Observations of the Lyman-a emissions from Interplanetary Hydrogen (IPH) atoms are made from Mars’ orbit using a high spectral resolution instrument in echelle configuration. The measurements can uniquely be used to resolve IPH from planetary H emissions and to subsequently determine the brightness, velocity, and thermal broadening of the IPH flow along the instrument line of sight. Planned as well as serendipitous observations, both upwind and downwind of the flow, are analyzed to determine these IPH properties and to examine the variability of IPH brightness with solar activity through the declining phase of Solar Cycle 24. A heliospheric interface model was used to simulate and interpret the derived IPH properties. The results show that the IPH brightness trends with solar irradiance, the flow is fainter downwind than upwind, the IPH brightness is variable and non-negligible compared with planetary emissions, and that deriving thermal properties of IPH requires higher spectral resolution than is presently available. These results can improve the theoretical understanding of solar system dynamics by providing empirical constraints to simulations from the inner boundary of the heliosphere and can guide the development of future interplanetary missions.