By Jim Shell, Space Surveillance (SSA/SDA) & orbital debris Subject Matter Expert
So what DID happen? How do we know what happened? Was it appropriate for China to invoke Article V of the United Nations Outer Space Treaty? What level of oversight should the US have with commercial space activities? What level of transparency is appropriate? These are just a few of the questions prompted by China’s 3 December 2021 note verbale to the United Nations.
China’s assertion that the Chinese Space Station station had to maneuver in order to avoid dangerous conjunctions with Starlink satellites has captured everyone’s attention and is simply a preview of what will become a more frequent occurrence with the ever growing population of objects in Low Earth Orbit. This is yet another use case highlighting the lack of international standards and protocols for such events.
This short analysis piece is not intended to be political. It is intended to be factual, based upon public information available provided by the US via space-track.org. So…my endeavor is simply to lay out the facts to the best of my ability. Yes, these facts are derived solely from a US government data source, but it is the standard by which the international community uses. The analysis uses two line element sets or “TLEs”. The US performs conjunction assessment using more accurate orbital states and propagation models than supported by TLEs. However, given the dynamic nature of these events (with maneuvering Starlink satellites) this higher order orbit theory would likely offer no improvement.
Many do not realize that the US provides emergency conjunction warnings to the all nations – not just those with an SSA Sharing Agreement. Both Russia and China receive conjunction notifications that meet the “Emergency Criteria”, the criteria for this case being a predicted miss distance of < 1 km and a probability of collision > 1E-4 with a time of close approach of three days or less (reference 18th Space Control Squadron’s Spaceflight Safety Handbook for Satellite Operators, Version 1.5, Aug 2020). As will be seen, it does not appear this emergency criteria threshold was ever reached.
The following examines both the 1 July 2021 and 21 October 2021 events investigating: 1) orbit of the Chinese Space Station before and after the conjunction, 2) orbit of the Starlink satellites before and after the conjunction, and 3) an assessment of the projected miss distances. Unfortunately the orbital data available (TLEs) do not lend themselves to uncertainty analysis and derivation of a realistic probability of collision, so one must be satisfied with a simple miss distance criteria.
1 July 2021 conjunction with Starlink-1095
Chinese Space Station behavior around 1 July
China stated in their note to the UN that they “took the initiative to conduct an evasive maneuvers in the evening of that day (1 July) to avoid a potential collision.” Indeed, examination of the Chinese Space Station orbit indicates that a maneuver was made late on 1 July. Further examination of the orbits before and after the maneuver indicate the center time of the maneuver was actually on 2 July, 0020 UTC. However, prior to the late 1 July or early 2 July maneuver, it appears that the Chinese Space Station also maneuvered on 29 June. Might this maneuver have set up a conjunction with Starlink-1095 (US object 44971) (Ref Fig 1)?
Figure 1: Mean motion of the Chinese Space Station around 1 July 2021.
Starlink-1095 behavior around 1 July
Starlink-1095 (US object 44971) appears to have not been maneuvering since 24 June 2021 going into 1 July 2021 (ref Fig 2). However, a maneuver by Starlink-1095 is evident between 0631 and 2151 UTC on 1 July. Might this maneuver to continue de-orbiting have been prompted by Starlink’s autonomous maneuvering strategy based upon a conjunction potential with the Chinese Space Station? It is important to note Starlink uses low-thrust electric propulsion for maneuvering. These long-duration thrust events are not accommodated by the force model of the SGP4 theory upon which TLEs are based, so it is difficult to maintain an accurate orbit state.
It is unknown what lead time is required for China to plan and conduct a maneuver of the Chinese Space Station. A typical strategy is to wait and understand how the probability of collision evolves as additional object tracking occurs and propagation errors decrease with a shorter time span prior to the conjunction. At the altitude of the Chinese Space Station, atmospheric drag is the most significant uncertainty introduced. Often three days (72 hrs) prior to a conjunction has been considered an optimal point for a maneuver decision which balances minimizing the required delta velocity to produce a safe miss distance along with sufficient state accuracies to make an informed decision. This decision timeline is likely key factor for this event given that Starlink-1095 appears to have also maneuvered on 1 July prior to the Chinese Space Station maneuvering.
Figure 2: Mean motion of Starlink-1095 around 1 July.
Examination of conjunctions with Starlink-1095 around 1 July
An examination of conjunction times and miss distances is now undertaken. For this event, there are no particularly close conjunctions identified. The most significant conjunction is that with a ~28 km miss distance on 01 July at 0756 UTC obtained by using the Chinese Space Station TLE with an epoch of 183.4658414 and the Starlink-1095 TLE with an epoch of 179.07755502. Again, without a realistic covariance associated with the objects states and an understanding of the potential Starlink maneuver during this conjunction window it is difficult to establish “truth” for this event.
We now turn to the 21 October event which has a more definitive assessment.
21 October 2021 conjunction with Starlink-2305
As with the 1 July event, a maneuver by the Chinese Space Station is evident on 21 October consistent with their UN note verbale. Examination of the pre-maneuver orbit with that of the post-maneuver orbit suggests a maneuver time of approximately 0312 UTC on 21 October.
Figure 3: Mean motion of the Chinese Space Station around 21 Oct 2021.
Starlink-2305 was continuously maneuvering as asserted by China. It appears Starlink-2305 had been raising the orbit (decreasing mean motion) since ~16 October and continued unabated through the conjunction event (ref Figure 4). It’s important to recall the discussion above on the difficulties of maintaining an accurate orbit state given the low-thrust maneuver technique employed by Starlink satellites.
Figure 4: Mean motion of Starlink-2305 around 21 Oct.
Examination of conjunctions with Starlink-2305 around 21 Oct 2021
An examination of miss distances between the Chinese Space Station and Starlink-2305 substantiates China’s concern for this event. A miss distance of ~7.3 km is indicated on 21 Oct at 0838:05 UTC. This miss distance, coupled with the fact that Starlink-2305 had been continuously maneuvering, likely had a large uncertainty associated with it and therefore a maneuver was deemed prudent given the risk of human life.
So, stepping back and attempting to be a neutral observer, the facts suggest China did have some legitimate concerns with Starlink satellite encounters. In particular the data support that the 21 October event had a relatively close approach (< 10 km) and with a continuously maneuvering Starlink satellite which introduced much uncertainty.
Now what? For starters, communication between owner operators across international and geopolitical boundaries is an imperative when there is potential risk to human life. At a minimum, when maneuvering satellites have encounters with crewed missions, some level of proactive communication is warranted. An eventual objective for the international community is to have independent data sources which lend themselves to corroboration and consensus building. However, this goal is challenged by numerous factors, the least of which are technical.
The certain future occurrence of similar events will continue to provide the gentle push necessary to mature this collaboration.
Jim Shell is a Scientist with broad Department of Defense acquisition and Science and Technology experience. Focus on distilling technical problems down to key variables, then developing innovative cost-effective means of satisfying requirements. PhD in Imaging Science with a strong background in remote sensing and reflectance phenomenology. Current research emphasis on use of optical telescopes for monitoring orbital debris, as well as space policy implementation. Specialities: Space Situational Awareness, orbital debris, radiometry, optical systems, polarimetric signatures, imaging.
This article is re-published with the permission of the author was originally published on LinkedIn. You can read the original here.