🚀 go-pugleaf

The major problem with using Mir for a manned Mars mission
is that Mir is not designed to protect its crew from the
interplanetary radiation environment.

At most times in the solar cycle, there would be a near
certainty that any unshielded astronauts on a two
year mission to Mars would be killed by solar flares. (At
the minimum of the cycle, the astronauts might make it
--some years have no major flares)

Adding a shielded "storm cellar" to Mir could greatly
reduce the danger.  Probably about 25 gm/cm^2 of shielding
would be needed to allow the crew to survive a major flare
in fairly good shape - extra mass would be roughly:
25 gm/cm^2 * 2*pi*200cm * 200 cm = somewhere around 6,500
kg for a 2meter long, 4 meter diameter "cellar".

Of course, there's also the cosmic ray radiation dose,
which is much more difficult to shield against and is
about 20-40 REM/yr depending on the time in the solar
cycle (more or less inverse to the flare probability)
This is below, but close to, the annual limit allowed for
shuttle astronauts (50 Rem/yr last I heard)

For more details I recommend looking at the AIAA's report
on assessment of technologies for the SEI -- it has a
lengthy section on the need for shielding and the effect
on total mission mass.