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In a recent question, uhoh asked about a dish mounted on the outside of the International Space Station (ISS):

What is the function of this (apparently) fixed dish on the ISS?

This turned out to be an ORU with a Space-to-Ground Antenna (SGANT) spare. That has me wondering--how good of condition do these spares stay in, while mounted on the outside of the ISS, when they are not in use?

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In general, pretty good condition. The ORUs are designed to live outside for their functional life (more less the same as station's), so there's generally no reason they can't live outside while they're waiting to be called upon. Many of them have flight support equipment (FSE) that provides some support to keep them alive. These are typically heaters, sometimes telemetry monitors.

They are evaluated for environmental risks just the same as the operational ORUs are -- most of them are better protected in their stored locations, by virtue of the location itself or by dedicated covers. Both of these options generally serve to create a more benign environment for the ORU than it would experience in its deployed, operational state.

Not all spare ORUs are stored outside. Some are small enough (the Sequential Shunt Unit, for example) to be kept indoors and transition through the airlock with the crew. There's a trade-off to be made, however. Keeping an ORU outside presents the opportunity to be able to replace it purely robotically, if it was designed with that in mind.

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  • $\begingroup$ Interesting, thanks! Roughly, what is the life expectancy of an ORU like an SGANT? How long is an SGANT spare expected to sit around before it will be put to use? $\endgroup$ – called2voyage May 12 '17 at 16:26
  • $\begingroup$ Life expectancy per se would be someone else's department, but they are designed to wait indefinitely as far as I know. As far as I understand, the primary causes of failure of a spare would be an MMOD strike (my specific area of expertise), unexpected thermal excursions, environmental materials degradation (vacuum outgassing, AO, and UV), and radiation effects on electronics (both permanent damage and correctable things like EEPROM bit flips). Not all of these are applicable to all the spares, and the deployed ORUs have these plus utilization-based wear and tear. $\endgroup$ – Tristan May 12 '17 at 16:48
  • $\begingroup$ Right, but it's the strikes, degradation, and radiation effects that I'm wondering about. Are these safeguarded against on spares? Or is it really not any more necessary than what is already done for the active ones? $\endgroup$ – called2voyage May 12 '17 at 16:50
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    $\begingroup$ They're designed to live outside for more or less the life of station. Generally the spare locations are more benign, as they have MLI covers (which also protect against AO and UV) and can use station structure for shielding in ways that they would not be able to while operating. $\endgroup$ – Tristan May 12 '17 at 17:05
  • $\begingroup$ Awesome, that's what I was wondering. If you roll that into the answer, I'll accept. $\endgroup$ – called2voyage May 12 '17 at 17:32

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