Radiation exposure to satellites in medium earth orbit

Lead Research Organisation: University of Cambridge
Department Name: Applied Maths and Theoretical Physics

Abstract

Research shows that the proton radiation belt varies with time, but at different rates depending on altitude. There are several sources and losses which depend on geomagnetic activity. The student will develop a dynamic radiation belt model that can take these variations into account. The model will be verified by comparing the results against data from the VAP satellites for selected events of interest. Using the SPENVIS system, the student will calculate the solar array degradation from the new model and compare against that obtained using the AP8 and AP9 models.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
NE/R009457/1 01/10/2017 30/09/2022
1940244 Studentship NE/R009457/1 01/10/2017 31/05/2021 Alexander Lozinski
 
Description (Summary of first paper:) Electric Orbit Raising is a technique introduced commercially in 2014 to launch satellites into geostationary orbit using electrical propulsion. After a satellite is put into low Earth orbit using a rocket, electric thrusters are used to raise the altitude of orbit over a period of 100 - 200 days until the satellite reaches geostationary altitude (~36000km). This is in contrast to conventional liquid propellant which takes only a few days to boost the satellite into geostationary orbit, but is less efficient.

This work found that electric orbit raising to geostationary orbit can result in significant solar cell degradation, if the satellite is launched during or shortly after a radiation belt enhancement. In this scenario, charged high-energy particles arriving from the Sun enter Earth's magnetic environment (magnetosphere) and become trapped by Earth's magnetic field, forming part of Earth's radiation belts. Of these particles, protons in particular occupy a region in space that satellites must traverse in order to transfer from low earth orbit to geostationary orbit. Because satellites undergoing electric orbit raising traverse this region more slowly in comparison to conventional launches, the increase in trapped energetic protons after an enhancement causes an increased rate of solar cell degradation as these particles bombard the spacecraft.

As well as quantifying the potential increase in degradation due to such an enhancement, this work analysed previously-used trajectories from electric orbit raising missions, and by comparing them found ways to mitigate the effects of such enhancements by carefully choosing an orbit that avoids enhanced regions of trapped protons as much as possible.


(Summary of second paper): Energetic particles in the Earth's radiation belts (both protons and electrons) undergo transport due to radial diffusion. This is a key process responsible for the distribution of radiation belt particles, and as such requires accurate modelling in order to predict the time evolution of flux/phase space density. The radial diffusion process is quantified by radial diffusion coefficients, which are required for such modelling. In our work, we estimate the value of proton radial diffusion coefficients using a steady state optimisation technique: by assuming that measurements of Earth's proton radiation belt represent a steady state, we can solve for steady state using our own numerical model, altering radial diffusion coefficients used during the calculation until the solution matches the measurements. This allows us to estimate the true values of radial diffusion coefficients.

By estimating radial diffusion coefficients, our work enables further modelling efforts. In addition, our work introduces some new modelling techniques and discusses the validity of the steady state assumption in detail.
Exploitation Route Satellite operators may consider the impacts of launching satellites using electric propulsion in light of these findings (for example, the expected degradation in solar cell performance and the amount of shielding required to plan a mission) and may take them into account when deciding on an optimal trajectory
Sectors Aerospace, Defence and Marine,Other

URL https://phys.org/news/2019-07-space-weather-years-satellites-electric.html