Spacewalkers successfully replace failed unit on ISS

Please consider donating to Behind the Black, by giving either a one-time contribution or a regular subscription, as outlined in the tip jar below. Your support will allow me to continue covering science and culture as I have for the past twenty years, independent and free from any outside influence.


Regular readers can support Behind The Black with a contribution via paypal:

Or with a subscription with regular donations from your Paypal or credit card account:


If Paypal doesn't work for you, you can support Behind The Black directly by sending your donation by check, payable to Robert Zimmerman, to
Behind The Black
c/o Robert Zimmerman
P.O.Box 1262
Cortaro, AZ 85652

In a a short spacewalk just under three hours two astronauts today successfully replaced the failed MDM data relay unit on the outside of ISS.

Some additional details about this unit:

The MDM that failed and an identical unit are part of the tier 2 command architecture and relay commands to a variety of critical station systems, including the station’s guidance, navigation and control system, the lab’s stabilizing gyros, the environmental control system, the station’s cooling system and others.

Both tier 2 computers were installed during spacewalks March 30 and March 24 respectively, replacing two older units with models featuring upgraded data processing cards. It is not yet known whether the problem with MDM-1 involved the upgraded components or some other circuitry or software

I suspect this upgraded but failed unit is going to be looked at very carefully.



  • pzatchok

    Yeah nice, but did they check the oil and get the windows also?

    Sort of like car tires. if one is bald and goes flat you can pretty much be sure the others are going to go flat soon also.
    Change them all at once or just drive on one at a time.

  • pzatchok: You should read the story at the link. The unit that failed was installed in March as part of routine maintenance, when they replaced them all because it was time to do it, just as you suggest they do (as if they didn’t). The issue here is that the new unit failed.

  • pzatchok

    But they are using all of the installed units at the same time. All of them are powered up and ready to go.

    This was one of three that do the very same job and only one is needed at any time. Why even keep the third one powered up?

    That is like buying three laptops to manage this web sight and leaving all of them on all the time. And then being surprised when they all fail at about the same time.

  • pzatchok: I do not see from the Honeywell pdf you linked to any solid evidence that all of the installed units are powered up and ready to go. Granted, they appear to be are operated in a manner that allows the back-up to instantly take over should the primary fail, but that does not necessarily require it to be fully operational all the time.

    Regardless, I still say the issue here is that this failed unit was less than two months old. It should not have failed that quickly, which also suggests a more fundamental flaw that needs to be looked at.

  • pzatchok

    They are run like any PC.

    Plugged in all the time and this powered up and ready to start.

    Like your TV. Turned off does not mean powered down. It just means the main components are not operating but the power supply and the remote receiver are powered up. Or your laptop in sleep mode.

    The PDF explains that three do the same job. One is operating fully and passing data. The second is co-processing data to double check the first and the third is in sleep mode waiting for the day its needed to backup one or two.

    Its the scourge of modern digital electronics. I have found that 90% of the time its that little digital remote on/off switch or the power supply that fails. It must be because they are constantly powered up and waiting.

  • Edward

    pzatchok wrote: “Why even keep the third one powered up?

    There can be a variety of reasons to keep electronics powered up while they are not used, but my favorite is thermal control. I have designed a few spacecraft electronics boxes (the physical enclosure, not the boards themselves), and keeping the electronics between the minimum and maximum acceptable operating temperatures is one of the big concerns.

    There is no atmosphere to circulate room temperature air, so the box’s temperature and the enclosed electronics’ temperatures are affected by thermal conduction with the mounting platform and thermal radiation to and from deep space, the sun (occasionally), and nearby objects (the station or the Earth). Because the electronics generate a lot of heat, the box is designed to eliminate the generated heat, keeping the temperature at or near optimal operational temperatures. This means that when the electronics are turned off, then the temperature will drop to low levels, perhaps levels too low for the electronics to operate properly.

    I won’t speculate on the cause of the failure, because I have seen such a large variety of reasons for failures of spacecraft electronics boxes that I don’t dare speculate.

    As pzatchok’s linked Honeywell pdf shows, that single part of the system, the MDMs, has a large number of electronics boxes associated with it (I counted 46). We can see that once on-orbit there is not a high rate of failure of electronics, but that is probably due to the ground testing as well as experience gained from previous on-orbit usage. It will be interesting to learn why that particular box failed so soon after installation.

  • wayne


    very well done period piece:

    “Computer for Apollo”
    M.I.T/Raytheon short, mid 60’s

Leave a Reply

Your email address will not be published. Required fields are marked *