In 2019 I presented a paper at the 1st International Orbital Debris Conference looking at implications of relocating satellites of a large constellation from high LEO altitudes to low LEO altitudes ( https://www.hou.usra.edu/meetings/orbitaldebris2019/orbital2019paper/pdf/6069.pdf). Here's a thread on some new analysis #spacedebris 1/n

In my presentation I pointed out that relocating satellites to lower altitudes might solve long-term space debris issues but wouldn't be risk-free & could introduce problems related to space safety. Adding traffic to an already busy region could increase conjunction events (2/n)

Space systems at altitudes below 600 km are typically small satellites such as cubesats. Often these do not have a propulsion capability and so cannot implement collision avoidance manoeuvres. The figure shows recent launch activity & spatial density (data courtesy of @ESA) (3/n)

Predicting conjunctions (i.e. close approaches between orbiting objects) is difficult. Due to uncertainties in solar EUV, for example, the error in satellite trajectories can be > 10s km. Figure showing errors in 7-day predictions from Emmert et al. (AMOS Conference 2014) (4/n)

Since my presentation at the IOC, @SpaceX have filed a modification request with @FCC to relocate all of the #Starlink satellites to altitudes below 600 km. One reason suggested for this change is to enhance space safety. (5/n)

In the last month I looked again at the implications of this proposed solution to long-term #spacedebris concerns. Specifically, I have investigated changes to the conjunction events that have been taking place since 1 October 2019, focusing on events involving #Starlink. (7/n)

I just want to add here that I have used #Starlink because the system is the largest constellation operating below 600 km and has been growing quite consistently since 1 October 2019 (at ~1.8 satellites per day on average) (8/n)

I have used conjunction data from SOCRATES ( https://celestrak.com/SOCRATES/ ) with thanks to the fantastic @TSKelso for support. Specifically, I used SOCRATES reports from the 1st and 15th of each month from 1 October 2019 through 15 June 2020, and 30 June in lieu of 1 July 2020 (9/n)

The figure shows the number of #Starlink satellites in orbit and the average number of conjunctions per day predicted by SOCRATES as a function of time. (10/n)

(quick note: the conjunctions shown exclude all Starlink v Starlink satellite events; i.e. only conjunctions involving a Starlink satellite and something else - payload or debris - are included) (11/n)

SOCRATES predicted 1 additional conjunction < 5 km per day for approximately every 2 #Starlink satellites added to orbit over the analysis period (12/n)

and 1 additional conjunction < 1 km per day for approximately every 50 #Starlink satellites added to orbit over the analysis period (13/n)

In the SOCRATES report from 30 June 2020: ~210 conjunctions < 5 km & ~6.5 conjunctions < 1 km per day, on average, for the 7-day period from 12:00 UTC 30 June 2020 (14/n)

In the SOCRATES report from 15 June 2020: 1,546 conjunctions < 5 km & 42 conjunctions < 1 km in total involving #Starlink (15/n)

The conjunctions predicted in this report were expected to occur consistently throughout the 7-day period. So, put another way: 1 #Starlink conjunction < 5 km every 3.9 minutes & 1 #Starlink conjunction < 1 km every 3.8 hours (16/n)

In the 15 June 2020 SOCRATES report there were 302 unique conjunction partners with #Starlink, many were debris: FENGYUN 1C DEB (208 events), COSMOS 2251 DEB (118 events) & IRIDIUM 33 DEB (46 events) (17/n)

Of the 302 unique conjunction partners in the 15 June 2020 SOCRATES report, I think 80 were cubesats (including FLOCK & LEMUR satellites) and these accounted for 204 conjunction events (18/n)

In the 15 June 2020 SOCRATES report, 407 out of 538 #Starlink satellites in orbit (75%) were identified as a conjunction partner with either another payload or debris. E.g. STARLINK-1201 was identified in 29 conjunction events. (19/n)

Based on extrapolation (using a linear model) from the trends shown: for a #Starlink system with 1,584 satellites SOCRATES might predict 715 conjunctions < 5 km per day & 28 conjunctions < 1 km per day (~10,200 per year) (20/n)

For a #Starlink system with 4,408 satellites SOCRATES might predict ~2,070 conjunctions < 5 km per day & ~85 conjunctions < 1 km per day (~31,000 per year) (21/n)

If 5% of all conjunctions < 1 km result in a collision avoidance manoeuvre current trends might predict ~500 to 1,500 manoeuvres per year (22/n)

It would be remiss of me not to remark on predictions for the long-term #spacedebris population. While our studies are still being undertaken, preliminary results show very little impact on the eventual debris population growth. (23/n)

So it does appear that relocating large numbers of satellites from high LEO orbits to low LEO orbits will have a beneficial effect over the long-term. However, it is hard to argue that space safety will be enhanced by such a move given the consequences we are already seeing (fin)