Several sources including the Orange County Register are reporting that a "spacecraft emergency" was declared today onboard the ISS due to the new MLM module's uncommanded thruster firings after docking.

humorous image of a man shouting into a megaphone. He wears an orange track suit and a helmet with a flashing light on it.

What does it mean that a "spacecraft emergency" was declared? What happens onboard the ISS? Is there a specific response?

Image source: Cropped screen capture from Kentucky Fried Movie

  • 1
    $\begingroup$ I'm assuming this is an ask-to-answer type of question. I await your answer. I work on the design side rather than the ops side of things, so I can't provide a good answer. We generally design thrusters to fail off: Multiple valves in series, software and firmware that are carefully scrutinized. A failed on thruster ranks right up there in the hierarchy of bad things we do not want to ever happen in space. $\endgroup$ Jul 30, 2021 at 3:30
  • 3
    $\begingroup$ @DavidHammen No, I don't know the answer in today's world. It was a pretty incredible scenario including a 45 minute LOAC. There hasn't been one of those in a long time. The design people swore up and down shuttle jets couldn't fail on. MLM appears to be different. $\endgroup$ Jul 30, 2021 at 4:04
  • 3
    $\begingroup$ I'm not ops, but I would call a thruster fail on that causes a 45 minute loss of attitude control an emergency. Thrusters are not supposed to turn on "inadvertently". And for 45 minutes? Wow. $\endgroup$ Jul 30, 2021 at 4:18
  • 2
    $\begingroup$ @GordonD Those inadvertent firings on Gemini 8 were a huge lesson-learned in how not to design thrusters. I worked on a much later vehicle on detecting and correcting for thruster failures. I asked whether I needed to worry about thrusters failing on. I was told thrusters don't fail on. A few weeks later, the avionics team said they had found some circumstances where thrusters could fail on. This was not a happy moment for the avionics team. It was easy to find the project lead: Listen for where all the shouting was coming from. There was a lot of shouting in that meeting. $\endgroup$ Jul 30, 2021 at 10:58
  • 1
    $\begingroup$ I fixed that mistake, @OrganicMarble. $\endgroup$ Jul 30, 2021 at 13:56

1 Answer 1


Possibly the term encompasses many protocols, but at least one effect is detailed here:

[Spacecraft Emergency] is a protocol that the Ground Controller (GC), who is responsible for the MCC infrastructure as well as interface to the NASA Space Network, invokes with the Space Network to make sure the ISS gets all available satellite communications assets and bumps other users from using those assets.

If the ISS Program suddenly needs TDRS coverage—such as for an emergency spacewalk—the NASA flight director can declare the TDRS time as critical, thereby forcing other users off the network. Due to the impacts to other uses, which can include loss of science, this is not done unless absolutely required.

The ISS is not unique in the ability to call a TDRS emergency; see for instance the THEMIS satellite.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.