Real-Time Atomic Oxygen Measurements—From Lab to Space

The atomic oxygen (AO) presence in a low Earth orbit (LEO) environment is one of the primary factors responsible for the degradation of spacecraft external surface due to its ability to cause severe oxidation and surface erosion. As a result, thermal, mechanical, and optical properties of AO-exposed materials, especially polymers, are vulnerable. Therefore, information concerning the flux of AO is very important for spacecraft design and mission lifetime assessment. Moreover, AO monitoring can be used to detect real-time changes in solar activity, thus providing data of great importance to “space weather” forecasts.<br/>Many spectrometric, optical, gravimetric, and semiconductor-based methods to measure AO flux have been developed over decades of space research. Advantages, as well as shortcomings of some of the most useful methods, will be reviewed. Furthermore, two new, elegant, and cost-effective methods for AO monitoring in space, developed in our laboratory, will be discussed.<br/>The first method is based on material erosion measurements using the on-orbit material degradation detector (ORMADD). The ORMADD is comprised of solar cells coated with a semi-transparent AO-sensitive layer. Recently an ORMADD, coated with amorphous carbon and Kapton, was integrated on the TAUSAT-1 nanosatellite. TAUSAT-1 mission was to study the low Earth orbit space environment and its effect on electronic components and materials. It was ejected from the international space station into its orbit in March 2021. Using ORMADD we were able to follow the erosion of the coatings in real-time and to calculate the AO flux at the TAUSAT-1 orbit. The flux calculated using ORMADD data was on-par when compared to the AO flux calculated using the SPENVIS software.<br/>The second method is based on the unique electrical properties of diamond substrate overcoated with a thin transition-metal oxide (TMO) layer. This device, abbreviated as DTMO sensor, was implemented using type IIa (001) single crystal diamond and atomic layer deposited (ALD) tungsten oxide (WO₃). The TMO serves as an electron acceptor, promoting hole conductivity on the diamond surface through a process of transfer doping. When exposed to AO, the TMO goes through a redox process, thereby changing its band structure and reducing the hole concentration in the diamond surface. This process results in an increase in the diamond surface resistivity that can be monitored in real-time. The results of AO flux and fluence measurements with the DTMO, obtained during ground-based exposure using 5 eV AO laser detonation source, are very promising. The change in the diamond resistance, while exposed, was measured <i>in-situ</i> and compared to the LEO equivalent AO fluence, calculated based on polyimide mass loss as a standard. The results showed a linear increase in the diamond resistance as a function of fluence up to 2×10²⁰ O-atoms/cm²without reaching saturation. The sensitivity of the DTMO sensor with 10 nm WO₃ coating was ~6×10¹⁴ O-atoms/cm²Ω. Our work demonstrated the potential of the DTMO sensor to enable fast and reliable real-time on-orbit AO flux monitoring.