Wikipedia Salyut 7 page
Perigee: 219 km
Apogee: 278 km
Wikipedia Skylab page
Perigee: 434 km
Apogee: 442 km

Also from wikipedia:
The mesosphere is located from about 50 km to 80-90 km altitude above Earth's surface.
The thermosphere, named from the Greek θερμός (thermos) for heat, begins about 80 km above the earth.

So, as I suspected, that has nothing to do with it.

__________________
At night the stars put on a show for free (Carole King)

One Earth, One Sky - IYA 2009
All moderation in purple