Its research program emphasizes astrodynamics, satellite meteorology, oceanography, geodesy, and terrestrial vegetation studies. Other areas are dark matter, cosmology, astrodynamics, binary stars, solar neighborhood, solar physics and astrobiology. Listed below is a circular orbit in astrodynamics or celestial mechanics under standard assumptions. Spacecraft are further studied in the subfield of astrodynamics. In astrodynamics, canonical units are defined in terms of an object's reference orbit. Astrodynamics was developed by astronomer Samuel Herrick beginning in the 1930s. His astrodynamics class was taught by Richard Battin. Ehricke was an accomplished practitioner in the field of astrodynamics and its applications. Different astrodynamics theories are used to maintain these catalogs. SEI also developed a free interactive Astrodynamics Lab for use by teachers and educators.