Nav mesh without ground?

I am wanting to make a game thats in space but im trying to make enemy movement and it apparently needs this nav mesh. ( is there a way to do it without nav mesh) or is there a way to set a nav mesh without having to walk on it

how about faking it?

lets say playarea is 500x500x500,
make navmesh in 500x500, and disable visible parts of it.

have enemy object that moves on navmesh on ground, and have another object parented under it that chagnes y position or something like that.