Instrumentation Solutions and Constraints for a Long Duration Interstellar Probe Mission

A mission that traverses through our solar system, past the boundaries of our heliosphere, and out of our habitable astrosphere to the very local interstellar medium (VLISM) provides a unique opportunity for various in-situ and remote observations during this long journey. The Interstellar Probe mission concept explores a near term, pragmatic basis for designing such a mission, prioritizing critical science measurements while identifying and working with the engineering constraints that come with a long duration mission operating far away from Earth. One of the many challenges of such a mission is selecting instrumentation that will collectively meet science requirements over a long baseline. In order to accomplish this, a variety of instruments will be need to be included in the payload, while keeping in mind size, mass, and power constraints for the mission. These may include particle and field sensors, imaging spectrometers, spectrographs, mass spectrometers, and dust analyzers.

Magnetometers (MAG), placed on a boom away from the spacecraft, will be one of the most critical instruments in the payload. With the exception of composition analysis and particle detection, magnetometers are capable of answering many questions related to the nature of the heliosphere, VLISM, and interactions between the two. While both vector helium magnetometers and fluxgate magnetometers have heritage, due to the lengthy duration of this mission fluxgates may provide a more reliable instrument.

Another set of critical instruments will be a particle suite that covers a wide range of energies. Particle sensors will play a key role in learning more about our heliosphere and VLISM, providing insight into everything but the neutral hydrogen wall. The suite would most likely include four sensors. First, a plasma system (PLS) would detect thermal ions and electrons up through light pick-up ions (PUI) with energies in the 10s-10000s eV. Detecting energetic ions, electrons, inner source PUIs, and PUI in the ISM would require an energetic particle system and dedicated pick-up ion instrument (EPS and PUI) for particles with energies 10s-1000s keV. A cosmic ray system (CRS) would account for the highest energy particles, observing anomalous cosmic rays (ACRs) and galactic cosmic rays (GCRs) with energies most likely ranging from 1-1000 MeV. Each of these systems would need as close to full coverage of the sky as possible, most likely achieved through angular coverage provided by a spinning spacecraft.

The final particle and field sensor that might be included on such a mission is a plasma wave instrument (PWI). This would support measurements made by the magnetometers and particle suite, enabling a better understanding of the size and shape of the heliosphere, particle acceleration in shock regions and the heliosheath, the structure and nature of the heliopause, and properties of the VLISM and GCR spectra outside the heliopause. While the measurements would most likely be made with four components spaced 90° from each other, all perpendicular to ram direction, determining the length and type of antenna used for this instrument is a trade between plasma wave science, guidance navigation and control capabilities, and mission operations.

Another critical sensor suite would involve energetic neutral atom (ENA) imagers, where the suite might include one or more imagers designed to image at different energy levels (the low energy ENA-L at 10-2000 eV, medium energy ENA-M at 0.5-15 keV, and high energy ENA-H at 1-100 keV). ENA imagers would result in a better understanding of the force balance and ENA ribbon, as well as solar/heliosphere/VLISM interaction and influence on each other. In particular, an ENA-H that has the capability to point back at our heliosphere once we are well into the VLISM would allow scientists to gain insight into what our astrosphere looks like from the outside. While the two lower energy ENA imagers would only require noseward hemisphere angular coverage, in order to perform the study of the heliosphere from the outside the ENA-H would need full sky coverage with a sun exclusion zone.

A neutral mass spectrometer (NMS) would provide key compositional insight during the mission by measuring neutral gas and dust in the VLISM, as well as the neutral hydrogen wall and neutral ISM gas and dust inside the heliosphere. Direct measurements of elemental and isotopic gas compositions of the VLISM would place an important constraint on models of stellar nucleosynthesis which holds implications for the formation of matter in the galaxy. This would enable a much better understanding of the properties and potential history of the ISM as a whole. The instrument would be placed facing the ram direction. Co-boresighted to perform complementary measurements to the NMS would be an Interstellar Dust Analyzer (IDA), which would further establish properties of the VLISM and how it affects our heliosphere. It would also provide important insight into the formation of planetary systems through the examination of interplanetary dust.

There are additional choices that could augment these core instruments, including a Lyman-alpha spectrograph (LYA) to provide vital information about interplanetary and VLISM hydrogen phasespace density, imaging spectrometers in the ultraviolet/visible/infrared (UVS/VIR) to study planet formation in the solar system by examining the debris disk and potential nearby Kuiper Belt objects and dwarf planets, and a visIR spectral mapper (IRM) to observe the diffuse red-shifted light emitted by the universe beyond the dominant Zodiacal cloud foreground that obfuscates such studies when performed within our heliosphere.

Taking the science objectives into account along with size, mass, and power constraints, two example payloads were developed for the Interstellar Probe concept study: one baseline payload which focuses on heliophysics objectives and an augmentation payload which accommodates a visNIR imager and the visIR mapper for performing a dwarf planet flyby and studying the extragalactic background light in addition to core heliophysics instrumentation. This presentation provides an overview of these example payloads, their accommodation on the spacecraft, and reliability issues associated with requiring up to 50 years of functionality.