BREAKING: Small but finite chance ISS to be hit by debris

By Phil Plait | March 12, 2009 10:01 am


First, everything is fine! The ISS is safe, and the astronauts are alive and back to work.

Here’s the story. Today at 16:39 (UT), a small piece of space debris passed by the International Space Station. Orbit predictions put it close enough to the ISS to be a real danger, though the actual collisions odds were low. Astronauts closed all the hatches in the station and moved to the Soyuz escape module to wait it out. After the debris passed safely, they emerged, opened up all the hatches, and got back to work.

It’s still not known as I write this just how close the debris passed. It was a piece of a used booster called a Payload Assist Module, a device used to boost satellites from low Earth orbit up to geosynchronous orbit. Robert Pearlman from CollectSpace told me:

This object is the “yo weight” from 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.

The mass is probably less than 1 kg, but because it’s basically a piece of string, the cross-section is large for its mass.

A 1 kg mass moving at a relative speed of several kilometers per second could have done vast damage to the ISS, especially since it was not compact like a chunk of metal, but extended. It could have torn a huge hole in the station, and I imagine the astronauts are breathing a huge sigh of relief.

The good news here is that this object, though small, was tracked well enough to give the astronauts on the ISS plenty of time to evacuate… and don’t overlook the fact that the astronauts did a great job handling the situation (whereas I would be very busy screaming and wetting myself). This was an extraordinary event handled by extraordinary people. My sincere thanks to those who took care of it, and congratulations as well.

Also, my thanks to Nancy Atkinson of Universe Today for breaking this story and updating Twitter so fervently.

Here is my original post on this:

Nancy Atkinson is on top of the story that a small piece of orbital debris might hit the space station at 11:39 Central (US) time, less than 40 minutes from the time I write this. Astronauts on board are preparing for this potential impact. Follow her Twitter feed for more info. You can also watch NASA TV for info.


Comments (64)

  1. Todd W.

    I smell a Hollywood blockbuster!

    Seriously, though, I really hope that if the debris hits, it doesn’t cause any major damage.

  2. gss_000

    Don’t forget to give credit to Chris Bergin over at NASA Spaceflight, who Atkinson credits in her post.

  3. While I hope everyone stays safe, maybe this will be the event that finally gets governments taking this subject seriously.

  4. Strange. NASA TV appears to be showing file footage.

  5. I hope everyone stays safe. Do we know what the debris is from or how big it is?

  6. jwwalker

    A finite chance as opposed to an infinite chance? I think you mean a small but nonzero chance. Sorry, pet peeve.

  7. tlivernois

    Steve, under Channels select “Live Space Station Video.” The channel is currently just showing a live map depicting where the ISS is, but it’s also playing the communications (which have been largely silent).

  8. Michelle

    Here,s to hoping it won’t cause too much damage or will totally miss.

  9. @Steve – Yeah, I see that too. I was hoping for updates or something

  10. BILL7718

    You have to switch channels on NASA TV to the Live Space Station feed. It is showing a map as the video, but there’s audio of the communication between the crew and the ground. They’ve moved the crew into the Soyuz.

  11. Confirmed, NASA TV is showing file footage of a bunch of Americans and Russians patting each other on the back and drinking.

    Which I endorse, of course.

    But I’d still like some updated news…

  12. Kyle

    Go to the ISS feed, its live, 2nd choice on the menu list

  13. Ray

    What if they do get hit? What’t the plan then?

  14. Kyle

    My bad 3rd choice

  15. dre

    NASA TV! AAAAAAARGH! Why are you so lame when you should be so great?!

  16. Kyle

    They are buttoned up in the Soyuz capsule, not sure if they closed the door, switched over to Russian control and I don’t speak Russian

  17. Michelle

    @Ray: That would’ve meant they bail with Soyuz. They were hiding in it.

  18. Woot, sounds good! No impact.

  19. dre

    No impact. Relieved.

  20. holastefan

    That was a really scary few minutes hitting the F5 refresh. Let’s not do that again. (I know, this will probably become more and more common)

  21. Whew, it’s OK. They’re starting to leave Soyuz. Then comes the leak test

  22. Kyle

    Yahoo they missed the debris!!!!!!!!!! Going back into the station. Naughty astronauts, left their checklist on the station. Oops.

  23. Michelle

    PHEW!!! they’re OK!

  24. Lee

    That was tense!

  25. Jon D

    that was really intense.. glad everything’s ok!

  26. Anton P. Nym

    Phew. Glad everything turned out okay.

    I caught part of me being relieved that the Shuttle launch yesterday was scrubbed so that the orbiter wouldn’t be vulnerable too… then I caught myself, as I take it they wouldn’t have rendezvoused with the station yet, would they?

    — Steve

  27. Until the next piece of space junk approaches…

    Really though, isn’t the risk just going to increase with time as more junk accumulates up there?

  28. holastefan

    Fincke communication from a few moments ago: “Houston, if you find out later, we would be very interested in knowing how close it came to us.” (paraphrased)

  29. holastefan

    Didn’t know that NASA TV had continuous live feeds from the ISS (thought it was only during launches). Pretty cool.

  30. If that PAM (Payload Assist Module) had hit the ISS it would have spun it like a top. This would have made the Soyuz hard to control at best, uncontrollable at worst. -YIKES!-
    Twitter was all over this one. Main stream media has largely missed it.

  31. Quiet_Desperation

    Finite chance? Can’t they just rotate the station 90 degrees and make the chance imaginary?

    Wow. Bad math humor. Ignore me. :-

    Seriously, though, I really hope that if the debris hits, it doesn’t cause any major damage.

    Isn’t it better (philosophically, at least) to hope that it doesn’t hit at all?

    I know! Let’s all pray! ūüėõ

  32. Todd W.


    I thought that the hoping it doesn’t hit went without saying.

  33. mosse

    Am I the only one that is just constantly amazed that we were even able to track a piece of debris so small, floating over the planet, and predict that there was a risk of it hitting the ISS? I’m guilty of losing my awe of computer technology (shouldn’t I be in a total state of amazement that I can send photos from my phone to a friend in Germany within seconds?), but stuff like this just makes me feel like a kid again.

  34. CNN was on it when the announcement was made. But I haven’t been watching since then.

  35. Whew! A near miss.

    “Isn’t that a near hit?”

    Shut up Carlin, you’re dead!

  36. Chris

    I too am amazed at the resolving power of their radar. By comparison, the lost toolbag must look like a 747.

  37. MH

    “Small, but finite”?

    Can you give an example of a number which is small but NOT finite?

  38. @MH:

    pi / 100000000000000000000000000000000000000000000000000000000000000

  39. Caleb

    Phil, you should do a write up on the Kessler Syndrome.

  40. Charles Boyer

    “Can you give an example of a number which is small but NOT finite?”

    Delved much in quantum mechanics? ūüėČ

  41. John


    I think “small but finite” simply means “not infinitely small” which, I guess, means “not zero”

    Though I never did well in math, so somebody please correct me if I’m wrong.

  42. holastefan

    Phil said: “whereas I would be very busy screaming and wetting myself”

    Funny! I believe that’s listed under “Top 10 Things to Not Say During Your Astronaut Interview”. It comes right after “I am afraid of rockets”.

  43. Caleb


    ‚ÄúTop 10 Things to Not Say During Your Astronaut Interview‚ÄĚ

    #1 has to be Jack Handy’s quote:

    “Fear can sometimes be a useful emotion. For instance, let’s say you’re an astronaut on the moon and you fear that your partner has been turned into Dracula. The next time he goes out for the moon pieces, wham!, you just slam the door behind him and blast off. He might call you on the radio and say he’s not Dracula, but you just say, ‘Think again, bat man.'”

  44. holastefan

    @Caleb: He might call you on the radio and say he’s not Dracula

    I possibly may have just soiled myself laughing. Classic!

    It’s a good thing I wasn’t an ISS crew member today — I would have just grabbed the keys to the Soyuz, and said “I’m outta here!!” instead of waiting for any Houstonian parental permission.

  45. RossD

    Has the ISS crew ever gotten this close to evac in response to space junk in the past? Surely NASA isn’t suddenly being more cautious about potential collisions just because of the recent Iridium collision raised public awareness of space debris, but it is interesting that the two happened so close together, and I suspect the mainstream will give more attention to this incident because of that coincidence.

  46. Sili

    Can’t we put big (friggin’) lasers on the ISS to deflect this stuff? They’re wasting tonnes of money of Bush’s Star Wars II (Electric Boogaloo), might as well put them to *good* use.

  47. kebsis

    How do they know what the piece of debris was from?

  48. Todd W.


    My guess is that they’ve been tracking it since it became space debris.

  49. Torbjörn Larsson, OM

    What a mess.

    Has the ISS crew ever gotten this close to evac in response to space junk in the past?

    The BBC article claims:

    It is unclear how many times crew members had been told to enter the Soyuz, but officials said this was “not the first” occasion. Flight controllers have moved the ISS eight times over the past ten years in order to avoid debris. [My bold.]

    I believe I read that it happened last in December, but I can’t seem to find it now.

    [OB BA pun:]


    Considering it happened over 100 km up, shouldn’t that be an up2date?

  50. Scott Belyea

    “A finite chance as opposed to an infinite chance? I think you mean a small but nonzero chance. Sorry, pet peeve.”

    “Small but nonzero chance “? As opposed to “small but zero chance”?

    I think he means “a small chance.” There’s no “…but x” qualification needed.

  51. Cairnos


    Lasers powered how? Also wouldn’t you get the same problems that you encounter with meteors, as in the mass is still moving towards you at a really unpleasant rate of knots? Not to mention that there might be some protest at turning the ISS into the ultimate satellite killer.

  52. coolstar

    Our BAD Astronomer mentions that the closing velocity was km/sec but I’ve not been able to find verification of that anywhere. Does anyone know what the deltaV relative to the ISS of this piece of junk REALLY was? On average, deltaV’s should be no where near several km/sec. Of course, hundreds of miles per HOUR for junk of this mass would still be quite dangerous.
    Maybe it’s time for the ISS to get it’s own AESA, so alarms can be sounded and the station even moved automatically for low velocity approaches undetected from the ground.

  53. MadScientist

    So, nuts and loose screws are threats to people in space as well?

  54. Mile

    Send out Chuck Norris to handle the debris :)

  55. Dunc

    whereas I would be very busy screaming and wetting myself

    Well, that’s one of the great things about working in a space suit – you can wet yourself and nobody notices.

  56. Wendy

    Astronauts, man…. They’re SO BRAVE!! They deserve more money. Much more.

  57. Torbjörn Larsson, OM

    Lasers powered how?

    Star wars x-ray lasers were powered by nuclear explosions. According to Wikipedia the tests were inconclusive.

    [But also, nations would take a dim view of launching nuclear material, as well as having nuclear devices in orbit.]

  58. Ben

    Perhaps a stupid question.

    We have three people on the station. So there is, roughly speaking, probably a 30 to 1 chance that passing space debris would hit have hitable parts of the station, not the lander. (Or do they reorient the ISS to try to keep the lander safe. What are the odds that the lander could still be hit?

    But more importantly. Wouldn’t it be better to leave one astronaut in the station and send two to the capsule?

    Indeed. I suspect If all modules were sealed, it might be better to spread the space explorers to the various sections with food, water, (Spacesuits?) in order to ensure a better survival chance of all?

    These are questions from someone who hasn’t done the math and (perhaps most stupidly) doesn’t know the orientation of the craft at any given time.

    But to put all your eggs in one basket. Why does NASA do that?


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