Orbital rescue "tug"... how practical?

Hi...
I am not at all versed in the complexities, energy
requirements, and economics of spacecraft orbits,
either near-earth or within the overall Lagrange
orbital zoo. But the recent demise of the Kepler
spacecraft mission leads me to ask a naive question.

What would it take to develop one or more Space-
Tugs, either to live in near-earth orbit or at
least launchable from the usual launch facilities?
Such an unmanned vehicle's only job would be to
tend ailing spacecraft, perhaps merely to journey
out to, say, the L2 point, grab the patient and
haul it slowly in to be adjusted by human crews
on perhaps the ISS. The tug's propulsion might
be an ion drive, solar and/or radioisotope powered,
with a tank of argon for propellant. I cannot
be the only one who's thought about such a way to
save or enhance missions, unworkable as it might
be.

[[this is really more about "space science" than astronomy per se,
so I've set followups to sci.space.science]]

stargene wrote:
What would it take to develop one or more Space-
Tugs, either to live in near-earth orbit or at
least launchable from the usual launch facilities?
Such an unmanned vehicle's only job would be to
tend ailing spacecraft, perhaps merely to journey
out to, say, the L2 point, grab the patient and
haul it slowly in to be adjusted by human crews
on perhaps the ISS.

This idea is a perennial "wouldn't it be nice if" proposal. It
sounds tempting, but there are a lot of potential (and expensive!)
problems, so it's never (yet) been built.

Some misc problems include:
* it's often hard for even human astronauts to grab a satellite;
doing this by long-distance remote control would be harder
* ISS doesn't really have a currently-empty "garage" facility where
astronauts could work on an ailing satellite
* ISS's astronauts have essentially no spare time available for
satellite repair work
* the USA might not allow ISS to be used for repair of "unfriendly"
country's satellites, and "unfriendly" countries might be worried
about possible sabotage of their satellites during the repair process

Propulsion for a space tug is a BIG problem:
* any low-thrust scheme like solar-electric ion propulsion is going
to spend a LONG time spiralling in and out through the Van Allen
radiation belts to get between ISS (in low orbit) and a high-orbit
satellite; this means expensive/heavy radiation shielding on the
space tug AND a high risk of the satellite suffering serious
radiation damage
* High-thrust chemical rockets would need a LOT of fuel for a space-tug
mission: the space tug needs (very roughly speaking) 4 major velocity
changes (rocket burns) for the mission
[#1 to leave parking orbit,
#2 to rendezvous with the to-be-rescued satellite,
#3 to enter an orbit back to the "garage", and
#4 to rendezvous with the "garage" again]
All that fuel would need to be carried up to ISS in the first place,
which further runs up the cost.

Apart from the fuel cost, actually using this system wouldn't be cheap:
astronaut training (for the remote-control "grab-a-satellite" maneuver
and for the "garage" repairs back at ISS) is VERY expensive. I seem
to recall that the first Hubble Space Telescope repair mission cost
something like US$ 450 million; maybe the later servicing missions
were a bit cheaper, but they were still not at all "cheap".

And finally, the typical malfunctioning satellite is close to the
end of its design lifespan. The satellite owner would get to choose
between the space-tug-repair of what would remain an old satellite,
and launching a new satellite (which takes much less rocket fuel,
because there's no return trip or ISS rendezvous involved) which
could use the latest technology.



Returning to something closer to astronomy, it's instructive to
consider the experience of the Hubble Space Telescope. Recall that
the HST primary mirror was beautifully made... with the wrong shape.
Being able to service it on-orbit saved the mission. And by replacing
the science instruments, the successive servicing missions have vastly
increased HST's science capabilities.

But I have heard it suggested that we might in the end have spent less
money if we'd just abandoned the defective HST and built and launched
a new one, and similarly launched new ones every N years with new
instruments. Alas, I have never seen a serious analysis of this
tradeoff.

It is worth noting that the next-generation James Webb Space Telescope
will be launched into a very high orbit which will NOT be acessible
for repairs if anything goes wrong.

ciao,

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
on sabbatical in Canada starting August 2012
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam