Can satellites be serviced?

At 550kM, I'd imagine it is next to negligible. The density is on the order of two ten-trillionths that of sea level (21.4 nanograms/cubic meter?).

"Is There Gravity in Space?
Gravity causes every object to pull every other object toward it. Some people think that there is no gravity in space. In fact, a small amount of gravity can be found everywhere in space. Gravity is what holds the moon in orbit around Earth. Gravity causes Earth to orbit the sun."