Thanks for cluing me in. I'd forgotten about energy release asymptotically approaching infinity. Same thing with mass, and consequentially gravity, which suggests that a starfaring civilization will need traffic control and speed limits: "slow to <.1 c inside a star system", for example, to avoid wreaking such havoc as dragging the planets out of their orbits.

Science fiction writer CJ Cherryh has done a fine job of realistically depicting problems like mass and navigation at relativistic speeds. She's written some vivid descriptions of a starship entering a system at high velocity, essentially blind and mute (overrunning its own electromagnetic wave front), and system traffic control frantically trying to plot its course within an expanding cone of uncertainty, while in-system craft desperately try to get out of reach of its huge effective mass. Her attention to detail leads to descriptions of space combat that ring true for their similarity to submarine combat, with all its extrapolation and guesswork; and hours of tedium punctuated by moments of extreme terror. Excellent stuff.

--Robert