In the event of something accidently being placed into the same orbital path as the ISS and on an imminent collision course, assuming it would be an object with the potential to cause some damage or loss of hull integrity, what would the procedure be.

Would the crew have to get suited up, are there enough suits? and assuming the object was fast approaching how would that work as for more than one person at a time it would be a slow process? Is there some sort of emergency suit?

Or would they climb into an attached resupply ship and undock?

Or is there a secure refuge somewhere in the structure where they could seek shelter?

Or would they try to move the ISS to a slightly different orbit given enough time to get out of the objects path?


1 Answer 1


This kind of situation, while not exactly frequent, has happened on ISS multiple times. With the large amount of debris out there, tracked objects do intersect the ISS orbit from time to time. There are procedures in place on precisely how this is handled.

Generally speaking, this is how it works:

  1. ISS flight controllers get regular conjunction updates from USSTRATCOM (United States Strategic Command), which is the group at Vandenberg Air Force Base that is responsible for tracking all in-orbit objects. USSTRATCOM notifies NASA if a tracked object is projected to enter the "pizza box", a 50 km by 50 km by 4 km zone centered around ISS. From this article, it seems that the ISS team gets one of these notifications about once a week on average. In general (but not always), these notifications provide a roughly three day warning.

  2. The TOPO flight controller takes these notifications and computes a probability of collision for each one and gives it a label. A collision probability between 0.001% and 0.01% results in a "yellow" label, and a probability higher than 0.01% results in a "red" label.

  3. What happens next depends on several factors. Mission rules specify that an avoidance maneuver is to be taken for "yellow" threats when the maneuver would not have a mission impact, e.g., affecting an upcoming Soyuz rendezvous. Action must be taken for "red" threats unless a maneuver would itself endanger station, e.g., if something is damaged on ISS and firing the thrusters would cause further damage.

    • If the ISS team gets warning more than 28.5 hours in advance (per flight rule -- the odd time period is necessary to coordinate between US and Russian flight control teams, as the Russian segment is what executes the maneuver), a custom debris avoidance maneuver (typically a 0.5 - 1.0 m/s nudge) is planned and executed that best alleviates the threat while minimizing the effects on mission success, vehicle consumables, and structural health.

    • If the warning gives less than 28.5 hours notice, the team can fall back to what is known as a PDAM -- a Predetermined Debris Avoidance Maneuver. This is a canned, 0.5 m/s maneuver that has been vetted ahead of time by the various engineering teams and can be executed very quickly. It may not be the most ideal option, but it's available quickly.

    • If warning comes within that 28.5 hour window and a PDAM cannot alleviate the threat, or if a PDAM will create a different conjunction, simply swapping one threat for another, the crew will "shelter in place" in whichever vehicle is their ride home. This means they will retreat to their return vehicle (Soyuz at present, soon to include Dragon and Starliner), seal up the hatches, and wait until the threat passes. In the unlikely event of a loss of station atmosphere, they will be protected by the closed hatches, and -- provided that station still has stable attitude control -- they can undock and go home if a full evacuation is necessary.

All of these options have been exercised multiple times in ISS's flight history.

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    $\begingroup$ Unless it's the Soyuz that gets hit...but that's the instructor in me talking... $\endgroup$ Commented Oct 2, 2017 at 16:47
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    $\begingroup$ @OrganicMarble This is true, but ultimately, it falls under a list of risks that have to be accepted. There are also situations where the crew may be safely inside their vehicles but unable to evacuate -- say, large strike creates a propulsive leak that results in complete loss of attitude control, leaving station tumbling, which would make undocking exceedingly dangerous. Ultimately, you can't eliminate all risks; you can only beat them down as low as you can get them. At some point, you reach a place where further action no longer lowers risk. $\endgroup$
    – Tristan
    Commented Oct 2, 2017 at 17:38
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    $\begingroup$ Totally agree!! $\endgroup$ Commented Oct 2, 2017 at 17:39
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    $\begingroup$ I think there's an assumption in the question that something might just "pop onto the radar" 10 minutes before impact, leading to a mad scramble to avoid it or take shelter. I gather that's not exactly realistic. If we spot something, we'll spot it in time for an organized response. $\endgroup$
    – hobbs
    Commented Oct 3, 2017 at 2:22
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    $\begingroup$ @hobbs, in July 2015 the crew was alerted about 90 minutes prior to an event (for which they performed shelter in place). I'm not sure how much earlier the ground team first became aware of the collision possibility. Objects with low perigee can have predicted orbital positions change significantly over just a couple of orbits, which can move it from yellow to red. $\endgroup$
    – BowlOfRed
    Commented Oct 3, 2017 at 4:17

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