Message-ID: <01a501c7805f$ceef82c0$36cf5ec8_at_0019110394>

Hi Jim, List,

Temperature in space and on other ET bodies has got to be the most
misunderstood subject on Earth if comments on web pages are any indication!

The radiative heating process has one somewhat unfamiliar effect in space.
Two objects right next to each other will reach different temperatures if
they are not in significant physical contact. In Space at Mars' distance
from the Sun, for example, a small Ordinary Chondritic Meteoroid will be at
about -62?C traveling next to a small Iron Meteoroid that will be a
comfortable 21?C.

It is quite reasonable to believe that the batteries overheated while other
vital components of the spacecraft froze (some even in the Sun!), when you
consider the above temperatures and materials. The temperatures I've given
you are based on assumptions that the object isn't to big and not rotating
in and out of Sunlight (being blocked, such as the case on a spacecraft
mistakenly orienting the battery at the Sun). If it rotates out of Sunlight
it will start cooling. But it won't feel cold since there is no conduction
or convection - it will just gradually decrease in temperature. Engineers
have made use of this fact since the early days of the space program to
regulate temperature of spacecrafts. The material they use to paint them
determines how much heat they absorb and emit, and the time they are rotated
into the Sunlight/shade is made into a thermostat control.

How is this? On Earth we live on a heat reservior - heat sink insulated by
the atmosphere where everything is locally mutually connected by the
mixed-gases soup (short circuiting heat flow everywhere). So our experience
is that the Sun has a little to moderate effect in warming things up, and
when we talk about the temperature, it is one number, oh say a nice day of
23?C or a refreshing night of 16?C.

But still - the temperature in the shade is less than the temperature in the
Sun - despite all of the wind and short circuiting.

In space, unless objects are in direct contact, there is no
convective/conductive heat exchange (Heat flowing from a greater to a lesser
temperature). Basically everything is inside its own thermos bottle.

The only way mass in space is warmed is by 'radiative' heating. For masses
not in physical contact in space (not even through gas), heating works like
this:
1) The Sun emits the whole spectrum of radiation. 2) The object is capable
of only absorbing a certain fraction of this energy depending upon which
wavelength it is, 3) The absorbed radiation is converted into increased
molecular motion which raises the temperature analogous to frictional
heating, 4) The object emits a certain fraction of the radiation it has
depending on which wavelength it is.

In our space there are only four general sources of heat: Direct Sunlight,
Reflected Sunlight, excess geothermal radiation from Jupiter, Saturn, and
Neptune (probably they are still in the throngs of accretion: gravitational
compression causes friction, too), re-emitted radiation from objects warmed
by all of the above.

That explains the situation of the spacecraft, next, your doubts about Mars:
Recall that the Martian atmosphere is about 1% of Earth's. Thus convection
and conduction are much lower and radiative heating has a much more
important contribution relatively, than it does on Earth. This is why snow
can melt in the Sun even when the ambient air temperature averages below
freezing. The ice absorbs enough radiation which is reradiated more slowly,
or not reflected, and the temperature of the ice rises over the melting
point. Just like when youn go into the mountains and notice the Sun-facing
slopes have no snow, but the shaded and northern slopes have two feet of
snow on them.

The ability of an object to have a higher temperature than its surroundings,
then, only requires it to be somewhat insulated from convective and
conductive heating, and have a higher absorption/emission ratio than objects
nearby. Of course, this is all due to radiative heating, so if it isn't in
the Sunlight the ambient heat reservior and its insulation against it
determine how quickly it will cool down. In the case of Mars, -47?C is the
temperature a perfect absorber/emitter would have at 1.52 AU where Mars
orbits if it were in the Sun. Mars' average surface temperature of -63?C.
Yet after reading all of this you can now see why a man in a space suit in
Mars' shadow needs no climate control in his suit as long as he is alive
(body generates heat). But, if he is in a massive spacecraft with an
artificial atmosphere and no heating system his body probably can't keep up
with the radiative emission of the entire outside surface of the spacecraft
that would be in conductive/convective contact with him. And if he is in
the Sun, even at Mars' distance, he could get quite warm inside if the
spacecraft is painted with high absorption low emission coatings.
This is a hot design topic for spacecraft engineers as it is part of the
heat exchanging systems to maintain desired temperatures.
More information is available at this link:
http://www.diogenite.com/met-temp.html

So, 'they' are probably telling us the truth about Mars...
Best Wishes and Great Health,
Doug



----- Original Message -----
From: jwb7772 at netzero.net
To: meteorite-list at meteoritecentral.com
Sent: Friday, April 13, 2007 4:40 PM
Subject: [meteorite-list] MARS off topic


They are saying that Mars is so HOT that direct sunlight can cause batteries
to over heat. Something is not right here! They are not telling us the
truth about Mars. They also say that the temp is quite cold, even in the
summer it will not get to 32* at the equator, so how come then that there is
melted ice even at the poles. Jim sent 4:40 PM 4-13-07



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Received on Mon 16 Apr 2007 03:45:34 PM PDT