The Challenges Of Heat Protection In Space
There is a reason why there is still no confirmed presence of life outside of Earth. Space is a hostile environment inimical to life. Conditions are widely different from those found here on Earth, so scientists and engineers have to remove some preconceived notions applicable only here on the planet when designing aerospace equipment.
One notable characteristic of space is its near-vacuum state, which has surprising complications as to how heat transfer to and from objects. Without special precautions, heat-related problems can easily undermine the integrity of space equipment.
You might remember from high school that there are three primary methods of heat transfer. There is conduction, where heat flows throughout solid objects or bodies of liquid in an attempt to distribute itself equally. Convection can also occur, where heat is carried and transferred by moving liquids and gases. Finally, there is radiation, where objects emit electromagnetic radiation that can travel even through a vacuum.
Space is empty and devoid of any atmosphere. It is not precisely accurate, but the air pressure in space can be less than a millionth of Earth’s atmosphere. It means that heat transfer due to conduction or convection is negligible for spacecraft. This factor is important because, on Earth, these two modes of heat transfer help stabilize temperatures. Hot objects quickly lose heat and cool down, while cold objects warm up within minutes
The only heat transfer available in space is through radiation, so temperatures of objects in space can vary wildly from boiling to freezing in a single day. The properties of its surface determine the amount of absorbed radiation and ultimately the temperature reached by an object in space since radiation interacts mostly with surfaces. Shiny surfaces that reflect light will not be affected much by radiation, while dull, rough, and dark surfaces will experience large temperature swings.
A different problem occurs during reentry. As spacecraft reenters the atmosphere, it compresses the air in front of it while the sides rub against stationary air. These effects generate a lot of heat, and temperatures can reach more than 1000°C. Without any way to dissipate this heat, exposed areas may melt and break apart, endangering the entire craft.
The Detrimental Effects Of Heat
That said, why exactly should we care about temperature swings? Materials change properties depending on their temperature. Most objects contract when cooled and expand when heated, and if a part of an object heats up too fast, there will not be enough time for the heat to distribute itself.
The hot parts will expand more than the rest of the object, which can cause stress to build up and eventually break it. Anyone who has poured boiling water on a thick mug and saw it break apart will understand this, and the same concept applies to spacecraft.
In addition to thermal shock, high temperatures can directly reduce the rigidity and strength of many materials used to create aerospace equipment. Gaskets and other polymer-based materials can become brittle when exposed to heat.
Electronics may fail when overheated, and batteries may malfunction and explode under hot conditions. Finally, excessive heat can endanger any crew on board. Hence, engineers need to do specific measures to protect the spacecraft from heat.
How Thermal Coatings Help
One objective of coatings used for space applications is to limit the buildup of heat either by suppressing heat transfer into the object or enhancing heat transfer out into space. These coating typically form white, reflective surfaces to prevent radiation absorption.
For other areas, dark and heat-conductive materials can be used to help disperse heat outward and to distribute any absorbed heat, preventing thermal shock evenly. Finally, such coatings usually have a base composed of insulating material, preventing heat from reaching the spacecraft interior.
It is evident that space presents unique challenges when it comes to heat. However, using the right coatings and materials can help make this critical job easier.