Why doesn't gravity accelerate things in space? If I'm on a planet like Mars with little air density its low gravity can accelerate me to higher speeds on earth because there's not enough resistance to stop it like on earth. So why don't objects in space accelerate continually? You can use gravity assists to accelerate like pic related when you're in the sphere of influence of a planet but objects in space are always under a sphere of influence mainly the suns.

How is it that while being in orbit you maintain a constant velocity?