Exploring Trans-Neptunian Objects with Interstellar Probe
Authors: Runyon Kirby , Holler Bryan , Bannister Michele 
Published: October 2020

Introduction. Interstellar Probe is a NASA Heliophysics Division mission concept that would leave the solar system on a 50-year prime mission with a primary objective to study the heliosphere and the local interstellar medium (>200 au; Brandt et al., 2020). In order to reach interstellar space, the spacecraft must pass through the trans-Neptunian region, presenting the opportunity to study planets and minor bodies (Brandt et al., 2020). We focus here on the opportunities for exploring trans-Neptunian objects (TNOs) with Interstellar Probe.&#160;TNO Geoscience Investigation. The primary science instrument for TNO flyby geoscience would be a visible/infrared imaging spectrometer to enable studying a wide range of physical, chemical, and geological processes at temperatures 400 km in diameter&#8212;beyond Neptune. In or near this region, between -20&#176; and 20&#176; in heliocentric ecliptic latitude, there are five potential dwarf planets that Interstellar Probe could fly by: 2002 MS4, Quaoar, Gonggong, Pluto, and Ixion (Figure 1).Potential dwarf planet targets represent a wide range of colors, compositions, and geological processes. Quaoar is dense (~2 g cm-3; Braga-Ribas et al., 2013) and may therefore have retained enough internal heat to remain geologically active and/or support a subsurface ocean (e.g., Shchuko et al., 2014). Gonggong and its satellite Xiangliu show one of the largest color dichotomies among a primary and satellite (Kiss et al., 2019), and Gonggong may be a contact binary (P&#225;l et al., 2016). Little is known about 2002 MS4 due to its current position in a crowded star field, frustrating ground-based observations, but it is large enough for possible differentiation and a surface dominated by H2O ice and the resultant geological features. Ixion has a flat near-IR spectrum (Barkume et al., 2008; Guilbert et al., 2009), meaning it could provide clues to the substrate present beneath the ice on all TNOs. Pluto would benefit from a follow-up 20+ years after New Horizons to reveal surficial and atmospheric changes.Figure 1.&#160;Positions of dwarf planets (black), CCKBOs (red), and extreme TNOs (purple) with respect to the ecliptic on January 1, 2040. Longitudinal regions for the heliospheric nose (pink), tail (yellow), and waist (blue), as well as the heliospheric nose (filled red star) and tail (open red star), are marked. We assumed the trajectory of Interstellar Probe will take it somewhere between ~300-340&#730; longitude and -20&#730; to 20&#730; latitude, opening up the possibility for a flyby of 2002 MS4, Quaoar, Pluto, Ixion, or Gonggong.&#160;Prospects for a secondary target flyby. New Horizons&#8217; flyby of Arrokoth (2014 MU69) in 2019 provided a valuable glimpse into the formation history of planetesimals and the pristine surfaces present beyond Neptune (Stern et al., 2019). The flyby of this small TNO (~35 km) revealed the building blocks of the largest TNO, Pluto, New Horizons&#8217; other target. Interstellar Probe could complement the Arrokoth and Pluto flybys via a serendipitous encounter with a TNO