Following on from the vibration testing of the last week, the vibrations team has been swapped for the thermal team. The last thing that the vibration team did before leaving was to put PROVE Pathfinder into an oven for a bakeout – taken to a low vacuum and held at a steady temperature for a duration of time to accelerate the outgassing process before the testing begins!
Outgassing occurs in two forms:
- Diffusion
- Desorption
Diffusion outgassing is the process by which gases trapped within materials, such as spacecraft components or materials used in spacecraft construction, slowly diffuse or migrate to the surface of the material and are released into space. This process occurs over time as gas molecules move through the material’s structure and eventually escape from the material’s surface into the vacuum of space. Diffusion outgassing is typically a slow process that occurs at relatively low temperatures. Water vapour is one of the most common gases released through diffusion outgassing but other volatile compounds such as organic molecules, volatile solvents, and other gases used in spacecraft materials or processes can also be released.
Desorption outgassing is the process by which gases are released from the surface of a material due to thermal energy or heating. When a material is exposed to elevated temperatures, such as during operation or manufacturing processes such as bakeout, gases absorbed or adsorbed on the material’s surface can be released through desorption. Desorption outgassing is typically a faster process compared to diffusion outgassing and can result in higher gas release rates.
We use a bakeout to ensure the cleanliness of the Thermal Vacuum chamber and our payload. We are particularly careful about outgassing as we are an optical payload. If a material were to outgas and cover our lenses, then this could negatively affect our image quality. This is why, when we bakeout PROVE, we covered its lenses with Kapton tape. The bakeout oven does two things, it decreases the pressure and increases the temperature. But why?
As can be seen in Figure 3, the boiling point of water decreases as the pressure decreases. This, therefore, allows us to outgas more at a lower temperature. This can be very useful when you have temperature-sensitive hardware like we do. You may then ask, why do you increase the temperature too? This is due to the speed of random motion where k is the Boltzmann constant, T is the temperature and m is the mass of the particle:
$$\sqrt{\frac{kT}{m}}$$
As you can see above, the speed of particle movement is dependent on temperature which is essentially a measure of kinetic energy. This means that the higher the temperature, the faster the outgassing!
Anyway, that’s enough physics for today! By means of local and international trains, hire cars and aeroplanes the 4 of us made the journey to southern Belgium, to Libramont-Chevigny – our home for the next 12 days.
Stay tuned – tomorrow we will have our briefing and then retrieve our payload from the oven, ready to begin testing!