Statement of relevance

This answer to Highest DC voltage ever intentionally produced in space? mentions particle beam experiments done from the Space Shuttle while it flew through Earth's atmosphere at an altitude of 990,000 feet in a region that was substantially ionized. In fact it was in the ionosphere and we can also call this area of Earth's atmosphere space.

Since this Shuttle-based experiment was exploring the nature of Earth's atmosphere, we can also say it was studying atmospheric effects... in space. We can even say that this effort was space exploration.

Thus, this question self-demonstrates that it is on-topic here, because it is relevant to space exploration.


This answer sent me searching for "SEPAC" and I found a copy of Space Experiments with Particle Accelerators (SEPAC) and the button labeled Deepblue.Lib.Umich.Edu provides an open access downloadable copy of the paper.


The Space Experiments with Particle Accelerators (SEPAC) were conducted as part of the ATLAS 1 Speeelab mission from March 24 through April 2, 1992. One of the objectives was to perform artificial aurora experiments from orbit using high-power electron beams and the optical imaging capability of the Atmospheric Emissions Photometric Imaging (AEPI) instrument/1/. The SEPAC electron beam accelerator (EBA) is capable of injecting electrons with beam energies up to 6.25 keY. At this energy the perveance-limited electron gun can emit beam currents of up to 1.21 A. The previous flight of SEPAC on Spacelab 1/2/ showed that at these levels special means of neutralizing the Shuttle are necessary; therefore, for ATLAS 1 the SEPAC instrument complement included three 122-cm diameter conductive spheres for charge collection and a 1.6 A hollow-cathode Xe+ plasma contactor. The flight data show that the effectiveness of these charge neutralization devices was sufficient for injection of electron beams up to the highest beam currents available with the SEPAC EBA. Another objective of SEPAC was to conduct neutral xenon releases from the plasma contactor (PC) to test the predictions of the critical ionization velocity theory.


  1. How do these two types of objects help the Shuttle remain neutral while its exploring the atmosphere by emitting a 1.2 amp electron beam?
  2. Are there any drawings or photos of the three 1.22 meter spheres, or the hollow cathode Xenon ion source?
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    $\begingroup$ There's a photo on the STS-45 wikipedia page en.wikipedia.org/wiki/STS-45 Some low res ones on the KSC mission page science.ksc.nasa.gov/mirrors/images/html/STS45.htm There are many papers about SEPAC on ntrs. $\endgroup$ – Organic Marble Sep 19 '20 at 14:11
  • $\begingroup$ @OrganicMarble intriguing! I expected either shiny metal or space-grade chickenwire for the spheres, there must be something interesting behind them being coated black. (can't get the link to KSC working right now, will try from another connection, happens from time to time). $\endgroup$ – uhoh Sep 19 '20 at 15:02
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    $\begingroup$ The KSC ones aren't really worth bothering with because of their terrible resolution. There's a nice one of pallet / igloo integration prelaunch here artsandculture.google.com/asset/… $\endgroup$ – Organic Marble Sep 19 '20 at 15:19

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