Chuck wrote in
:

Jorge, what do you think about John's idea in an earlier post about
using an ion propulsion system to push Hubble into an ISS compatible
orbit? Would that be a viable option or would the development/
transition fuel costs, etc. be too prohibitive?

As far as the ion propulsion goes, the basic concepts were proven with DS1,
but that was a smaller beast than HST. To transfer HST to ISS inclination
in a reasonable time (on the order of months rather than years), you'd need
a larger engine than has been successfully developed so far. Some
development cost but no show-stoppers that I can see.

Likewise, using DS1-class ion engines with HST might work but would require
longer transfer times (on the order of years), so you'd need an engine that
can work reliably over a very long period, or multiply-redundant engines.
Again, no showstoppers, just development work and tradeoffs to be made.

In either case, the fuel costs are negligible compared to development
costs. Take all of the above with a grain of salt - I'm a rendezvous guy,
not a low-thrust propulsion guy. I haven't actually run any numbers on how
long these transfers would take.

The biggest development headache may well be automated rendezvous and
capture of HST. All the automated systems developed to date require their
targets to be cooperative to some degree (i.e. equipped with navaids for
the chaser vehicle). The Russian Kurs system requires a rather elaborate
set of RF antennas/avionics on the target vehicle. The systems under
development for ATV and HTV require the target vehicle to have a GPS
receiver and an array of laser retroreflectors.

Trouble is, HST has *none* of those things - it's a completely passive
target, save the (visual) aids on its RMS grapple fixtures and berthing
mechanism. Attaching the navaids to HST becomes a chicken-and-egg problem -
any automated system that could attach the navaids to HST needs the navaids
to be already there in order to get there in the first place.

The *only* vehicle in existence which can rendezvous with and capture such
a completely passive target is the very vehicle which O'Keefe has ground-
ruled out of the solution: the space shuttle. It was designed to be able to
rendezvous with passive targets - hell, it can even rendezvous with debris,
if the situation required it. Its rendezvous radar is capable of skin-
tracking a target with no transponder, and the crew is equipped with
police-style handheld lidars that are likewise capable of skin-tracking an
inert target.

That is not to say that it's impossible to develop an automated system to
rendezvous and capture a non-cooperative, inert, and (by 2007-08) possibly
slowly tumbling target. But the challenge of developing such a system will,
IMO, be greater than that of getting the ion propulsion to work.

--
JRF

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