In article ,
Christopher M. Jones wrote:
That's why I mentioned arcjets...

Are there arcjets large enough for this task? I was
under the impression that there were none large enough
for main propulsion tasks. Although for simple orbital
maintenance, something that large might not be needed.

Some quite large ones have been tested, although not recently. The
off-the-shelf ones *are* small, but you could use a cluster of them.

Even resistojets would be worth considering. You don't actually *need*
terribly high Isp for this; the point of the electric thrusters is more
that they can run on storable non-contaminating fluids.

Agreed (or solids, of course). And perhaps solar sails or
electrodynamic tethers, though those are much less easily
compatible with telescope operation than even ion engines.

Solar sails almost certainly can't be made to work for this because of
air drag. Except in very favorable circumstances, it's difficult to get
an excess of thrust over drag with a solar sail below 1000km or so.

Electrodynamic tethers are a possibility, although as you say, making
them compatible with telescope operations would be tricky.
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