Environmental sustainability of large satellite constellations in low earth orbit

A specific criticality index, the collision rate percentage increase, was introduced in 2017 to assess the environmental impact of large satellite constellations in low Earth orbit (LEO). That index was estimated in this paper for various constellation arrangements, ranging in altitude from 800 km to 1400 km. The results obtained clearly show that in the regions of space where the current density of cataloged debris is already significant, such as around 800 km, just one hundred more abandoned satellites would increase the current collision rate by ~10%. In less congested LEO regions, as near 1110 km and 1325 km, a comparable increase in the collision rate could be achieved by a number of abandoned satellites between 200 and 500. Taking into account the new planned constellations from 800 km to 1400 km (consisting of approximately 6000 satellites), an increase by nearly 20-30% of the total collision rate among cataloged objects in LEO might be expected, assuming an immediate spacecraft de-orbiting at the end-of-life, with a success probability of 90%. Of course, a greater number of satellites, as well as a reduced probability of successful disposal, would affect the environment even more negatively. Moreover, if the many disposed satellites were not de-orbited immediately, or in a relatively short time, the collision rate in LEO would further increase, at least in the medium term, unless the satellites do not continue to be controlled and maneuverable until they reenter the atmosphere. As an example, if a thousand satellites were disposed on elliptical orbits between 300 km and 1000 km, the collision rate among cataloged objects in LEO might grow by an additional 30% during the few years needed to decay. That said, even assuming a willingness to endure a maximum 50% increase in the collision rate in LEO among objects greater than 10 cm, in the next 25 years, it is clear that an extended and expanded use of large constellations would be consistent with the environment sustainability only if it were possible to increase the post-mission disposal success probability to at least 95%, and hopefully to 99%. At the same time, the de-orbiting phase should be either quite short or fully controlled, in order to avoid the prolonged presence of several hundred or thousands of abandoned satellites in disposal orbits, further increasing the collision rate in low LEO.