A potentially under-explored aspect of SSA is the extent to which satellite launch activities can have an effect on the ionosphere.
Rocket exhaust plumes have been observed to cause large-scale depletions of ionospheric plasmas (known as “ionospheric holes”). In the F-region, charge exchange reactions occur between O+ ions and rocket exhaust species such as H2O, H2, and CO2 to form ions which then undergo rapid dissociative recombination.
A range of measurement techniques have been used to observe these depletions of the ionosphere by rocket exhausts. The first detection of an ionospheric hole was made using ionosonde measurements in 1959. Subsequently, depletions have been studied using geostationary satellite signal Faraday rotation; airglow emission; incoherent scatter radar; satellite Langmuir probes; and GNSS total electron content (TEC) measurements.
What is not clear is whether rocket launches could lead to longer term effects in the Earth’s upper atmosphere, (analogous, perhaps, to the issues allegedly caused to the ozone layer by the use of CFCs). Monitoring these effects over time represents a novel challenge, as the evolution of these chemical species is driven by diffusion and electrostatic forces, and not by the more familiar Keplerian and perturbative ones.
This chemical evolution has potential implications for orbit modelling, signal propagation, and possibly even future launch vehicle design if a long-term problem is identified.
There , of course, other precedents for “unintended consequences” arising from human activity in space. Measurements of the radiation belts suggest that some of particles trapped there, which have been slowly dissipating over the past 40 years, are not natural, but actually the products of exo-atmospheric nuclear weapon tests in the 1960s, (which famously “fried” various satellites at the time, and may have shortened satellite lifetimes for many years afterwards).
And if you think that probably wasn’t humanity’s finest hour, consider the West Ford needles experiments from the early 1960s, which placed 480 million copper needles in orbit around the Earth to create an artificial ionosphere…..and a major debris issue for our descendants.
So although a great deal of SSA data involves tracking short term variations in particle fluxes, satellite orbits, etc., the conclusion that one can draw from these examples is that there is a long-term SSA monitoring problem that needs to be addressed too.
(The author would like to thank Lauchie Scott from Canada and Melrose Brown from Australia for bringing these issues to his attention, and contributing the “Global” element of the GNOSIS acronym).