Measurement of the Composition of the Local Interstellar Cloud with the Interstellar Probe Mission

The proposed Interstellar Probe (IP) spacecraft of NASA will travel through the heliosphere and advance into the local interstellar medium (LISM) within roughly 16 years, i.e., at twice the speed as the Voyager spacecraft. IP will enable the dedicated exploration of the heliospheric boundary by imaging the heliosphere from inside and outside the heliopause, and by directly sampling the unknown LISM. IP will also enable in situ measurements in the undisturbed LISM beyond the heliospheric bow shock or bow wave. The measurement of the chemical composition of the neutral gas in the local interstellar cloud is an important element of the scientific investigations of IP. So far, the chemical composition of the LISM was mostly inferred from pickup ions in the solar wind, from anomalous cosmic rays, and from spectroscopic observations of nearby stars. We are designing a highly specialized mass spectrometer to measure the neutral gas of the LISM in situ at these extremely low densities. The expected species to be recorded are H, He, C, N, O, Ne, Na, Mg, Al, Si, P, S, Ar, Ca, and Fe. This list of species allows to derive astrophysical important element ratios, like the Ne/O ratio. In addition, this mass spectrometer will measure the isotope composition of D/H, ³He/⁴He, ²²Ne/²⁰Ne, and ³⁶Ar/³⁸Ar of the LISM with unprecedented accuracy. These measurements will take advantage of the long duration of the IP mission, allowing for long integration times. The design of the instrument will be presented, together with estimated signals, and the operation scenario for the 50-year IP mission.