Issue 135

Separation Anxiety: SJ-21 and SJ-25 Part Ways

5 Dec 2025: After having docked on 2 July 2025 it appears SJ-21 (49330) and SJ-25 (62485) successfully detached from one another. The Joint Commercial Operations Cell first reported the potential separation on 28 November, then the amazing s2a systems published an image showing two distinct dots confirming separation…

5 Dec 2025: After having docked on 2 July 2025 it appears SJ-21 (49330) and SJ-25 (62485) successfully detached from one another. The Joint Commercial Operations Cell first reported the potential separation on 28 November, then the amazing s2a systems published an image showing two distinct dots confirming separation on 29 November. Based on published TLE data from Spacetrack.org and SBMS from Saberastro, the two satellites were separated by ~30km on 30 November. From 1-5 December both satellites maneuvered to initiate RPO conditions with one another with occasional close approaches of <1km. The situation remains fluid and I expect more maneuvering from both satellites in the coming days/weeks. Watch COMSPOC Video.

Background:

  • China launched SJ-25 on 6 Jan 2025 and announced that it would be “used for the verification of satellite fuel replenishment and life extension service technologies.”
    • China launched SJ-25 into a co-planar orbit with SJ-21 which had been used to demonstrate “debris mitigation technology” (it captured a defunct Chinese satellite in GEO and dragged it into a graveyard orbit in early 2022.)
  • June-July 2025: China maneuvered both SJ-21 & SJ-25 into extremely close proximity with one another and made several docking attempts. The two satellites entered into merge conditions on 2 July.
  • August 2025: China conducted significant plane change maneuvers with the docked SJ-21/25 satellites. The pair made minor maneuvers from 2-7 Aug, then from 8 – 15 August, Chinese space operators conducted several plane-changing maneuvers which resulted in a 6° reduction of the spacecraft’s inclination (from 10.4° to 4.4°)and a significant change in RAAN as well. Such maneuvers are highly fuel/energy intensive.


Recent Activity

  • Using only TLE data it is impossible to determine maneuvers with any reliable precision.
  • On 30 Nov, SJ-25 had an SMA ~1km greater than SJ-21. As a result, SJ-25 was drifting westward at 0.097°/day compared with SJ-21’s westward drift of 0.086°/day.
  • From 2-4 Dec Chinese space operators maneuvered both SJ-21 and SJ-25 and the two spacecraft entered into RPO conditions separated by 1-7km. (see graphic next page)
  • On 5 Dec the satellites again maneuvered and began to increase their absolute separation distance. The satellites are separated primarily in the In-Track vector.





Editor’s Comment: Now we wait to see what China does next with each of these satellites. Barring any official announcement from China related to refueling success, our only indications will be from subsequent maneuvering. I’ll be watching for indications of SJ-25 maneuvering to get co-planar with other Chinese satellites. I believe the most likely re-fueling targets are SJ-23, SY-12 01 or 02, & TJS-3. Currently SJ-23 is in the closest orbital plane with SJ-25 with a ~1.5° inclination difference & 18.3° RAAN difference. I will also be looking for potential orbital debris test objects for SJ-21 to attempt to capture (this assumes a successful refueling mission).