Investigation of the energetic and radiative transfer properties of exospheric hydrogen with the Hydrogen Emission Line Interferometric eXplorer (HELIX)

The Hydrogen Emission Line Interferometric eXplorer (HELIX) is a SmallSat mission concept to study the vertical distribution and transport flux of geocoronal hydrogen that is transitioning from Earth's upper atmosphere into the exosphere. This investigation is conducted using a dual channel, all-reflective Spatial Heterodyne Spectrometer (SHS). A SHS is a type of Fourier Transform Spectrometer that divides incoming light into 2-beams with a diffraction grating that interfere to produce a linear fringe pattern from which spectral power is obtained. The HELIX SHS has a dual ruled grating that is can be simultaneously aligned to observe the Ly-α and Ly-β transitions of hydrogen. It has a spectral resolving power of ~200000 that separates the geocoronal lines from contaminating background emissions and allows exploration of the thermal and non-thermal populations of hydrogen. HELIX is designed for incorporation into a high-heritage SmallSat spacecraft platform that would be launched into low Earth orbit for a 12-month mission. This presentation describes the SHS concept, the mission requirements for HELIX, and the top-level on-orbit performance of the instrument.