Starlink originally planned to put their first shell at 1100 km altitude. They changed to 550km for several reasons. First, because it offered slightly reduce

Starlink originally planned to put their first shell at 1100 km altitude. They changed to 550km for several reasons. First, because it offered slightly reduced latency. Second, so long as you launch enough satellites, the reduced coverage region for each satellite is offset by having more satellites and hence (other things being equal) more bandwidth per area of land. Third, satellites at 550 km will naturally deorbit in a few years if something fails. So although they plan to actively deorbit the first satellites after 6 or 7 years to replace them with newer ones, if they get something wrong and have a lot of satellite failures, they really won't cause a long term problem. At 1100 km the orbit won't decay for centuries. If you have satellites fail, the rest of the constellation will be doing avoidance maneouvers for a very long time. Thus if your launch costs are low enough and you can mass-produce satellites cheap enough, you want them as low as possible. Somewhere around 500 km is about as low as you want to go, before too much of the satellite mass ends up being fuel to maintain orbit.