WHAT IS HEAT?
It seems an odd question, but a glimpse of the scientific
understanding of heat is a helpful background to discussing operating
in space and on the lunar surface. The average person's intuitive
understanding of heat may not apply very well. What follows is a
simplified discussion of heat and heat transfer.
Heat, put simply, is the vibration of molecules in a substance.
Even in solid objects the molecules that make them up move around.
The hotter an object is, the more the molecules jump and jive. When
they are very excited, they will even break the solid structure and
the substance then undergoes a phase shift from solid to liquid.
Similarly when the molecular motion is too vigorous for the liquid
phase, the substance enters the gaseous phase.
It's possible for different areas of an object to have different
heat levels. The difference between the hot part of an object and the
cold part is called its "thermal gradient". When a molecule vibrates
it passes along a little of its exuberance to neighboring molecules.
They too begin to vibrate but the original molecule now vibrates a
little less because some of its excitement has been taken by its
neighbors. This is how heat spreads through a substance.
The ability of an object to move heat from one part of itself to
another is called "thermal conductivity". It depends on the substance
the object is made of. Certain substances like metals pass heat very
readily. That means when a metal molecule (atom) vibrates, its
neighbors quickly begin to vibrate too. If a substance doesn't pass
heat well, it can be used as thermal insulation. The surface atoms
(or molecules) vibrate, but nearby atoms aren't as apt to start.
Transferring heat from one object to another is as simple as
passing the molecular vibration from one object to another. As you
can imagine, the most basic method is "conductive heat transfer".
Simply place the two objects in contact with each other, and the
molecular vibrations from one object will case the molecules in the
other object to begin vibrating.
The thermal conductivity of the objects involved plays a big part
in how much heat is transferred. As a general rule, solids have the
highest heat conductivity. Liquids have less conductivity. Why?
Because in most liquids the molecules are farther apart than in
solids. Since the molecules are more spread out, vibration in one of
them isn't as likely to spread to nearby molecules. Gasses have the
poorest thermal conductivity because their molecules are even more
When the transfer medium is a fluid (i.e., a liquid or a gas) you
have a slightly different form called convective heat transfer. This
is the notion of a "coolant" that "carries away" heat. Convective
heat transfer is what cools your car engine by circulating water
through the hot parts and then through the radiator where it is
transferred to the air.
The air around us plays a big part in our everyday encounters with
conductive heat transfer. The science of meteorology is largely based
on the heat transfer properties of earth's atmosphere. The
temperatures reported daily are the temperatures of the air at various
places around the earth. The earth's atmosphere is the primary
conductor of heat in our daily experiences.
A SNEAKIER FORM OF
Conductive heat transfer is pretty easy to understand. But
there's another important phenomenon. Excited molecules release
electromagnetic radiation (e.g., visible light, infrared light,
x-rays, microwaves, or radio waves). This release of energy slows
their vibration and helps them shed heat.
Conversely, when a molecule absorbs electromagnetic radiation, it
becomes more excited and vibrates faster. It's easy to see that by
using this mechanism objects can transfer heat between each other
without even touching. This mechanism is called "radiative heat
transfer". Objects transfer heat between each other through
Electromagnetic radiation includes visible light. We often see
hot objects giving off electromagnetic radiation in the visible
spectrum. The wavelength of light emitted depends on the substance
and how vigorously it is heated. Most hot objects will emit light in
the infrared spectrum. This is why infrared sensors are used in
security applications to detect the presence of warm human bodies
where they aren't necessarily supposed to be.
PUTTING IT ALL
These two forms of heat transfer account for just about everything
we observe relating to heat.
The sun warms the earth through radiative heat transfer. Vast
amounts of electromagnetic radiation all across the spectrum travels
from the sun and hits the earth. The various substances on earth
(dirt, rocks, water, concrete, sand, etc.) absorb this energy and
their heat level is raised. They transmit that heat through
conductive heat transfer to the surrounding atmosphere, and eventually
The daily temperature is reported as air temperature. On a
pleasant summer day, the air temperature may be 80 F (21 C). But the
various solid surface substances on earth may have been quite a bit
hotter that day. Have you ever walked barefoot on dark asphalt on a
hot day? It usually feels very, very warm to your feet. Since your
body temperature is about 99 F (37 C), you know that pavement must be
considerably hotter than that, perhaps 150 F (52 C). This difference
in surface temperature versus air temperature is very important to
discussing the lunar environment.
Place your hand near a hot object such as a pan on the stove. You
can feel the heat from it, even though you aren't physically touching
it. The air between your hand and the pan is conducting the heat
between the air and the pan. The farther you move your hand away, the
less heat can be transmitted that distance through the air.
If you've ever stood on a stage under full lighting, you realize
how hot that can get. That's radiative heat transfer -- the same as
from the sun. The very hot coils inside the light bulb send out lots
of electromagnetic radiation which hits your skin. Absorbing this
radiation heats your skin up, and you feel it as heat.
Microwave ovens are a special case of this phenomenon. Microwave
radiation is part of the electromagnetic spectrum. It happens to be a
wavelength that causes water molecules to vibrate especially
Now in space there's no air. That means conductive heat transfer
doesn't occur between objects that are not physically touching. Only
radiative heat transfer can occur. This is important for two
reasons. First, you can be very, very close to something that's very
hot, and you won't feel a lot of heat. (Radiative heat transfer
typically moves less heat than conductive heat transfer.)
Second, objects take longer to cool off. This is because
conductive heat transfer to the atmosphere is the primary means for
keeping things cool on earth. Objects in a vacuum can only get rid of
heat through radiative heat transfer, and since that moves less heat
it isn't as good.
FINDING A HAPPY
MEDIUM: THE STEADY STATE
So we have two means by which objects can acquire heat and pass it
on to other objects. In practice, any given object is both receiving
heat and passing it on. If it acquires heat faster than it passes it
on, it heats up. If it passes it along faster than it receives it, it
cools down. An object at a constant temperature is receiving heat
just as fast as it is getting rid of it. This is called "thermal
An object at equilibrium can still have a thermal gradient. Shine
a bright light on an object. The side facing the light will be heated
by radiative heat transfer. The shaded side will still be cooler.
But as long as the temperature at each point in the object remains the
same over time, the object is said to be at equilibrium.
A more complicated version of this example would be a concrete
highway on a still day. The sun warms the pavement to perhaps 150 F
(52 C). It would be hotter, but some of the heat is drawn away by the
air on top of it. The air may be cooler because it's less dense than
the pavement -- say only 80 F (21 C). But very close to the pavement
it's significantly hotter. As long as the wind doesn't stir things
up this system will be at equilibrium even though we can observe
several different temperatures at different places in the system.
In space our ability to get rid of heat is limited. Since an
object can only use radiative heat transfer and not conductive heat
transfer, it will absorb heat faster than it can radiate it. That
means equilibrium temperatures will be significantly higher for
objects in a vacuum. The same concrete highway in a vacuum may be
heated to 250 F (121 C).
Intuitively we know that things in the shade don't heat up as
much. Without the radiative heat transfer from the sun, objects can
only receive heat through conductive heat transfer. Since the vacuum
of space limits how we can get rid of heat, the best way to keep cool
in space is not to be heated in the first place. Fortunately the
vacuum of space also limits how we can receive heat, so by reducing or
eliminating radiative heat transfer to an object, we can keep
Intuition tells us that wearing a black shirt on a sunny summer
day is unwise. White colors prevail in summer because they reflect
away the electromagnetic radiation that heats us up. Similar
principles apply in space. Painting something flat black would cause
it to absorb sunlight and heat up. Covering it with reflective
material has the opposite effect of reducing the absorption and
keeping it from heating up.
We use the Fahrenheit and Celsius temperature scales for everyday
temperatures. But since they have both positive and negative values,
it makes them hard to use for scientific equations. And so when we
discuss heat transfer we use a special temperature scale called
Kelvins. The Kelvin temperature of an object is simply the number 273
added to its Celsius temperature. This makes all the temperature
Why 273? Because scientists have shown that at -273 C, all
molecular vibration ceases. That is, there is no heat present
in a substance at that temperature. Nothing can be colder than the
complete absence of molecular vibration, so -273 C (or 0 Kelvin) is
called "absolute zero" -- the coldest an object can possibly be. If
that represents zero on our temperature scale, then no pesky negative
numbers will clutter up our calculations.