Orbital Responsibility: The Critical Role of High Disposal Success Rates in Mitigating the Environmental Impact of Commercial Satellite Operations

For a long time, the vastness of outer space has been perceived as infinite. But, latest since Starlink and OneWeb have been dominating the public attention of commercial spaceflight with ever-more satellites being deployed into increasingly congested orbits, this has changed. And, in fact, adherence to internationally agreed and widely accepted rules to mitigate space debris, using the IADC Space Debris Mitigation Guidelines as a proxy, is increasing. In general, though, activities to regulate (be it through soft or hard law) are too slow, even though just recently, there have been a lot of positive signs in the right direction. Unfortunately, guidelines-adherent behavior is hard to enforce, as space law by its definition and purpose needs to operate globally, but global changes in law require time and compromises. Luckily, though, there are other means by which environmentally responsible behavior can be enforced and/or incentivised: managing market access.

Source: Our World in Data

In space, the poster child for market access control is the American Federal Communications Commission (FCC). In the mid-2010s, they faced an initial surge of frequency applications for non-geosynchronous(NGSO) satellite constellations, prompting the need for an environmental impact analysis under the US Government’s Orbital Debris Mitigation Standard Practices (known as “ODMSP”). However, they have since taken a leading role in advocating for stricter and more environmentally protective requirements for satellite operators seeking licenses to operate in the US market, as perceived by the general public. A highly recognised change was the planned adoption of the “5-year rule”, which is meant to require operators to de-orbit their spacecraft within 5 years after their operational lifetime to replace the good-old 25-year rule. The latter one was often argued as being set arbitrarily and as way too long, but, in its defense, was, when defined in the early 1990s, based on extensive studies trading off cost of disposal vs. impact on the environment, and was a sensible choice at that time. Shortly after this, they went further and fined a satellite operator for not complying with the disposal of the satellite, which was set out in the license agreement.

A question that has not been discussed much though is: how relevant is the change from requiring a 5-year remaining orbital lifetime vs. a 25-year orbit?
In the end, the main feature of the 25-year guideline was not only that it reduced the lifetime of objects to 25 years, but rather the fact that once an object had been put into an orbit with a limited lifetime, if part of a fragmentation, also more-or-less all fragments created would be in orbits with a limited lifetime. Recalling that the impact of an object in orbit on its environment is a function of its collision probability, mass, and orbital lifetime, limiting either of the factors leads to a massive reduction in the overall impact.

The graph presents the average orbital lifespan of constellation/non constellation objects and shows its decay over the years.
Source: ESA's Space Environment Report

In fact: the reduction of the orbital lifetime of a disposal orbit from 25 to 5 years rather has a secondary effect: Of much higher importance is that objects reduce their orbital lifetime at all (cf. Section 3.2.1 of this paper by Hugh Lewis). Some care must be taken as the study mentioned assumed a much lower number of satellite launch rates than currently observed, so the lifetime impact might be higher than shown.

What will remain though is the core finding: Much more important than the precise orbital lifetime (5, 10 or 20 years) is the adherence of satellites to any meaningful lifetime reduction. Here comes the catch: Usually, a 90% success rate (or reliability of the satellite to be able to perform end-of-life disposal) was deemed as high and sufficient. But, with the large number of satellites being launched now, this is not the case anymore.

When OneWeb, Starlink, Boeing and others announced their constellation plans in 2015, several European space agencies (including ESA, CNES, DLR, and UKSA) in a common effort investigated the impact of different operational scenarios of a 1080-object constellation as a proxy on the environment. In the study, the details of which, even though relevant, are omitted here, a 95% success rate was required; the more satellites, the higher the rate required. The lifetime is important but still, secondary.

Luckily, producing much less attention than the suggested change to FCC guidelines, both the IADC Space Debris Mitigation Guidelines and the US ODMSP received already before updates, stating that higher reliabilities might be required; in the case of ODMSP even stating a 99% or better reliability of disposal as a goal for large constellations. Similarly, ESA has adopted higher disposal success rates in the context of the Zero Debris Charter. And, rest assured, even though international law takes its time, these formulations will find their way into other guidelines, industry standards such as ISO24113, and national space laws.

What to take from this? First, there is progress. Even though from the outside, these changes appear marginal, a lot of effort is put into place to define guidelines that are feasible but also effective. And last – if you are just designing your satellite constellation, keep in mind to aim for a very high disposal reliability first – and a short lifetime second, for the environment (but of course consider both).

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