When it is hot, we take off our coats and open windows. When it is cold, we close windows and put on our coats. In this way, we produce the environment at a comfortable temperature. For spacecraft such as satellites and explorers, there are hot and cold environments as well. When a spacecraft is under strong sunlight, the temperature goes up. On the other hand, in the shadow of the Sun, the spacecrafts temperature drops rapidly. Under this harsh thermal environment, how does a spacecraft produce comfortable temperature environment by itself?
This article discusses the pastEand futureEof spacecraft thermal control.
Current status of spacecraft thermal control and stricter demands in the future Spacecraft is exposed to many heat environments including external heat energy sources such as sunlight, albedo, and infrared radiation from planets. Meanwhile, the internal heat source includes heat generated by onboard instruments. The spacecrafts temperature is determined by the energy balance between heat output (radiation) and heat input (sum of heat coming from outside and heat internally-generated). Movement of heat is hard to occur in the space environment where no atmosphere exists. Accordingly, the problem of how to transfer heat efficiently from onboard equipment and expel it into outer space, i.e. heat transportEand heat radiationEtechnology, becomes very important.
There are two methods of heat transport. One simple way is to prompt heat transfer with highly heat-conductive materials. When even more heat transport is required, a heat transport element called the heat pipeEis used. To release the heat into outer space, heat-release planes called radiatorsEare employed. We can see that all spacecraft, including explorers, Space Shuttles, and the International Space Station (ISS), is equipped with radiators to expel heat into outer space (Fig. 1). The size of the radiators is determined so that the spacecrafts temperature would stay optimal in the hottest environment. With this design, however, in a low temperature environment, excessive heat radiation occurs, making the spacecraft too cold. In this situation, onboard heaters warm the onboard instruments to maintain their temperature. In conclusion, the method is widely adopted that spacecraft heat is continuously released by radiators and excessively-expelled heat is filled up by heaters. This method is indeed simple but it is inefficient from the view point of energy use. In future space missions, it will be difficult for us to perform thermal design using only this method.
For example, inner-planet explorers are bound for planets where sunlight is several times stronger than that on earth. While the explorer orbits the earth before setting off to a hot target planet, warming by heaters is essential because its thermal design was built to meet the environment around the hot planet. In some missions, most of the power generated onboard is consumed by the heaters to maintain the explorers temperature. Thus, a problem not to allocate enough power for the mission comes up. Speaking of lunar lander, it is exposed to a temperature cycle of about 30 days when the temperature peaks about 120 deg. C in daytime and falls to -180 deg. C at night. If thermal design is set to suit daytime, heat retention during the night would become too difficult. The issue of how to endure a night extending almost two weeks is a critical technical one.
Space missions have evolved over time. As discussed above, requirements for thermal control come stricter with the advances. The traditional thermal design using heaters to make up for thermal loss is no longer useful for future missions. What technology will be required in the future?