Priorities:
P1 - major functionality, IRAD users waiting
P2 - major functionality, external users waiting
P3 - specialized functionality, potential current or future use
P4 - specialized functionality, historical or minor use
(-) GUI updates for all
(P1) Solar System Barycenter
Ephemeris Models
(P2) Add DE421
(P3) Support user-selected DE files
(P3) Add DE423
(P4) Add other DE-file ephemerides
(P1) Gravity model for user-defined bodies
(P1) Variational terms for non-spherical gravity
(P3) MASCON gravity model
SRP
(P1) Update SRP to handle variational terms
Density models
(P2) Update drag models to handle Variational Terms
(P2) Read solar flux files/geomag for drag models
(P2) NRLMSISE
(P2) Mars-GRAM 2005
(P3) MSISE86
(P3) Mars-GRAM 2010
(P3) Venus-GRAM
(P4) U.S. Standard Atmosphere
(P4) MET
(P4) Mars-GRAM (others)
Finite Burn
(P2) Variational terms for spacecraft mass
(P2) Albedo/Thermal
(P2) Relativity
(P2) Earth tides
(P3) Spacecraft box-wing model
(P3) Spacecraft outgassing
(P3) Celestial body outgassing pressure
(P3) Tri-axial ellipsoid shape model
Body ephemeris
(P3) Variational terms for ephemeris errors (?)