In article om,
says...

Update:

Gennady Padalka took pictures of the Progress passing underneath the
station, but too far to reveal sufficient details on what could have
happened. They will try again.

A russian news source said that Progress' orbit decays by 100m per day.
I assume this number will grow somewhat exponentially ?

Yes, it's a positive feedback loop. You don't say if that's the drop in
apogee or perigee.

In the case of an elliptical orbit (Progress is said to have 190/270km)
would eccentricity increase during orbital decay since at perigee, the
ship slows down more than at apogee ? Or does the loss of energy ar
perigee cause both apogee and perigee altitudes to drop at same rate ?

Yes, the apogee drops more, in altitude, than the perigee due to he
reduction in velocity at perigee.

Would "re-entry" be defined as the time when the current
perigee becomes apogee and perigee becomes "0" ? (lasting
about half orbit) ?

When it crashes to the ground, that's re-entry, IMHO. Since the
atmosphere tapers off as altitude increases, there is no "hard line"
which denotes where atmospheric entry "begins", other than on the last
orbit, which the satellite never escapes again.

Is this a sudden/violent change once perigee hits a certain
altitude with perhaps the change of orbit to peridee=0
happening over 1 or two orbits, or is that smooth transformation
over multiple days which ends up circularizing the orbit ?

Are there situations where perigee would cause some heating
of the ship which would then emerge from atmosphere to reach
apogee and cool down, and drop into atmosphere again ?

There is always some heating of the ship due to atmospheric drag. Less
drag means less heating, but if you have drag, heating is present.

References:

Orbital decay
http://en.wikipedia.org/wiki/Orbital_decay

This links to an interesting paper by the Australians (Skylab, anyone?).
See the last page of this document for what happens to an eliptical
orbit which is (obviously) subject to more atmospheric drag at its
perigee.

Satellite Orbital Decay Calculations
http://www.ips.gov.au/Category/Educa...Weather/Space%
20Weather%20Effects/SatelliteOrbitalDecayCalculations.pdf

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer