This is nearly the same format as BDATAx.DAT:

 +0     WORD    ? (unused?)
 +2   n BYTEs   500 starbase records of 156 bytes each, see BDATAx.DAT.
+78000  WORD    Turn number. If this number does not match the number in
                NEXTTURN.HST, THost prints an error message and uses
                the number from BDATAx.HST.
This is a testing version.
It may be incomplete, and have more bugs (or features) than the public live version at planetscentral.com.