Francisco (moon)

Template:Short description Template:Use dmy dates Template:Infobox planet
22 km<ref name="SheppardMoons"/> | rotation = | albedo = | magnitude = 25.0 (average)<ref name="SheppardMoons"/> | abs_magnitude = 12.9<ref name="MPC-NatSats"/> }}

Francisco, also known as Uranus XXII and previously as S/2001 U 3, is the innermost irregular satellite or moon of Uranus, orbiting in the retrograde direction. It was discovered on 13 August 2001 by John J. Kavelaars, Matthew J. Holman, Dan Milisavljevic, and Tommy Grav using the 4.0-meter Víctor M. Blanco Telescope at Cerro Tololo Observatory, Chile. It was named after Francisco, a lord in William Shakespeare's play The Tempest. Francisco orbits Uranus at an average distance of Template:Convert and takes about 267 Earth days (0.73 Earth years) to complete one orbit. Francisco is estimated to be up to Template:Cvt in diameter, though many of its physical properties are unknown.

Discovery

Because Francisco is extremely faint, it is better seen by shifting-and-adding multiple images of its location. The result is Francisco appearing as a single dot against trailed background stars.

Francisco was discovered on 13 August 2001 by a group of astronomers consisting of John J. Kavelaars, Matthew J. Holman, Dan Milisavljevic, and Tommy Grav, who at the time were searching for distant moons of Uranus.<ref name="press"/><ref name="Kavelaars2004"/> The discovery observations were made at Cerro Tololo Observatory, Chile,<ref name="MPEC-2003-T29"/> using the 4.0-m Víctor M. Blanco Telescope equipped with a wide-field camera.<ref name="Kavelaars2004"/>Template:Rp The group discovered Francisco and three other Uranian irregular moons (Trinculo, Ferdinand, and MargaretTemplate:Efn) in the telescope images by using a computer algorithm that followed the motion of Uranus and then combined the images to enhance the faint moons—a technique known as shift-and-adding.<ref name="Kavelaars2004"/>Template:Rp

To determine the orbit of Francisco, the group continued observing the moon from different observatories, including the 200-inch Hale Telescope at Palomar Observatory and 8.2-m Very Large Telescope at Paranal Observatory.<ref name="MPEC-2003-T29"/><ref name="Kavelaars2004"/>Template:Rp These follow-up observations lasted until 5 September 2002.<ref name="MPEC-2003-T29"/><ref name="IAUC8216"/> The Minor Planet Center announced the discovery of Francisco on 6 October 2003.<ref name="MPEC-2003-T29"/>

Name

When the discovery of Francisco was announced, it was given the temporary provisional designation S/2001 U 3 by the Minor Planet Center.<ref name="MPEC-2003-T29"/> 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.<ref name="IAUC8648"/> The moon was named after the character Francisco from William Shakespeare's play, The Tempest. In the play, Francisco is a lord who is shipwrecked with King Alonso and others.<ref name="NASA"/><ref name="usgs-disc"/>

Orbit

Francisco is an irregular moon of Uranus, which have very wide, elliptical, and inclined orbits in contrast to the regular moons of Uranus.<ref name="Brozovic2022"/>Template:Rp Irregular moons like Francisco are thought to be former asteroids that were captured by their planet during the Solar System's formation.<ref name="Kavelaars2004"/>Template:Rp 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.<ref name="Brozovic2022"/>Template:Rp This results in significant changes in the orbits of irregular moons over short periods of time, so a simple Keplerian elliptical orbit cannot accurately describe the long-term orbital motions of irregular moons. 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 a long period of time.<ref name="Brozovic2022"/>Template:Rp

Over a 30,000-year time period, Francisco's average semi-major axis or orbital distance from Uranus is Template:Convert, with an average orbital period of Template:Convert.<ref name="Brozovic2022"/>Template:Rp Francisco is the innermost irregular moon of Uranus.<ref name="NASA"/> Francisco 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.<ref name="jplsats-elem"/><ref name="Brozovic2022"/>Template:Rp Since Francisco's orbital inclination is greater than 90°, the moon has a retrograde orbit, meaning it orbits in the opposite direction of Uranus' orbit around the Sun.<ref name="NASA"/> Francisco's orbital elements fluctuate over time due to perturbations: its semi-major axis varies by ±Template:Cvt, eccentricity varies by ±0.05, and inclination varies by ±2°.<ref name="Brozovic2022"/>Template:Rp<ref name="Sheppard2024"/>Template:Rp Compared to other Uranian irregular moons, Francisco's eccentricity variations are significant, although they occur over relatively slow periods of several thousand years.<ref name="Brozovic2022"/>Template:Rp Francisco's orbit exhibits nodal and apsidal precession with average periods over 10,000 Earth years.Template:Efn

Francisco is not known to be part of a collisional family or group; its orbit is apparently unique among the known Uranian irregular moons.<ref name="Cuk2004"/>Template:Rp<ref name="Sheppard2024"/>Template:Rp 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.<ref name="Sheppard2024"/>Template:Rp

• 
  Animation of Francisco's orbit (cyan) around Uranus.
  Template:Legend2Template:·Template:Legend2Template:·Template:Legend2Template:·Template:Legend2Template:·Template:Legend2Template:·Template:Legend2
• 
  Irregular satellites of all four giant planets, plotted by average distance from their planet (semi-major axis in Hill radii) and orbital inclination (in degrees wrt ecliptic). Data as of February 2024.

Physical characteristics

Francisco is very faint with an average apparent magnitude of 25.0,<ref name="SheppardMoons"/> so it could only be observed with long-exposure imaging by large-aperture telescopes.<ref name="press"/><ref name="Kavelaars2004"/> Nothing is known about Francisco's physical properties other than its absolute magnitude of 12.9, which can be used to estimate the moon's diameter.<ref name="MPC-NatSats"/> Francisco is expected to have a dark surface<ref name="NASA"/> composed of water ice, hydrated silicates, and organic compounds, similar to other irregular moons.<ref name="Cartwright2021"/>Template:Rp Assuming a geometric albedo range of 0.04–0.10 that is typical for most irregular moons,<ref name="Sharkey2023"/> Francisco should have a diameter between Template:Cvt.Template:Efn Scott Sheppard, who has discovered many irregular moons around other giant planets, estimates Francisco's diameter to be Template:Cvt.<ref name="SheppardMoons"/>

Exploration

Francisco has not been imaged up close by a space probe, although Voyager 2 did pass within Template:Convert of Francisco during its flyby of Uranus in 1986.<ref name="Verbiscer2023"/> All Uranian irregular moons including Francisco are planned to be distant observation targets for the upcoming Uranus Orbiter and Probe (UOP), which will measure the Uranian irregular moons' rotation periods and shapes by watching their brightness change over time.<ref name="Verbiscer2023"/><ref name="Denk2023"/> The UOP may not be able to do a close flyby of Francisco because the moon does not orbit near the ecliptic plane.<ref name="Denk2023"/>

See also

• Moons of Uranus

External links

• Uranus Moons – Francisco, NASA Science, 25 January 2024
• Three more minor moons for Uranus, J. J. Kavelaars, 27 November 2001
• Moons of Uranus, Scott S. Sheppard, Carnegie Institution for Science