Thread by @ProfHughLewis

I've been thinking about the new proposed @FCC "five-year rule" for #SpaceDebris mitigation & wanted to share some analysis & thoughts. Whilst I think the intentions are good I believe the implications of the change are poorly understood. Let me explain... [1/n]

As @brianweeden's excellent thread explains, "The new proposed ruling would require all FCC licensed satellites that end their life in LEO to re-enter the atmosphere within 5 years, and ideally ASAP." [2/n]

https://twitter.com/brianweeden/status/1568232780386934792

The justification provided by the @FCC is that "a shorter benchmark would promote a safer orbital debris environment." [3/n]

The justification appears to be based on "growth of the commercial space industry" & "disruption to satellite operations due to the increased need for collision avoidance maneuvers" & not on any lack of effectiveness of the "25-year rule" to limit the rate of debris growth [4/n]

Indeed, the @NASA ODPO study cited in the proposed ruling shows that the "25-year rule" is highly effective at reducing the orbital debris population & any gains coming from a change to a "5-year rule" are not significant [5/n]

Unfortunately, the reporting of the ODPO study in the Orbital Debris Quarterly Newsletter (here: orbitaldebris.jsc.nasa.gov/quarterly-news…) is limited so I offer some additional insights below using results from simulation studies conducted using our DAMAGE model [6/n]

Firstly, it's important to have some understanding of how the "25-year rule" & the "5-year rule" can be implemented by operators [7/n]

A satellite ending its mission at a high altitude (e.g. higher than 650 km) will likely have a residual orbital lifetime greater than 25 years. With a manoeuvre to a different orbit, the remaining orbital lifetime can be reduced below the 25-year limit. [8/n]

In the example shown above, the new orbit has an apogee at the same altitude as the original orbit but the perigee is lower, where the atmospheric density is greater. This higher atmospheric density leads to a higher level of drag & a swifter orbital decay. [9/n]

The use of an elliptical orbit is efficient from a propellant/delta-V perspective but is not the only solution. Another option is to manoeuvre to a circular disposal orbit, again taking advantage of higher atmospheric density at lower altitudes, but this is costly. [10/n]

Keeping these explanation in mind, we can now look at the results of simulations where these manoeuvres have (and have not) been implemented by spacecraft & rocket stages. Here are results equivalent to those in the @NASA ODPO study [11/n]

@NASA The results show that the "25-year rule" is highly effective at reducing the growth of the debris population, regardless of how the rule is implemented, but the adoption of a "5-year rule" doesn't provide a significant additional benefit. [12/n]

@NASA Focusing on collisions we see a slightly different picture: yes the "25-year rule" is still effective, but now we see there is a difference depending on how the rule is implemented. At the same time, there is still only a marginal benefit from a change to a "5-year rule" [13/n]

So what is going on? Well, by looking at the perigee altitude of the disposal orbits (used in the simulation) we can start to understand how the lifetime (25 or 5 years) & how the rule is implemented might make a difference to the outcome. [14/n]

Mindful that in our simulations, satellites target orbital lifetimes of 25 years (or 5 years) & do not aim to de-orbit as soon as practically possible, & that they have variable mass & area characteristics, we see how the residual lifetime determines the perigee altitude [15/n]

Implementing a "25-year rule" via circular orbits tends to put orbits around 625 km but for elliptical orbits the perigees are around 500 km. A "5-year rule" tends to put the perigees at around 400 km altitude (caveat: depends on the atmospheric model & other assumptions) [16/n]

So now when we look at *where* the collisions occur in the simulations, for each of the cases, a pattern/correlation emerges [17/n]

There is a lot to unpack in this plot! I've repeated the annotations in the thread below & offered some additional insights [18/n]

Firstly, the "25-year rule" & the "5-year rule" lead to a decrease in the number of collisions above 625 km altitude. It is this reduction that makes post-mission disposal so effective as a #SpaceDebris mitigation measure. [19/n]

The "5-year rule" is generally no more effective than the "25-year rule" above 875 km altitude. Here, the original orbital lifetimes are measured in millenia so a change to orbits with lifetimes of 25 years is, in relative terms, the same as a change to 5 years [20/n]

The "5-year rule" is generally a little more effective than the "25-year rule" at altitudes between 425 km and 875 km, where the original orbital lifetimes are measured in decades to perhaps centuries. [21/n]

The "25-year rule" leads to an *increase* in the number of collisions below 625 km (compared with the case where no post-mission disposal occurs). When the "25-year rule" was first investigated, this effect was observed by @NASA & others. [22/n]

@NASA If circular disposal orbits are used in conjunction with the "25-year rule" these lead to an increase in the number of collisions in the region between 525 km & 625 km. [23/n]

@NASA Although difficult to see in the plot, the results also show that the "5-year rule" leads to an increase in the number of collisions at and below 425 km, roughly where the ISS can be found. [24/n]

@NASA So, in the process of significantly reducing the number of collisions at higher altitudes, the "25-year rule" & the "5-year rule" lead to a small increase in the number of collisions at lower altitudes. It can be argued that the benefits outweigh the drawbacks. [25/n]

@NASA Considering close approaches - the events that feature so prominently in the arguments presented by the @FCC in its proposed ruling - we see patterns similar to those previously seen for collisions: significant benefits at higher altitudes but drawbacks at lower altitudes. [26/n]

What we don't see in these results is a significant reduction in close approaches or collisions at the altitudes currently being dominated by commercial space industry & human spaceflight. Instead we see the opposite. [27/n]

In addition, the simulations did not include the large constellations that are expected to populate the lower regions of LEO, at the altitudes where we see the drawbacks arising from the "25-year rule" & the "5-year rule" [28/n]

The @FCC concludes that the arguments for a change to a "5-year rule" are "persuasive" & that the change is "warranted". I'm probably going out on a limb by saying that the evidence doesn't appear to support these statements. At the very least, further research is needed. [29/n]

@FCC With other agencies, such as @esa, looking to adopt an immediate de-orbit approach - which would resolve some/all of the issues I identified in the thread above - I think the proposed ruling represents a missed opportunity & falls short of what is needed. [30/n]

@FCC @esa I hope this thread has been interesting and useful! I'd like to thank my MSc student, Chloe Price, for some of the simulation results & for helpful discussions. [31/31; fin]