On the following image, why is it written “astronaut use only”?

Since this power control unit is in space, it can only be used by astronauts. So why write this message?

enter image description here

Cropped from this image:


Description of the image:

A close-up view of the power control unit or PCU, which is the heart of the Hubble Space Telescope (HST) power system. Astronauts John M. Grunsfeld and Richard M. Linnehan, STS-109 payload commander and mission specialist, respectively, replaced the PCU on March 6, 2002, during the third space walk of the mission.

Source: https://commons.wikimedia.org/wiki/File:STS-109_power_control_unit_of_Hubble.jpg

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    $\begingroup$ IDK but those look like handles, and maybe they (or whatever they are attached to) were not strong enough to be used as handles on the ground when the thing was being assembled. $\endgroup$ Commented Jun 14 at 12:20
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    $\begingroup$ @SolomonSlow - I'm pretty sure that's the answer, if you can find any confirmation of this even second hand you could post it as an answer. Reminds me somewhat of the "No Step" signs on airplanes, obviously a different scenario but it's a place on the wing where workers might naturally want to step but it's not strong enough. $\endgroup$ Commented Jun 14 at 13:11
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    $\begingroup$ The other answers make compelling arguments, but I think they're actually to warn aliens that they aren't allowed to climb on HST. $\endgroup$
    – 0xDBFB7
    Commented Jun 14 at 16:11
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    $\begingroup$ @SolomonSlow - or maybe they foresaw the advent of space tourism and they didn't want any "spaceflight participants" messing with Hubble. $\endgroup$ Commented Jun 14 at 18:18
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    $\begingroup$ As you can (barely) see from this answer, some of the handles on the ISS cargo transfer bag say "Zero-G Only". They had to be inclusive to cosmonauts. $\endgroup$
    – user71659
    Commented Jun 14 at 21:16

1 Answer 1


Those handles are very lightweight, suitable for moving the PCU around in a zero-g environment, but on the ground, during construction of the Hubble, trying to pick it up by those handles would probably bend or break the handles and might damage the PCU (either directly by tearing loose, or indirectly when it unexpectedly drops to the floor). By the time the Hubble is in orbit, those labels are no longer relevant, but it would be an important reminder for the (non-astronaut) engineering crew assembling and testing the thing prior to launch.

There are similar handles all over Hubble, and they are not strong. In 2009, Mike Massimino had to rip one of them off with brute force because one of the screw heads had become stripped, and the rail he was pulling on bent significantly before reaching the 60 lbs necessary to to tear it off.

Here's a photo from Jeff Roddin of the ground team testing how much force Mike would need to apply by means of a strain gauge.

A NASA engineer using a strain gauge to bend a metal bar.

  • $\begingroup$ Do you have a reference for your claims? $\endgroup$ Commented Jun 14 at 23:35
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    $\begingroup$ Hubble - From Concept to Success says "Placed in Bays 2 and 3 of the SSM Equipment Section , these units were designed to be safely handled during the service missions. Each module had two large yellow handrails to enable astronauts to easily and safely extract it from its housing and insert the replacement. After the originals had powered Hubble for 19 years, they were replaced by the final servicing mission in 2009. $\endgroup$
    – Richard
    Commented Jun 14 at 23:44
  • $\begingroup$ Great quote, you should include it in the answer. $\endgroup$
    – Eugene
    Commented Jun 15 at 0:39
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    $\begingroup$ @OrganicMarble Added an anecdote from SM4 showing how weak those rails are. $\endgroup$ Commented Jun 15 at 0:53

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