Accurate geometric albedo, shape, and size of Hi’iaka from a stellar occultation

Stellar occultations, when a distant object passes in front of a star and casts a shadow on Earth, are a powerful tool to probe the physical properties of solar system bodies. They enable precise size and shape measurements and can reveal rings, satellites, atmospheres, or surface features. Here we present results from two stellar occultations by Hi'iaka, dwarf planet Haumea's largest satellite, observed in April 2021. The second event yielded the first documented multi-chord occultation involving a trans-Neptunian satellite other than Charon. Combining these data with photometric observations, we find that Hi'iaka is a triaxial ellipsoid with a volume-equivalent diameter of 370 ± 20 km and a density of 640 ± 80 kg/m³, indicating a porous interior. The visible geometric albedo is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>0.76</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.15</mml:mn></mml:math> , (∼ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mn>30</mml:mn> <mml:mi>%</mml:mi></mml:math> higher than Haumea's). The combination of low density and rapid rotation (9.68 ± 0.02 hours), under the assumption of a homogeneous body, indicates that Hi'iaka deviates from hydrostatic equilibrium.