This article originally appeared in Orbital Debris Quarterly News, Volume 19, Issue 4, October 2015.

The 24th and 25th collision avoidance maneuvers in the history of the International Space Station (ISS) were performed this quarter. These were the 3rd and 4th of this calendar year.

Figure 1. A view of the ISS (SNN 25544, Zarya)-34356 encounter geometry. Note the apogee of 34356 had almost cleared the ISS orbit at the time of the PDAM.

The first maneuver this quarter was performed for a conjunction with a debris fragment from Iridium 33 (International Designator 1997-051EY, U.S. Strategic Command [USSTRATCOM] Space Surveillance Network [SSN] catalog number 34356). The fragment was created during the February 2009 accidental collision between Cosmos 2251 and Iridium 33 (ODQN, vol. 13, issue 2, April 2009, pp. 1-2). The debris fragment was experiencing much higher drag than the ISS. Due to the high drag profile, the debris was not a concern until less than 8 hours prior to the time of closest approach. The probability of collision exceeded the red threshold for a maneuver before the initiation of the pre-determined avoidance maneuver (PDAM) on 26 July at 03:48 GMT, as shown in Fig. 1. The 0.5 m/s maneuver was executed using thrusters from the Progress 58P visiting vehicle. An update using additional tracking data from the SSN was received within a few minutes of the initiation of the PDAM, which showed that the probability of collision had fallen to acceptable levels. However, at that time the ISS was already under thruster control for attitude which could perturb the trajectory, making the risk unknown. Since there were no major consequences in performing the burn, the maneuver sequence was continued.

Figure 2. A view of the ISS-debris encounter geometry that prompted PDAM-25.

At the time of the second maneuver, flight controllers were tracking two potential conjunctions approximately 6 hours apart in time. Fig. 2 depicts the encounter geometry. Both objects were debris fragments. The first fragment (International Designator 1994-029AGH, USSTRATCOM SSN catalog number 40241) was created from the Hydrazine Auxiliary Propulsion System (HAPS) Pegasus fragmentation from June 1996 (ODQN, vol. 1, issue 2, September 1996, pp. 2, 11). Until the Chinese ASAT test in 2007 and the Iridium/Cosmos collision in 2009, the HAPS rocket body explosion had produced the largest number of SSN trackable breakup debris despite the stage’s relatively small size.

The second debris piece being tracked was from the Chinese ASAT test target, Fengyun-1C (International Designator 1999-025AZ, USSTRATCOM SSN catalog number 29759) (ODQN, vol. 11, issue 2, April 2007, pp. 2-3). The probability of collision for conjunction with the Fengyun debris was below the threshold for performing a maneuver; however, the HAPS Pegasus debris probability exceeded the red threshold requiring a maneuver. The 0.3 m/s PDAM was performed on 27 September at 9:06 GMT using thrusters from a Progress resupply vehicle.