A possible constraint to Uranusʹ great collision

The system of the five major satellites of Uranus resembles those of Jupiter and Saturn in many ways. However, although Uranusʹ Jacobi limit is almost 7×107 km, the outermost known satellite of Uranus is Oberon (Zytkow et al., Icarus, 102, 298–306, 1993), which is found orbiting at only 5.8×105 km from the planet (nearly 22.3 planetary radii). It stands in contrast to the rich system of outer satellites of Jupiter, and the outermost satellites of Saturn (Phoebe) and Neptune (Nereid) also orbit far beyond 200 planetary radii. Another remarkable property of Uranus is its anomalous obliquity, that is the inclination of the spin axis with respect to the orbital plane, of about 98°. It is generally attributed to a supposed collision with another proto-planet whose mass is a few percent of Uranusʹ mass (Safronov, NASA TTF-667, 1972; Koricansky et al., Icarus, 84, 528–541, 1990; Slattery et al., Icarus, 99, 167–174, 1992, among others). Theoretical results, confirmed by numerical simulations, show that it is possible that such a massive collision not only tilted Uranusʹ spin axis but also imparted enough orbital impulse to unbound the orbits of preexisting distant satellites (if they ever existed). An intensive search for faint objects orbiting Uranus beyond 250–300 planetary radii would provide a constraint to the giant collision scenario.