Prospero (moon)

Prospero (Uranus XVIII, formerly S/1999 U 3) is an irregular moon of Uranus. Matthew Holman and his team discovered it on 18 July 1999, alongside Setebos and Stephano. The Minor Planet Center gave it the temporary designation S/1999 U 3, and on 21 August 2000 the International Astronomical Union's Working Group for Planetary System Nomenclature assigned it the Roman numeral Uranus XVIII and named it after the sorcerer from Shakespeare's The Tempest.

Orbit

Prospero orbits Uranus at a mean distance of about 16,256,000 km, taking roughly 1,978 days (5.4 Earth years) to complete one revolution. Its orbit is retrograde, highly inclined (about 152° to the ecliptic), and has a high eccentricity of 0.445. These orbital elements shift over time due to solar and planetary perturbations. Prospero is not known to belong to any collisional family, but its similar orbital characteristics and color to Setebos suggest a possible shared origin, though further observations are needed.

Physical characteristics

Physical properties are poorly known. Assuming a low albedo of 0.04, typical of Uranian irregular moons, its brightness indicates a diameter of about 50 km (31 mi).

In visible light, it appears neutral grey, with color indices B−V=0.80 and R−V=0.39, similar to Setebos. Its brightness varies periodically, implying an elongated shape; Kepler observations in 2017 measured a variation of 0.41 magnitudes, giving an equatorial semi-axis ratio (a/b) of at least 1.4. The rotation period is ambiguous: if the light curve is single-peaked, the period is 7.145±0.092 hours; if double-peaked, it is 14.29±0.183 hours.

Prospero likely did not form near Uranus but was captured later. Its orbital parameters place it in the same dynamic cluster as Sycorax and Setebos, suggesting a common origin, though Sycorax is considerably redder than the predominantly gray others. No space probe has imaged Prospero up close. The proposed Uranus Orbiter and Probe (UOP) plans to observe all Uranian irregular moons from a distance, measuring their rotation periods and shapes via brightness changes, but a close flyby of Prospero is unlikely because its orbit lies far from the ecliptic plane.