Scientific Opportunities in the Kuiper Belt and Inner Oort Cloud with an Interstellar Probe

Proposed to pierce the heliopause and explore proximal interstellar space, the Interstellar Probe (IP) mission would have extraordinarily valuable flyby opportunities with other planetary bodies along its flight path. Gas and ice giant encounters are one kind of opportunity, but the most likely such flyby would be with Jupiter, New Horizons style. Jupiter and the Galilean satellites should be well covered scientifically in the years to come, but the jovian magnetosphere and especially the long magnetotail would be prime targets for IP instruments. Superior opportunities for scientific discovery would come with the Kuiper Belt (KB) and beyond, however, as reaching such distant, unexplored worlds remains challenging otherwise. Whatever the heliospheric community decides as the best and most scientifically advantageous outbound asymptote for IP, there will be multiple possible KB encounters, both with dwarf planets and smaller bodies, on either the chosen or very similar trajectories. Given the likely timeframe for the launch of IP (2030 or later), many more potential targets will have been discovered in the intervening years. The discoveries of New Horizons at the Pluto system (dwarf planet) and Arrokoth (cold classical KB object) have proven revolutionary [1,2]; IP would no doubt achieve the same, even before reaching the heliopause, but only if it carries the necessary instrumentation. The minimum would be a visible wavelength camera and a near-infrared imaging spectrometer.

Fast flyby speeds are manageable; New Horizons proved that with the Arrokoth encounter. IP will likely be a spinning spacecraft, however, to maximize the capabilities of its primary fields and particle instrument suite, which poses a challenge to any KB object flyby. One strategy is simply to adapt to the spin rate, in the manner that Juno has at Jupiter (i.e., line scanning). Another would be for IP to have the ability to vary (slow) its spin rate, or even to go into 3-axis mode for encounters. In either case, a gimbal in the optical path would greatly reduce (or eliminate) the need for additional spacecraft repointings. Bodies in the so-called detached region of the KB, or inner Oort Cloud (OC), such as Sedna, would be especially tempting targets, as the likelihood of other missions to this region of the solar system are essentially zero. That is, unless there really is a major planet on a distant eccentric orbit shepherding the orbits of the detached/inner OC bodies. This is a big if of course, but if such a planet exists, the drive to visit it will be great, which alone could be used to leverage the launch of IP. Such an encounter would demand a more comprehensive remote sensing suite, in which case a daughter spacecraft to IP might be considered as an engineering option. A fully autonomous RTG-powered daughter craft would probably put too much of a lean on IP’s mass and C3, but a battery-powered 3-axis stabilized daughter craft could, potentially, carry out the critical rendezvous measurements and transmit the results to IP, which from there would be transmitted to the Earth.

Finally, the dream of sending a full IP (1000 AU in 50 yr, which would require speeds of ~100 km/s) should not preclude thinking about tempering ambitions in the face of financial or engineering realities. Even a mission that “simply” doubles the asymptotic velocity of Voyager 1 (the fastest of the 5 interstellar precursor missions) could accomplish much of IP’s science in a similar timeframe. Plus there may be a speed which is “too fast,” even for fields and particles measurements. The technological ability to reach ~35 km/s would also allow us to rendezvous with, or even ride along with interstellar visitors (e.g., 1I/ 'Oumuamua and 2I/Borisov) as they head back into the depths of space.

[1] Stern S. A., Grundy W. M., McKinnon W. B., Weaver H. A., and Young L. A. (2018) The Pluto system after New Horizons. Annu. Rev. Astron. Astrophys., 56, 357–392, doi: 0.1146/annurevastro-081817-051935.

[2] Stern S. A., et al. (2019) Initial results from the New Horizons exploration of 2014 MU₆₉, a small Kuiper Belt object. Science, 364, eaaw9771, doi: 10.1126/science.aaw9771.