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Temperatures in Vacuum
Name: Mel W.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 10/24/2004
Question:
Why does temperature vary so much in the absence of air
and how does this affect daily life in space?
>
Replies:
Hi Mel,
First we need to make sure that you understand the difference between heat
and temperature. Heat is a form of energy or the amount of energy being
transferred. Temperature is how fast particles are moving. While most of the
time, if an object has a lot of energy, it also has a high temperature,
there are some cases where an object can have high energy but low
temperature or vice-versa. For example, the flame of a candle will have a
high temperature (the particles at the candle are moving very fast), but it
has very little energy or heat (you can't power machinery with a candle).
Now think of air on Earth. Since air is attracted by gravity to the Earth it
is fair to assume that the ones that are closer to sea-level (closer to the
center of the Earth) must be moving slower than those that are up high (like
on a mountain-top) - after all a gas particle has to be moving really fast
(have a high temperature) in order to fight the pull of gravity. So we can
say that air is moving slower at sea-level then on a mountain-top - which
means that the temperature of the air is higher on a mountain-top then at
sea-level.
But it is colder on a mountain-top.
This means that when we measure temperature (such as with a thermometer) we
are actually measuring the amount of energy (heat) the air particles are
transferring to our thermometer. But since there are fewer gas particles on
a mountain-top (it's harder to breath), even if the air is moving fast (high
temperature) it has little energy (low heat) and so our thermometers will
give a low reading - since little of that temperature is being transferred
as heat to our thermometer.
Greg (Roberto Gregorius)
Mel,
Thanks for your question. There are 3 methods of heat transfer: conduction,
convection, and radiation. Conduction involves direct contact between a
hotter object and a colder object -- so, for example, when you place a
frying pan on the heating element of a stove, the frying pan gets hot
because it is in direct contact with the hotter heating element. In
convection, a "medium" such as air or water is used to transfer heat due to
changes in the medium's density. So if you go into a room with a radiator
or a space heater, the entire room gets warmer because the air in the room
moves around the room (in "convection currents") from the heat source to the
other parts of the room. In radiation, heat moves directly from the heat
source as a form of electromagnetic energy -- infrared -- that can travel
through a vacuum. No convection medium or direct contact between the heat
source and the colder object is required.
In outer space, colder objects can conduct heat from stars (like our sun)
through radiation. Objects receiving heat from the sun without any
protective thermal insulation (like from our earth's atmosphere or a space
suit) can get very hot. Objects in shadows that are blocked from the sun's
radiation will quickly lose heat to the colder surroundings unless they have
some type of thermal insulation or another heat source.
This is why astronauts outside of the earth's atmosphere must have thermal
insulation in their space suits and in their space vehicles. To keep them
cool when they are exposed to the sun's direct rays and to keep them warm
when they are outside of the sun's direct rays.
Todd Clark, Office of Science
U.S. Department of Energy
Mel ~
Let me answer your question by proposing a related question. The question
that you should be asking is not why does temperature vary in space; it is
what does not it vary on Earth (as much). The ground gets heated by
infrared radiation
(IR radiation) that the sun gives off. Although, the term radiation
sounds bad, IR radiation is a good thing. Anyway, the Earth is massive
and gets heated by the sun due to this radiation. Although lots of IR
radiation is deflected by the atmosphere, enough still gets through to
heat the ground and the atmosphere. It turns out the amount of IR
radiation getting through the outer parts of the atmosphere has to do with
the angle the sun is to the Earth-- which causes seasons. The sun is
"high in the sky" in the summer, which allows more IR radiation to get to
the ground (because the sun is more directly over you), which in turn
heats the air, causing warmer weather. (This is also why the equator is
so hot, because the sun is directly over it, generically speaking.) In
the winter, the sun is "low in the sky"-- that is to say, much closer to
the horizon even in mid day. As a direct result, more IR radiation is
deflected away from Earth by Earth's outer atmosphere. Just as a hot cup
of coffee takes time to cool off, so does the Earth. So, at night, the
air and ground cool off slowly. But, as soon as the Sun comes back out,
so too does the IR radiation warming the earth. If it were to turn to
night, and sun never came back out, the temperature would slowly get
colder and colder as you would expect.
Now, with these principles down, I can answer your question. When you are
in space, there is no atmosphere around you. Thus, IR radiation is not
deflected away at all... so sunlight is a lot more intense than on Earth.
Also, when you are in the shade, the Earth's surface is not there to keep
radiating heat like that hot cup of coffee, so with no heat source, you
would be cold. The way this affects daily life in space is that an
astronaut wears a suit to protect himself from the presence and absence of
radiation (deflecting unwanted IR radiation, and insulating from the
intense cold). There is a mask that can be lowered that is a "one-way
mirror". The radiation is reflected away, the same way visible light is
with a mirror, but the astronaut can still see out of the mask. Also,
space stations have very good insulation from IR radiation.
James Wayne Taylor, II
Energy is transferred by three mechanisms: conduction, convection, and
radiation. Conduction occurs by the collision of molecules at the
atomic/molecular level. Convection occurs by the transport of masses of air
(or a liquid) that results from a difference in density (or by physical
mixing), and radiation which is transferred by electromagnetic radiation.
In a vacuum conduction and convection do not occur to any appreciable
extent and radiative transfer is the dominant mechanism. As the temperature
of a body increases, radiative energy is proportional to T^4 power where
'T' is the temperature expressed in Kelvins. So for very hot objects, like
stars, this becomes the major path for the transfer of energy. However, if
there is no object to absorb the radiant energy it continues to travel from
the source at the speed of light.
Vince Calder
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