Thermal systems are crucial components in the operation of spacecraft in micro-gravity. With respect to unmanned spacecraft, for example, satellites must maintain operable temperatures for their equipment to function and must execute station keeping techniques through the use of ‘burns’ (course corrections in which the main engine is lit).
Figure 1: A satellite in the midst of course correction 
Manned spacecraft have other concerns such as maintaining adequate pressure levels at a comfortable temperature for their crew (it gets very hot and very cold in space, there are no moderate temperatures), and must provide the essentials for sustaining life (i.e. water, oxygen, etc.) As expected, thermal systems in micro-gravity are not the same as thermal systems on the Earth; the common systems and configurations which are standard for many power, heating, and cooling cycles are not applicable in zero gravity conditions. Upon comparison to terrestrial conditions, several important differences affect the design and configurations of thermal systems in outer space. A lack of gravity creates a system governed by a set of laws that differs considerably, consequentially causing many Earthly thermal systems to be inoperable in outer space.
Thermal Systems in Space Suits
A type of thermal system ecountered in manned spaceflight is the astronaut’s space suit itself. This portable thermal system pressurizes the astronauts during liftoff and Earth Orbit Insertion. The suit regulates temeperature (giving the astronaut extra heat or cooling as required) and provides oxygen as well. Astronauts are usually not wearing their pressurized suits except during Exta-Vehicular Activity (EVA). During an EVA an astronaut’s suit is critical in protecting the explorer from the extreme temperatures and vacuum conditions of outer space. These same concepts, but modified for the appropriate applications will be required when voyaging back to the Moon and to Mars. Creating compact and efficient thermal systems to accompany the space/Moon/Mars suits will improve the quality of exploration for the astronaut as well as the length of each outing.
Figure 2: Astronauts establishing a moon base with the aid of their space suits (a walking thermal system) 
Modes of Heat Transfer
Firstly, there is no convection in space, which is the main form of heat transfer for thermal systems in Earth’s gravity . Natural convection is negligible in micro-gravity because there is no such thing as ‘heavier’ or ‘lighter’ (a concept created by the gravitational force field). Without the concept of gravity there can be no natural convection currents (responsible for flame shape and propagation on Earth). Natural convection currents are also what cause hot air to rise. Therefore, boiling a working fluid in a thermal cycle is quite different than boiling a working fluid in the same cycle on Earth. The fluid does not rise because of the lack of buoyancy force (a direct result of losing the gravitational field). Note: the buoyancy force derived in a terrestrial environment equals:(1)
Clearly, as the gravity term approaches zero, this force becomes negligible compared to the other forces present in the system. Therefore, buoyancy driven flows must be stricken from design options in zero gravity thermal systems design.
Figure 3: The right panel shows bubbles forming but NOT rising in micro-gravity while the left panel shows the bubbles rising because of the gravity force .
Without the force of gravity to shape and drive fluids, less effective physical forces (which are negligible in Earth fluids) take over in fluid response. These forces, such as the surface tension force, are usually difficult to study in regular gravity. The surface tension force is the dominant force in micro-gravity and it plays a key role in shaping a fluid (as well as creating more issues for thermal systems designers). For example, another concern in micro-gravity is that water cannot be transported using its gravitational potential energy as the flow driver. Instead, all fluids must be pumped in order to transport them (resulting in a larger power requirement for thermal systems in outer space).
The pumping of fluids introduces a variety of issues involving two phase flows in micro-gravity. Two phase flow refers to a flow that is a mixture of liquid and gas. For pumps in micro-gravity to function, they must introduce some gas (helium in some cases) to the pump since there is no air in order to displace the water. This flow type can be seen in spacecraft cooling systems, spacecraft life support systems, and spacecraft fuel systems. One issue with multiphase flow is that the bubbles of the gas phase can begin to collect along the walls of the systems piping. Moreover, because of the relative strength of the surface tension force, these wall bound bubbles can grow to the point where they totally occlude the liquid phases’ fluid flow . Several solutions have been proposed and are currently being researched. For example, it has been posited that using fuel lines with constant bend radii (creating a coil-like shape) will reduce the tendency of the gas phase to collect to the pipe walls . Solutions to this problem may lead to more efficient and reliable thermal systems in spacecraft in the future. Currently, University of Pittsburgh undergraduate students have been selected to conduct research in conjunction with NASA, in June 2008, to delve deeper into bubble topology in tubes with constant bend radii in a micro-gravity environment.
The need for piping to be air-tight is magnified in a micro-gravity system. To illustrate this concern, one can imagine a fluid dripping from a leak in a gravitationally impacted thermal system. The fluid would collect directly underneath the breach, and although a large amount of seepage could have detrimental effects, a small outflow should have no major impact. However, in a system that is devoid of gravity, working fluid that escapes from the confines of the system is not confined to remain in a certain location. Liquids, which often have negative effects on electrical systems, can be harmful in even small quantities in outer space, and it is also possible for harmful substances to poison the environment of living beings more easily.
Another concern that is inflated in the confines of space is that of insulation. As previously stated, the temperature minimum in space is roughly 173 K in the shade and 393 K in the sunlight . This gives an overall temperature difference of 220 K. When compared to a common Earthly temperature difference of 85 K, one can clearly see that insulation must deal with an additional order of magnitude when attempting to stabilize temperatures in outer space. This is especially important since radiation is proportional to the absolute temperature to the fourth power of a given object.
Main Differences Between Earthly and Micro-gravitational Thermal Systems
Thermal systems are a conglomerate of many different physical attributes which span much farther than simply fluid physics. The lack of natural convection (mentioned earlier), for example suggests that the majority of effective heat transfer in low gravity must be achieved through forced convection or radiation. This greatly reduces the efficiency of several thermal systems components with respect to their performance in regular gravity.
The same phenomenon causes flames (which can be modeled as a fluid) to react differently as well because a fire’s main mode of transportation is through natural convection. Thermal systems observe several barriers in micro-gravity that are not present in Earth’s gravity, and it will be necessary to develop and evolve more efficient solutions to maintain the future of space exploration a reality.
Figure 4: The pipe on the left shows water flowing through an Earthly system, whereas the picture on the right is one in micro-gravity 
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