Modelling the interstellar dust detections by DESTINY+ I: Instrumental constraints and detectability of organic compounds

The DESTINY+ 30 spacecraft will be launched to the active asteroid (3200) Phaethon in 2025. The spacecraft will be equipped with the DESTINY+ 31 32 Dust Analyzer (DDA) which will be a time-of-flight impact ionization mass 33 spectrometer. In addition to the composition of impacting dust particles, 34 the instrument will measure the particle mass, velocity vector, and surface 35 charge. Here, we study the detection conditions of DDA for interstellar 36 dust during the DESTINY+ mission. We use the interstellar dust module 37 of the Interplanetary Meteoroid environment for EXploration model (IMEX 38 Sterken et al., 2013; Strub et al., 2019) to simulate the flow of interstellar dust 39 through the Solar System. Extending earlier work by Krüger et al. (2019b) 40 we consider the entire DESTINY+ mission, i.e. the Earth-orbiting phase of 41 the spacecraft during the initial approximately 1.5 years after launch, the 42 nominal interplanetary mission phase up to the Phaethon flyby, and a four43 years mission extension beyond the Phaethon flyby. The latter may include 44 additional asteroid flybys. For predicting dust fluxes and fluences we take into account a technical constraint for DDA not to point closer than 90◦ 45 46 towards the Sun direction for health and safety reasons of the instrument 47 and in order to avoid electrical noise generated by photoelectrons. For the Earth orbiting phase after launch of DESTINY+ 48 our simulations predict that 49 up to 28 interstellar particles will be detectable with DDA in 2026. In the 50 following years the interplanetary magnetic field changes to a focussing con51 figuration for small (≲ 0.1 µm) interstellar dust particles. This increases the 52 total number of detectable particles to 50 during the interplanetary mission of DESTINY+ 53 in 2027. In 2028 and 2029/30 approximately 160 and 190 par54 ticles will be detectable, respectively, followed by about 500 in 2030/31. We 55 also make predictions for the detectability of organic compounds contained 56 in the interstellar particles which is a strong function of the particle impact 57 speed onto the detector. While organic compounds will be measurable only 58 in a negligible number of particles during the Earth orbiting and the nomi59 nal interplanetary mission phases, a few 10s of interstellar particle detections 60 with measurable organic compounds are predicted for the extended mission 61 from 2028 to 2031.