RED threshold late notice conjunction threat misses ISS – Crew egress Soyuz

by Chris Bergin

The three members of Expedition 18 carried out the contingency of  “evacuation” into the Soyuz spacecraft attached to the International Space Station (ISS), following a “RED threshold late notice conjunction threat” alert, related to Object “25090 PAM-D” debris. The threat passed with no impact, allowing the crew to egress back into the Station.

Tracking of the small piece of debris/MMOD (Micrometeoroid Orbital Debris) was initially classed as a low threat of collision with the ISS. However, latest tracking suggests the threat is red.As a contingency, NASA’s Expedition 18 Commander Mike Fincke, Russian Flight Engineer Yury Lonchakov, and NASA’s Sandra Magnus were asked to “evacuate” into the Russian Soyuz, which would serve as their means of departing from the Station – had it been required.

The debris object is called a “yo weight” – which was originally part of a Delta PAM-D stage – used to launch GPS 37 in 1993. The yo weight is a small mass attached to a 1-meter-long cable, used to tumble the stage after separation from the payload so it doesn’t recontact. Although it would have a mass less than 1kg, travelling at 17,500 mph makes even the smallest object a serious threat.

Updating the current status of the threat, several memos – acquired by L2 – relay the probability level of the conjunction, with the latest noting “It’s now in the red threshold and if it doesn’t improve between now and TCA (Time of Closest Approach) at 11:39am CDT today they will put the crew in the Soyuz per the rules.”

a111“The plan that has been coordinated with MCC-M (Moscow) is that if required the crew will enter the Soyuz and be in place by 16:35 staying there until at least 5 minutes post TCA,” added another memo. 

“WE will not be closing any additional hatches to enable us to immediately run the nominal depress procedures should they be needed.

“If the PC remains red the course of action will be to place the crew in the Soyuz from 5 minutes before TCA until 5 minutes after TCA.  This is ~2 hours after crew post sleep activities (crew will be awake). “

Update at 11am Central: Crew now closing the hatches and preparing to evac to Soyuz as a contingency in case of debris collision with ISS. Now in Soyuz.

Post Conjunction update: Debris passed by the ISS without impact, allowing the crew to egress Soyuz to re-open the hatches and continue with their tasks.


The opening notification memos appeared to show the threat was low, even though tracking was expected to show the “miss distance” would reduce as the numbers were refined.

“Received a low concern, late notification (42 hours to TCA) conjunction for a small piece of PAM-D debris,” noted the opening memo relating to the threat. “This object showed good tracking and a miss outside the notification box. Subsequent updates showed miss distances to increase until the latest update.

“(Tracking) explained that due to the low perigee of this object (154 km) they were in error running Solar Radiation pressure as they do for typical objects we encounter, correcting this value for this object this afternoon caused the miss distances to shrink, however (tracking) feels the radial miss should stay stable from here on out.

“(In the event of a) Red threshold violation, the crew will enter the return vehicle in preparation for departure and be prepared to close the hatch quickly if required.”

The following update noted the threat has increased to “medium” – following what appears to be an error in the tracking models – at a time deemed too late for the ISS to alter its position to decrease the collision threat.

“Medium concern conjunction: Object 25090 PAM-D debris – this conjunction alert has not gone out to the usual FD distribution because we’re past the maneuver go/no-time.

“(Tracking) detected an error in their processing that caused a big update, with resulting miss distances 0.5 x 4.6 x -2.6 km at TCA 71/16:39:39 GMT (1139 CDT Thurs),” noted the memo updating status.

“This object is quite elliptical with a low perigee, so we’re not getting a lot of tracks on it, but (Tracking) has increased tasking as much as they can. Hopefully the radial miss will hang in there at 0.5 km so this won’t be an issue.”

By Thursday morning, uncertainty still existed as to how big a threat the debris would be for a collision with the ISS.

“(Tracking) has an Invalid Red Threshold Violation of 2.34e-4. The large changes in downtrack are causing the 2 sigma downtrack requirement to be invalid, thus causing an invalid PC.

“These swings in downtrack can be typical of a high eccentricity (4220 x 150 km, ecc=0.24; ISS ecc= 0.001) high energy dissipation rate (0.2; ISS=0.002) object. The covariance are stable and well behaved, however are larger than typically seen at this stage of a conjunction (exact location of object has higher uncertainty).”

This article was updated during the contingency. Post conjunction notes will be added as they arrive.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

Related Articles