Most of our planet fits that description. As long as you stay away from the South Pole and don't fall into a volcano, Earth is a pretty comfortable world. But now that humans are venturing into space -- not as visitors, but as homesteaders -- finding the right balance is more of a challenge.

Consider, for example, the International Space Station (ISS).

Without thermal controls, the temperature of the orbiting Space Station's Sun-facing side would soar to 250 degrees F (121 C), while thermometers on the dark side would plunge to minus 250 degrees F (-157 C). There might be a comfortable spot somewhere in the middle of the Station, but searching for it wouldn't be much fun!

Fortunately for the crew and all the Station's hardware, the ISS is designed and built with thermal balance in mind -- and it is equipped with a thermal control system that keeps the astronauts in their orbiting home cool and comfortable.

The first design consideration for thermal control is insulation -- to keep heat in for warmth and to keep it out for cooling.

Here on Earth, environmental heat is transferred in the air primarily by conduction (collisions between individual air molecules) and convection (the circulation or bulk motion of air).

"This is why you can insulate your house basically using the air trapped inside your insulation," said Andrew Hong, an engineer and thermal control specialist at NASA's Johnson Space Center. "Air is a poor conductor of heat, and the fibers of home insulation that hold the air still minimize convection."

"In space there is no air for conduction or convection," he added. Space is a radiation-dominated environment. Objects heat up by absorbing sunlight and they cool off by emitting infrared energy, a form of radiation which is invisible to the human eye.

As a result, insulation for the International Space Station doesn't look like the fluffy mat of pink fibers you often find in Earth homes. The Station's insulation is instead a highly-reflective blanket called Multi-Layer Insulation (or MLI) made of Mylar and dacron.

"The Mylar is aluminized so that solar thermal radiation can't get through it," explains Hong. Here on Earth, we use blankets containing aluminized Mylar to wrap people who have been exposed to cold or trauma. Such blankets are especially popular among hunters and campers!

"Layers of dacron fabric keep the Mylar sheets separated, which prevents heat from being conducted between layers," he continued. "This ensures radiation will be the most dominant heat transfer method through the blanket."

Except for its windows, most of the ISS is covered with the radiation-stopping MLI.

"Windows are a tremendous heat leak," said Hong, "but astronauts need them for ergonomics and also for their research. It's something we have to design around."

MLI insulation does a double-duty job: keeping solar radiation out, and keeping the bitter cold of space from penetrating the Station's metal skin.

It does its work so well that the ISS presents another thermal challenge for engineers -- dealing with internal temperatures that are always on the rise inside this super-insulated orbiting laboratory fully stocked with many kinds of heat-producing instruments.

Imagine that "your house was really, really well insulated and you closed it up and shut off the air-conditioning," said Gene Ungar, a thermal fluid analysis specialist at NASA's Johnson Space Center. "Almost every watt of power that came through the electric wires would end up as heat."

This is just what happens on the Space Station. Energy from the solar arrays flows into the ISS to run avionics, electronics ... all of the Station's many systems. They all produce heat, and something has to be done to get rid of the excess.

The basic answer is to install heat exchangers. Designers created the Active Thermal Control System, or ATCS for short, to take the heat out of the spacecraft.

Waste heat is removed in two ways, through cold plates and heat exchangers, both of which are cooled by a circulating water loop. Air and water heat exchangers cool and dehumidify the spacecraft's internal atmosphere. High heat generators are attached to custom-built cold plates. Cold water -- circulated by a 17,000-rpm impeller the size of a quarter -- courses through these heat-exchanging devices to cool the equipment.

"The excess heat is removed by this very efficient liquid heat-exchange system," said Ungar. "Then we send the energy to radiators to reject that heat into space."

But water circulated in pipes outside the space station would quickly freeze. To make this fluid-based system work, waste heat is exchanged a second time to another loop containing ammonia in place of water. Ammonia freezes at -107 degrees F (-77 C) at standard atmospheric pressure. The heated ammonia circulates through huge radiators located on the exterior of the Space Station, releasing the heat as infrared radiation and cooling as it flows.

The Station's outstretched radiators are made of honeycomb aluminum panels. There are 14 panels, each measuring 6 by 10 feet (1.8 by 3 meters), for a total of 1680 square feet (156 square meters) of ammonia-tubing-filled heat exchange area.

Compare that majestic radiator with the 3-square-foot grid of coils found in typical home air conditioners and you can begin to appreciate the scope and challenge of doing "routine" things in space.

Finally, thermal control engineers must address air flow within the Space Station. The movement of air is a major factor in achieving the balance between hot and cold.

The ATCS works in tandem with the Environmental Control and Life Support System (ECLSS) that controls air quality and flow in the ISS. In orbital free-fall conditions -- equivalent to zero-G -- hot and cold air don't rise and fall as they do on Earth.

Proper air circulation helps prevent unwanted cold spots that could produce condensation, electrical shocks, serious corrosion and even biological problems such as microbial growth. Corrosive fungi were a nagging problem on Russia's Mir space station, and ISS mission planners want to avoid a repeat infestation.

It is indeed a strange new world on the ISS. Hot air that doesn't rise ... heat that doesn't conduct ... radiators too cold for liquid water ... it's enough to give a thermal engineer gray hairs! But thanks to the Station's efficient integrated thermal control systems, the crew needn't worry -- staying cool on the ISS is no problem!

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