Jorge R. Frank wrote:

That plot is not as alarming as it might appear. ISS is now flying through
solar minimum, so it can fly at a lower altitude while maintaining the same
orbital lifetime. And as you pointed out yourself, Zvezda has plenty of
propellant reserves that could be interconnected to Progress to boost ISS
all the way back up to its maximum altitude in a contingency. Right now the
Russians and NASA are taking advantage of solar minimum to optimize
Progress and shuttle payload.



Thought problem:
Something doesn't go right on the next Shuttle flight and it gets
grounded again.
The Progress reboosts are helping, but how long can they keep kicking it
up on there own once the Zvezda's fuel is expended?
Do they have to wait around for the new ESA Jules Verne module to raise
its orbit in lieu of the Shuttle if that occurs?
There is one ironic aspect to all this- if the ISS really had been
completed to the point it was supposed to be today on the original
schedule, it would have been a lot more massive than it presently is,
and those Progress reboosts would be having far less effect than they
presently are.
Having it lag so far behind schedule has kept it light enough for the
Progress' to keep its orbit up despite the two groundings of the Shuttle.

Pat