What Science Can an Interstellar Probe Mission at Large Heliocentric Distances Achieve With Remote Imaging and In Situ Dust Measurements?

Introduction. We present the scientific potential for using an Interstellar Probe (ISP) spacecraft traveling to > 500 AU from the Sun to study the dust in the solar system's debris disks and the nearby ISM, as well as to map the surface of a flyby KBO and study the Cosmic Background Light (CBL).

Figure 1 – Interstellar Probe Explorer payload at its design goal location of 1000 AU with respect to the planets, the heliopause, Alpha Centauri, and the Oort Cloud.

 

Discussion. The solar system is known to house two planetesimal belts, the inner Jupiter Family Comet (JFC) + Asteroid belt and the outer Edgeworth-Kuiper Belt (EKB), and at least one debris cloud, the Zodiacal Cloud, sourced by planetesimal collisions and comet evaporative sublimation. However, these are poorly understood in toto because we live inside of them. It is not understood well how much dust is produced from the EKB since the near-Sun comet contributions dominate the inner cloud and only New Horizons (NH) has ever flown a dust counter through the EKB. New estimates from NH put the EKB disk mass at at 30 – 40 times the inner disk mass [1]. Better understanding EKB dust production will improve our estimates of the number of EKB bodies, especially the smallest ones, and their dynamical collisional state. For the innermost Zodiacal cloud, questions remain concerning its overall shape and orientation with respect to the invariable plane - these are not explainable from perturbations caused by the known planets alone.

 

Imaging Studies. Using new technologies and passively cooled detectors, a suitable low system size/mass/power VISNIR spectrometer/FIR imager + 10 cm class primary has been specified using a CubeSat study baseline design [2]. The VISNIR spectrometer could provide maps of the cloud's dust particle size and composition, while FIR imagery would map the dust cloud's density. 3-D cloud mapping would occur during flythrough via tomographic inversion, and via lookback imaging once the s/c is beyond 200 AU. The lookback imaging will allow ISP to measure for the 1^st time in history the entire extent of the Zodiacal Cloud, and determine whether its inner JFC/asteroidal & outer KBO parts connect smoothly, as predicted by Stark & Kuchner [3-4] and detected by Piquette, Poppe et al. [5-8] (Figs. 2-3). This would also allow direct comparison of the solar system’s debris disks with those observed around other nearby stars, and test theories that suggest that our solar system is planet rich but dust-poor [9].

 

 

 

Figure 2 – Predicted dust cloud morphologies arising from solar system JFC (JFC) & Oort Cloud (OCC) comets & Kuiper Belt (EKB) sources. (Top) Looking down on the solar system. (Bottom) Looking through the plane of the solar system. After [1].

 

Looking back towards the Sun from >100 AU, ISP will perform deep searches for the presence of rings and dust clouds around discrete sources, like Planet X, the Haumea family of icy collisional fragments, the rings of the Centaur Chariklo, or dust emitted from spallation off the larger KBOs. The same instrument will map the surfaces of KBOs encountered along the way. Measurement of the cloud’s total brightness will allow removal of its signal from near-Earth CBL measurements looking at all the starlight ever formed in the Universe, and the same instrument will return its own remote CBL measurements.

 

 

Figure 3 – NH in situ measurements (black data pts) and predicted dust flux contributions (colored curves) for the solar system’s debris disks [1,8]. Measured PIONEER 10 dust fluxes are in the upper left corner of the plot, so the predicted crossover at ~10 AU from JFC dominated to EKB dominated is seen. ISP will help us determine if another crossover from EKB dominated to OCC dominated occurs at ~100 AU, and if the EKB dust is ice, rock, & organics rich like KBOs and comets.

 

First Ever Outer Solar System In Situ Dust Characterization. ISP can also carry the first ever in situ dust chemical analyzer past Saturn. Based on the Europa Clipper SUDA instrument [10], it will compositionally and directionally characterize the solar system’s dust clouds and will help isolate their sources, like the rocky asteroidal dust bands and the icy Haumea family fragments. Using measured dust particle masses and velocities, dust input & loss rates from these sources will be derived. Direct dust sampling will return the first ever in situ chemical analysis of EKB dust, the first ever in situ sampling of dust beyond 200 AU, and provide calibrated ground truth for cloud models produced from our imagery. It should also resolve the tension between the expected makeup of inflowing ISM microdust as determined by remote sensing and the mesasured ISM dust component found at Jupiter and Saturn by Galileo, Ulysses, and Cassini ([11] & Figure 4).

 

Understanding a G2V’s Astropause ISM Bowshock. At the outermost solar system edges, the role dust plays in shaping and energizing the heliosphere’s boundary with the local galactic medium is almost completely unknown. Estimates range up to 1/3 of the heliopause and heliosheath energy density is in the dust. Current heliopause/sheath models do not allow for dusty plasma physics because the dust component is so poorly known. We do know that submicron sized dust is streaming into the solar system's ram direction through the local ISM, and the large deficit between remote sensing models of local ISM dust and ISM dust measured inside the solar system suggests a large amount of energy is involved in diverting much of the impinging dust around the edges of the solar system.

 

  

 

Figure 4 – Disconnect between the nearby ISM dust size distribution predicted from remote sensing measurements (blue) and ISM dust counts measured inside the solar system (red). After [11].

 

References: [1] Poppe+2019, ApJLett881, L12 [2] Zemcov+2019, AASMeeting#233, id.#171.06 [3] Stark&Kuchner 2009, ApJ707, 543 [4] Stark&Kuchner 2010, AJ140, 1007 [5] Poppe+2010, Geophys.Res.Lett.37, L11101 [6] Poppe&Horányi 2012, Geophys.Res.Lett. 39, 1 [7] Poppe2016, Icarus264, 369 [8] Piquette+2019, Icarus321, 116 [9] Greaves&Wyatt 2010, MNRAS404, 1944 [10] Kempf+2014, EPSCAbstracts9, EPSC2014-229 [11] Weingartner&Draine, ApJ548, 296; Draine&Hensley 2016, ApJ831, 59