In this image (NASA original source) from the International Space Station. what is the "lightning rod"-like instrument at the end of the mast in the upper right of the photo?

enter image description here

enter image description here

  • 9
    $\begingroup$ Emitter antenna for the Orbital Mind Control Lasers. $\endgroup$ Mar 30, 2017 at 2:37
  • 2
    $\begingroup$ It looks more like the Space Trailer Hitch Ball.... $\endgroup$ Mar 30, 2017 at 2:53
  • $\begingroup$ Fascinating. Does anyone know what landmass that is on the photo? $\endgroup$
    – r41n
    Mar 30, 2017 at 10:09
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    $\begingroup$ @r41n It's New Zealand looking North over Pegasus Bay with Christchurch to the bottom-left of the bay. goo.gl/maps/A2niy3xogG92 $\endgroup$
    – spender
    Mar 30, 2017 at 13:00
  • $\begingroup$ Looks more like the subspace transceiver antenna array to me... $\endgroup$
    – user
    Mar 30, 2017 at 14:31

1 Answer 1


This is the Floating Potential Measurement Unit (FPMU).

Specifically, the ball is the Wide Langmuir Probe.

enter image description here

picture from here

The FPMU was installed by ISS crewmembers, during an Extra Vehicular Activity (EVA), on the starboard (Sl) truss of the ISS in early August 2006. The system is based on past USU/SDL space instruments and incorporates a Floating Potential Probe, multiple Langmuir Probes, and a Plasma Frequency Probe into one sensor suite.

The FPMU is used to correct the charging models of the station each time a new section is added to the structure. By providing feedback on the effectiveness of steps taken to control charging, the FPMU helps ensure a safe working environment for the astronauts.

USU = Utah State University

SDL = Space Dynamics Laboratory

For more info about how it works, not what it is, see my answer to this question: How do spacecraft measure their own charge? .

  • 12
    $\begingroup$ So it's a space lightning rod! $\endgroup$
    – SF.
    Mar 30, 2017 at 9:18

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