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, since Starlink and OneWeb began deploying ever-more satellites into increasingly congested orbits, this has changed. In fact, adherence to internationally agreed debris mitigation guidelines, using the IADC Space Debris Mitigation Guidelines as a proxy, is increasing. In general, though, regulatory activities are too slow, even though there have been many positive signs in the right direction.

Luckily, there are other means by which environmentally responsible behavior can be enforced and incentivised: managing market access. In space, the poster child for market access control is the American Federal Communications Commission (FCC). The FCC has taken a leading role in advocating for stricter and more environmentally protective requirements for satellite operators seeking licenses to operate in the US market. A highly recognised change was the planned adoption of the "5-year rule", which requires operators to de-orbit their spacecraft within 5 years after their operational lifetime, replacing the old 25-year rule. Shortly after this, they fined a satellite operator for not complying with the disposal requirements set out in the license agreement.

How relevant is the change from 25 to 5 years?

A question that has not been discussed much is: how relevant is the change from requiring a 5-year remaining orbital lifetime vs. a 25-year orbit?

The main feature of the 25-year guideline was not only that it reduced the lifetime of objects to 25 years, but that once an object had been put into an orbit with a limited lifetime, more-or-less all fragments created in a fragmentation would also be in orbits with a limited lifetime. The impact of an object in orbit on its environment is a function of its collision probability, mass, and orbital lifetime, limiting any of these factors leads to a massive reduction in overall environmental impact.

In fact, the reduction of the orbital lifetime from 25 to 5 years has a secondary effect: of much higher importance is that objects reduce their orbital lifetime at all. Much more important than the precise orbital lifetime (5, 10, or 20 years) is the adherence of satellites to any meaningful lifetime reduction.

The disposal success rate problem

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 (ESA, CNES, DLR, UKSA) jointly investigated the impact of different operational scenarios of a 1,080-object constellation on the environment. A 95% success rate was required in that study, and the more satellites, the higher the rate required. Orbital lifetime is important, but secondary.

Luckily, both the IADC Space Debris Mitigation Guidelines and the US ODMSP have already been updated to state that higher reliabilities may be required, in the case of ODMSP, even stating a 99% or better reliability of disposal as a goal for large constellations. ESA has adopted higher disposal success rates in the context of the Zero Debris Charter. These formulations will find their way into industry standards such as ISO 24113 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 being put into defining guidelines that are both feasible and effective. If you are designing a satellite constellation, keep in mind: aim for a very high disposal reliability first, and a short lifetime second, for the environment. Both matter, but reliability is paramount.