#SpaceWatchGL Share: Chinese Space Station and Starlink Conjunctions: A Closer Look
By Jim Shell, Space Surveillance (SSA / SDA) and Orbital Debris Specialist
So what happened? How do we know what happened? Was it appropriate for China to invoke Article V of the United Nations Outer Space Treaty? How much surveillance should the United States exercise over commercial space activities? What level of transparency is appropriate? These are just a few of the questions raised by Note verbale from China of 3 December 2021 at the United Nations.
China’s claim that the Chinese space station had to maneuver in order to avoid dangerous conjunctions with Starlink satellites has caught everyone’s attention and is just a glimpse of what will become increasingly common with the ever-growing population of objects in low earth orbit. This is another use case highlighting the lack of international standards and protocols for such events.
This short analysis is not intended to be political. It is intended to be factual, based on publicly available information provided by the United States via space-track.org. So… my effort is simply to state the facts to the best of my ability. Yes, these facts are only from a US government data source, but this is the standard used by the international community. The analysis uses two sets of line items or “TLE”. The United States performs a conjunction assessment using more accurate orbital states and propagation models than those supported by TLEs. However, given the dynamic nature of these events (with the maneuver of the Starlink satellites), this higher-order orbit theory would likely offer no improvement.
Many don’t realize that the United States is providing emergency conjunction warnings to all nations – not just those with an SSA sharing agreement. Russia and China both receive conjunction notifications that meet the “urgency criteria,” the criteria for this case being an expected failure distance. 1E-4 with a close approach time of three days or less (reference 18th Space Control Squadron’s Space Flight Safety Manual for Satellite Operators, Version 1.5, August 2020). As we will see, it does not seem that this threshold of emergency criteria has ever been reached.
The following examines both the events of July 1, 2021 and October 21, 2021 investigating: 1) the orbit of the Chinese Space Station before and after the conjunction, 2) the orbit of the Starlink satellites before and after the conjunction , and 3) an evaluation of the projection to miss the distances. Unfortunately, the available orbital data (TLE) does not lend itself to an uncertainty analysis and derivation of a realistic probability of collision, so one has to be satisfied with a simple criterion of failure distance.
July 1, 2021 in conjunction with Starlink-1095
Behavior of the Chinese space station around July 1
China said in its note to the UN that it “took the initiative to conduct evasive maneuvers on the evening of today (July 1) to avoid a possible collision.” Indeed, examination of the orbit of the Chinese Space Station indicates that a maneuver was carried out late on July 1. A closer examination of the orbits before and after the maneuver indicates that the central time of the maneuver was in fact July 2, 0020 UTC. However, prior to the late July 1 or early July 2 maneuver, it appears the Chinese Space Station maneuvered on June 29 as well. Could this maneuver have established a conjunction with Starlink-1095 (object US 44971) (Ref Fig 1)?
Figure 1: Average movement of the Chinese Space Station around July 1, 2021.
Starlink-1095 behavior around July 1
Starlink-1095 (American object 44971) seems not to have operated from June 24, 2021 until July 1, 2021 (ref. Fig 2). However, a Starlink-1095 maneuver is evident between 0631 and 2151 UTC on July 1. Would this maneuver to continue the desorbit have been motivated by Starlink’s autonomous maneuver strategy based on a potential for conjunction with the Chinese Space Station? It is important to note that Starlink uses low thrust electric propulsion for the maneuvers. These long lasting thrust events are not supported by the SGP4 theory force model on which TLEs are based, so it is difficult to maintain an accurate orbit state.
It is not known how much time it takes China to plan and conduct a Chinese Space Station maneuver. A typical strategy is to wait and understand how the collision probability changes as additional object tracking occurs and propagation errors decrease with a shorter time period before conjunction. At the height of the Chinese Space Station, atmospheric drag is the most important uncertainty introduced. Often three days (72 hours) before a conjunction has been considered an optimal point for a maneuver decision that balances the minimization of the delta speed required to produce a missed safe distance as well as sufficient state accuracies to take a informed decision. This decision timeline is likely a key factor for this event given that Starlink-1095 also appears to have maneuvered on July 1 prior to the Chinese space station maneuver.
Figure 2: Average movement of Starlink-1095 around July 1.
Examining conjunctions with Starlink-1095 around July 1
A review of conjunction times and missed distances is now undertaken. For this event, there are no particularly close conjunctions identified. The most significant conjunction is the one with a missed distance of about 28 km on July 1 at 0756 UTC obtained 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 states of the objects and an understanding of Starlink’s potential maneuver during this conjunction window, it is difficult to establish the “truth” for this event.
We now move on to the October 21 event which has a more definitive rating.
October 21, 2021 in collaboration with Starlink-2305
As with the July 1 event, a Chinese Space Station maneuver is evident on October 21, per 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 October 21.
Figure 3: Average movement of the Chinese Space Station around October 21, 2021.
Starlink-2305 continuously maneuvered as claimed by China. It appears that Starlink-2305 has been lifting orbit (decreasing average movement) since ~ October 16 and continued unabated throughout the conjunction event (ref. Figure 4). It is important to recall the above discussion of the difficulties of maintaining an accurate orbit state given the low thrust maneuvering technique used by Starlink satellites.
Figure 4: Average movement of Starlink-2305 around Oct 21
Examining conjunctions with Starlink-2305 around Oct. 21, 2021
A review of the miss distances between the Chinese Space Station and Starlink-2305 confirms China’s concern for this event. A missed distance of approximately 7.3 km is reported on October 21 at 0838:05 UTC. This missed distance, coupled with the fact that Starlink-2305 had been maneuvering constantly, was likely associated with great uncertainty and, therefore, a maneuver was considered prudent given the risk to human life.
So, taking a step back and trying to be a neutral observer, the facts suggest that China had legitimate concerns about Starlink satellite encounters. In particular, the data confirms that the event of October 21 had a relatively close approach (
Now what? For starters, communication between owner-operators across international and geopolitical boundaries is imperative when there is a potential risk to human life. At a minimum, when maneuvering satellites encounter manned missions, some level of proactive communication is warranted. A possible goal for the international community is to have independent data sources that lend themselves to corroboration and consensus building. However, this objective is called into question by many factors, the least of which are technical.
The certain future occurrence of similar events will continue to provide the gentle impetus necessary for the maturation of this collaboration.
Jim shell is a scientist with extensive experience in acquisition and science and technology at the Ministry of Defense. Focus on distilling the technical issues down to key variables, then developing innovative and cost-effective ways to meet the requirements. PhD in imaging sciences with a solid background in remote sensing and reflectance phenomenology. Current research emphasizes the use of optical telescopes to monitor orbital debris, as well as the implementation of space policy. Specialties: Awareness of the spatial situation, orbital debris, radiometry, optical systems, polarimetric signatures, imagery.
This article is republished with permission of the author originally posted on LinkedIn. You can read the original here.