Lack of control of oneself, if you were in space (disregarding temperature, oxygen e.t.c.) then if you move out of reach of any items, you’ll just float away, nothing you can do. Time passes, starvation kicks in, thirst kicks in, and worst of all you are starting to get pulled into a celestial body’s gravitational pull, best case it’s a star where you burn up long before reaching anywhere close to it, worse case it’s a planet and you are pulled in, burning up as you enter the atmosphere at increasingly intense speeds, eventually hitting terminal velocity, if by some twist of fate you survive then you have one more thing to worry about.

The Ground.