# Implementation of a Cryogenic Facility for Space Debris Analysis
## 1 Introduction  
Satellites are exposed to continuous temperature changes, e.g., due to changing sun irradiation during orbits. For materials to be suitable for space applications, their performance under thermal cycling in vacuum must be validated. Thermal cycling tests are usually employed to assess a material's endurance to the space environment for a specified mission duration. However, when a dismissed spacecraft continues in orbit, its external materials continue to be exposed to the space environment beyond the mission duration and are degraded to the point of generating debris [1]. In the case of low-Earth-orbit (LEO) satellites, the existence of space debris is considered a serious problem for operational space missions [2]. A thermal cycling setup consists of a heating/cooling source, a sample/item holder, and a thermal switch in a vacuum chamber. The main options to achieve the desired functionality are described below:  
* Heating/cooling source options  
To achieve the high temperature needed, an easy solution is the use of heating resistors. They are available for vacuum applications, and the output power can be adjusted, as needed. To achieve the low-temperature requirement, the most common and widely used material in cryogenic applications for temperatures from \(-100\)\({}^{\circ}\)C to \(-200\)\({}^{\circ}\)C is liquidnitrogen, with a reservoir of liquid nitrogen inside the chamber. This can generate a cold source at the nitrogen vaporization temperature, 78 K. When envisaging applications where temperatures are maintained constant, this approach works very well; however, to generate thermal cycles with a large temperature interval, like the one described in the requirements, the nitrogen consumption becomes too high, and the price of the test increases, while requiring a large-enough Dewar to keep the liquid nitrogen storage for many cycles. Another solution could make use of the Peltier effect (thermoelectric cooling). For low temperatures, commercial Peltier cells are not available, and the efficiency is well below 10% [3]. Yet another solution could be to use the vapor-compression refrigeration cycle comprising four steps: compression, condensation, throttling, and evaporation. The drawback of this approach is that for low temperatures, below \(-60\)\({}^{\circ}\)C, fluids based on ammonia are not suitable and an alternative fluid is not available [4]. Helium-based compressor/expansion cryocooler systems are normally used to achieve lower temperatures. Most of these systems are, however, mainly developed to cool down small material samples to enable physical studies, and our application requires a rather powerful helium compressor and cold-finger system. * Thermal switch options Thermal switching between heating and cooling stages also constitutes a challenge. One possible technology is to use thermal switches where a small, but large-area, material gap achieves variable thermal conduction by varying the pressure of a thermal conducting gas. The pressure is usually varied inside the cryostat by including an adsorption pump in the gas reservoir. The heat flow in these systems is, however, highly limited [5]. A mechanical switch between hot and cold masses to be implemented inside the cryostat system typically requires the use of cryogenic motors or, at least, cryogenic mechanisms or magnetic coils to exert appreciable contact forces between the temperature-coupling parts. The option used in the present work involves an external mechanical system that directly exerts mechanical contact forces through a mechanical connection inside a vacuum/air mechanical interface. Thermal isolation and careful vacuum wall design were a design challenge. * Spacecraft materials One of the goals of external spacecraft materials is to support spacecraft thermal control. This is achieved through the optimization of the reflection or absorption of electromagnetic radiation from the sun for surfaces at different temperatures.