HIRSL: a High-Resolution Spectrograph in Lyman-α (HIRSL)

A recent multi-year NASA study of an Interstellar Probe mission to the solar system and into the interstellar medium (ISM) resulted in a consensus for a set of baseline scientific instruments for the Probe mission. One of these instruments is a UV spectrograph to measure the velocity distribution of neutral hydrogen atoms both from the ISM and from charge exchange reactions with solar wind protons. The properties of these atoms reveal much of the interaction dynamics between the solar wind and ISM at the heliospheric boundary – a high priority for NASA as detailed in the decadal survey objectives. We propose a UV instrument that will significantly improve past/existing measurements, as it will include a high dispersion capability to resolve atomic motions through their doppler shift in scattering the bright solar Lyman-a emission. The instrument concept described herein is referred to as HIRSL: a High-Resolution Spectrograph in Lyman-a. Such an instrument has heritage from being flown on a sounding rocket and on a Mars mission. Including such an instrument for an interstellar probe places strict limits on the mass, size, power, and data rates beyond what has been flown to date. It is critically important to develop the instrument concept to show what is feasible in the context of the Probe mission, and the result of the proposed work would be a flight-ready, small-sat-friendly design module suited for the Ly-a investigation for the NASA Interstellar Probe mission. We propose to perform a design study in the parameter space of size and capability and to then breadboard a high spectral resolution echelle spectrograph for observations of interplanetary hydrogen at Lyman-a. The instrument concept proposed herein will first study a number of optical/mechanical configurations including ray tracing to discover the best system in terms of mass, size, sensitivity, and spectral resolution. We will assess the advantages of an optically-folded echelle spectrograph system over that of a compact spatial heterodyne spectrometer as well as over a double monochromator high-resolution grating system. The proposed efforts raise the instrument concept TRL from 3 to 5. The resulting instrument will allow for unprecedented resolution of the Lyman-a emissions of separate populations of H atoms. The measurements will facilitate determination of the flow velocities, number densities, and temperatures that all directly reflect interaction dynamics at the heliospheric boundary. The proposed instrument is relevant to NASA Heliophysics as described by the 2018 Strategic Plan and Science 2019-2024 that emphasizes the need to understand the Sun and its interactions with the solar system and the interstellar medium. Specifically, a recommendation from the 2003 decadal survey calls for "Understanding heliospheric structure... and the interaction of the solar atmosphere with the local interstellar medium. What is the nature of the interstellar medium, and how does the heliosphere interact with it?". HIRSL is relevant to H-TIDeS, ITD-NHT element as it will “innovate instrument and technology development that may be proposed as candidate experiments for future space flight opportunities”, per ITD requirements; specifically, in supporting "the development of instruments, sensors/detectors concepts that show promise for use in scientific investigations on, or give rise to, future heliophysics missions." Additionally, the PI of the team is a scientist from outside of the Heliophysics community and will be leading the instrument development effort while bringing in her experience with UV high spectral resolution planetary studies and instrument calibration to a heliospheric application. Co-I/Mentor Walsh is well versed in Heliophysics applications and he is integrated within the community. The instrument concept has low technological risk due to the high heritage and experience of the investigating team.

Support NASA's Heliophysics strategic science objectives to understand the Sun and its interactions with Earth and the solar system, including space weather. This will be achieved by developing/demonstrating instrumentation technology necessary to address the following science goals: Explore the physical processes in the space environment from the Sun to the Earth and throughout the solar system; Advance our understanding of the connections that link the Sun, the Earth, planetary space environments, and the outer reaches of our solar system; Develop the knowledge and capability to detect and predict extreme conditions in space to protect life and society and to safeguard human and robotic explorers beyond Earth.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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