*Athena* is a grid-based code for astrophysical gas dynamics being
developed with support of the NSF Information Technology Research (ITR)
program. This *User's Guide* describes version 1.0 (hereafter
referred to as `athena1.0`); the first publically released version of the code.

The primary goal of the *Athena* project is to develop a robust and
flexible code with multiple-physics options in a variety of geometries,
using flexible gridding methods, and optimized for modern shared or
distributed memory parallel machines. This code is necessary to enable
increasingly sophisticated investigations of a wide variety of problems
in astrophysical gas dynamics. As part of the project, *Athena*
will be made freely available to the astrophysics community, along with
complete documentation and web-based training material. Although the
capabilities of `athena1.0` are quite restricted compared to the ultimate
goals of the project, it is being freely distributed consistent with
this open source philosophy.

The `athena1.0` code contains algorithms for
the following:

- compressible hydrodynamics and ideal MHD in one spatial dimension
- ideal gas equation of state with arbitrary (including , an isothermal EOS)
- second- or third-order characteristic interpolation in the primitive variables
- numerical fluxes computed using Roe's, or the HLLE, approximate Riemann solver (the HLLC solver is also available but for hydrodynamics only)

There are four basic sources of documentation for *Athena*

*The Method Paper:*(in preparation)*The User's Guide:*(this document) gives an overview of how to install, configure, compile, and run`athena1.0`and visualize the resulting output. Necessary reading for all users.*The Programmers's Guide:*(in preparation)*Web-based Tutorials:*(in preparation)

Users of `athena1.0` should have a basic, working knowledge of the Unix
operating system, access to a C compiler, and a graphics package for
plotting one-dimensional tabular data. Some familiarity with code
management using Makefiles is helpful but not necessary.

The equations solved by `athena1.0` can be written in conservative form as

(1) |

(2) |

(3) |

For isothermal MHD, the fifth components of and are dropped, and equation (3) is replaced with , where is a constant (the isothermal speed of sound). For adiabatic hydrodynamics, the last two components of and are dropped, as well as all terms involving the magnetic field in equations (2) and (3). For isothermal hydrodynamics, the last three components of and are dropped, as well as all terms involving the magnetic field in equations (2) and (3).