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

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 spread out.

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 TRANSFER

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.

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 TOGETHER

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 to us.

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 vigorously.

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 equilibrium".

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).

HEDGING THE BET

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 it cool.

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.

THE KELVIN SCALE

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 measurements positive.

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.