Actually, they weren't, the ship was decelerating to get into an orbit, but the decreasing distance created the illusion of increasing speed.
"Space Viking"
Henry Beam Piper
We'll worry about decelerating when we're in shooting distance."
"Space Viking"
Henry Beam Piper
There'd be a Gehenna of a bill to pay when they started decelerating.
"Space Viking"
Henry Beam Piper