The goal of atmospheric data assimilation is to produce accurate gridded datasets of fields by assimilating a range of observations along with physically consistent model forecasts. The NASA Data Assimilation Office (DAO) is currently upgrading its end-to-end data assimilation system (GEOS DAS) to support NASA's Mission To Planet Earth (MTPE) Enterprise. This effort is also part of a NASA HPCC Earth and Space Sciences (ESS) Grand Challenge PI project. Future Core computing, using a modular Fortran 90 design and distributed memory (MPI) software, will be carried out at Ames Research Center. The algorithmic and performance issues involved in the Core system are the main subjects of this presentation.