Francisco (moon)

Francisco, also known as Uranus XXII and previously as S/2001 U 3, is the innermost known irregular moon of Uranus. It was discovered on 13 August 2001 by JJ Kavelaars, Matthew J. Holman, Dan Milisavljevic, and Tommy Grav at Cerro Tololo Inter-American Observatory in Chile. It was named after Francisco, a lord in William Shakespeare's play The Tempest. Francisco orbits Uranus at an average distance of and takes about 267 Earth days (0.73 Earth years) to complete one orbit. It follows a retrograde orbit around the planet, moving in the opposite direction to the planet's orbit around the Sun. Francisco has not been studied up close by any spacecraft, though Voyager 2 coincidentally passed nearby during its 1986 Uranus flyby. Francisco is estimated to have a diameter of , though most of its physical properties are unknown.

Discovery Francisco was discovered on 13 August 2001 by astronomers JJ Kavelaars, Matthew J. Holman, Dan Milisavljevic, and Tommy Grav, who at the time were searching for distant moons of Uranus. The discovery observations were made at Cerro Tololo Inter-American Observatory, Chile, using the 4-meter Víctor M. Blanco Telescope equipped with a wide-field camera. The group identified Francisco and three other Uranian irregular moons (Trinculo, Ferdinand, and Margaret) in the telescope images. The images were processed using the shift-and-add technique, which combines multiple exposures aligned to Uranus's motion to enhance the faint moons.

To determine the orbit of Francisco, the group continued observing the moon from different observatories, including the Hale Telescope at Palomar Observatory and Very Large Telescope at Paranal Observatory. These follow-up observations lasted until 5 September 2002. The Minor Planet Center announced the discovery of Francisco on 6 October 2003.

Name When the discovery of Francisco was announced, it was given the temporary provisional designation "S/2001 U 3" by the Minor Planet Center. It was later named and given the Roman numeral designation Uranus XXII by the International Astronomical Union's Working Group for Planetary System Nomenclature on 29 December 2005. The moon was named after Francisco, a character in William Shakespeare's play The Tempest. In the play, Francisco is a lord who is shipwrecked with King Alonso and others. In a 2014 interview with the Folger Shakespeare Library, Kavelaars mentioned that he picked the name because it resembled his younger daughter's middle name, Frances.

Orbit

Francisco is one of Uranus's irregular moons, which have very wide, elliptical, and inclined orbits in contrast to the regular moons of Uranus. Irregular moons like Francisco are thought to have been captured by their planet during the Solar System's formation. The Uranian irregular moons are loosely bound by Uranus's gravity because of their great distance from the planet, so their orbits are frequently perturbed by the gravity of the Sun and other planets. This results in significant changes in their orbits over short periods of time, so a simple Keplerian elliptical orbit cannot accurately describe their long-term orbital motions. Instead, proper or mean orbital elements are used to describe the long-term orbits of irregular moons more accurately, since these are calculated by averaging out the perturbed orbit over an extended period of time.

Francisco is the innermost known irregular moon of Uranus, having an average semi-major axis of and an average orbital period of . It has an average orbital eccentricity of 0.14 and an average inclination of 147° with respect to the ecliptic, or the plane of Earth's orbit. Since Francisco's orbital inclination is greater than 90°, the moon's orbit is retrograde, meaning it orbits in the opposite direction to Uranus's orbit around the Sun.

Over a 10,000-year time span, perturbations cause Francisco's semi-major axis to vary by ±, while its orbital period varies by ±1 day. Over a period of several thousand years, Francisco's eccentricity slowly varies from 0.09 to 0.19. Its inclination with respect to the ecliptic also varies from 146° to 149°. Its orbit exhibits nodal and apsidal precession with average periods between 10,000 and 20,000 Earth years.

Francisco is not known to be part of a collisional family or group; its orbit is apparently unique among the known Uranian irregular moons. If there is a collisional family associated with Francisco, then any Uranian irregular moons related to it are likely undiscovered because they are presumably small and faint.

Physical characteristics In red light (R-band), Francisco appears very faint with an apparent magnitude of 25.0. It can only be observed through long-exposure imaging by large-aperture telescopes. Little is definitively known about Francisco's physical properties other than its absolute magnitude of 12.9, which can be used to estimate the moon's diameter. Assuming Francisco has a low geometric albedo of 0.04 like other Uranian irregular moons, Francisco's diameter has been estimated to be . Like these moons, Francisco is expected to have a dark surface composed of water ice, hydrated silicates, and organic compounds.

Exploration Francisco has not been imaged at close range by a space probe, although Voyager 2 did coincidentally pass within of Francisco during its flyby of Uranus in 1986. All Uranian irregular moons, including Francisco, have been identified as distant observation targets for the proposed Uranus Orbiter and Probe (UOP), which would measure the Uranian irregular moons' rotation periods and shapes by monitoring their brightness change over time. However, the UOP may not be able to perform a close flyby of Francisco because the moon does not orbit near the ecliptic plane.

Francisco (moon)

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