Please consider the following excerpts from the necronaut's entry:
Gargantuan Undead (Chaotic, Evil, Extraplanar)Environment: Infinite Layers of the AbyssAlignment: Always chaotic evilNecronauts are created by demonsWelcome to WotC, where double-checking whether you got the right fiendish language is taboo.A necronaut speaks Infernal and rarely bothers to learn other languages.
Anyways, other than that the necronaut is a fairly uninteresting monster. It has a fairly unoriginal 'eat corpse to get HP back' ability, Trample, Unholy Toughness, and a weird vulnerability to undead-creating spells, all of which is made meaningless by 32 undead RHD. -0 LA unless this somehow loses a few dozen hit dice.
also why couldn't a 'necronaut' have been an undead astronaut? they wasted a perfectly good name!