In situ stellar occultations are needed to address priority science at Uranus

High speed stellar occultations will be required to address the priority science questions and strategic research outlined for Uranus in the 2023–2032 Planetary Sciences and Astrobiology Decadal Survey but with the exception of ring spectra, the Uranus orbiter strawman payload and traceability matrix suggests that this research will be entirely addressed with the imaging system and uplink X, Ka band radio occultations. While both techniques are invaluable, imaging will be limited in spatial resolution (>100 m/px) in the rings and radio occultations will be highly restricted in available viewing geometry by the low ring opening angle during the nominal mission period. We highlight cross-cutting science which is only possible with high speed stellar occultations of the Uranian rings and we provide instrument design constraints ideally suited for stellar occultations. Unlike Saturn's broad and bright rings, the Uranian ring system is very dark, compact, and the rings are extremely narrow (all but ɛ ranging between 2 - 10 km in width, excluding dust rings). The low albedo of the Uranian rings (AB ~ 2%) permits visible light star occultations in addition to UV and NIR without introducing unwanted background signal from ring shine. ~6,000 stars with mv < 6 spread across the celestial sphere provide a wide range of viewing geometries that are not available with radio occultations. Resolving particle aggregates and narrow wave trains in the narrowest rings requires integration times of ≲0.5 ms for spatial sampling of ~10 – 50 m. In the limit of zero integration time, resolution is limited to the Fresnel zone (√λD) projected into the rings where λ is the effective wavelength of starlight and D is the distance from the instrument to the point where the line-of-sight to the star intersects the ring plane. Single photon counters are preferred because SNR is determined solely by Poisson counting statistics. In this case, the statistical moments beyond the mean occulted star signal provide important constraints on the sizes of particles and structure below the resolution of the occultation. Stellar occultations enable cross-cutting science such as:

Constraining Uranian deep interior models with ring waves

Constraining the Uranian ring mass, age, and origin

Determining the orbital evolution of the moons

which may not otherwise be possible.