A review of satellite-based atomic oxygen sensing methods

The space industry has recently seen rising research interest in satellites developed to operate for extended periods at lower altitudes than ever before. The combination of lower launch costs, reduced radiation effects, and ease of deorbit, among other benefits, outlines very low Earth orbit (VLEO) missions as a key prospect of the space sector in the near future.

Neutral atomic oxygen (ATOX), the dominant gas species in lower orbital altitudes, is a major consideration for VLEO missions. The oxidising nature of ATOX with relative speeds at least 7.8 km/s can easily cause an early end to a satellite mission through generation of drag forces, as well as degradation of solar array surfaces and optical instruments. During a mission, accurate quantification of this erosion damage aboard the spacecraft may be gained though gathering data regarding ATOX gas flux impinging on susceptible surfaces at a given point in time.

Additionally, increased aerodynamic loading, caused by greater gas densities at lower altitudes, plays a substantial role in satellite station keeping and attitude control; an aerodynamically unstable spacecraft may experience undesirable rotational forces or even enter a tumbling state if left unchecked. Real-time measurement of ATOX flux allows a mission to determine short-term variations in ATOX flux and therefore estimate fluctuations of satellite lift and drag, enabling a satellite to react appropriately. Long-term recording of these fluctuations may also hold influence over future satellite aerodynamic design.

Atomic oxygen sensing methods are a crucial aspect of future VLEO satellite design as we begin to tackle the obstacles of reduced altitude orbits. These sensors are able to measure the quantity of atomic oxygen atoms impinging on a satellite per unit area. In the interest of reacting to, and designing for the erosive environment and aerodynamic forces presented by ATOX and gas species in VLEO, we review available ATOX sensing methods and their respective applications.

Each ATOX sensing method has its own benefits and drawbacks. The most appropriate method for a given mission depends on factors such as altitude, mission lifetime, and mass budget. A review of the most common methods will clarify design options, and outline suitable areas for future research.

This paper builds upon a review of ATOX measurement methods performed by Osborne, et al. in 2001, updating with technologies from the past two decades as well as providing further examples of each method’s real-world applications.

It also conducts a semiquantitative analysis of spacecraft-based atomic oxygen sensing methods, describing the relative merits of each and their relevant applications. Generalised recommendations are made with regards to the most appropriate sensing method for a range of satellite mission scenarios.

This review finds that proven methods, such as mass spectroscopy, remain the most appropriate sensing methods for many missions. However, the progress seen within technologies of lower heritage, such as renewable actinometers, suggests that we may see missions exhibiting more recent ATOX sensing methods in the near future.