Francisco (moon)

Discovery

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. The discovery observations were made at Cerro Tololo Observatory, Chile, using the 4.0-m Víctor M. Blanco Telescope equipped with a wide-field camera. The group discovered Francisco and three other Uranian irregular moons (Trinculo, Ferdinand, and Margaret) 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.

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. 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 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.

Orbit

Francisco is an irregular moon of Uranus, which have very wide, elliptical, and inclined orbits in contrast to the regular moons of Uranus. Irregular moons like Francisco are thought to be former asteroids that were 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 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.

Over a 30,000-year time period, Francisco's average semi-major axis or orbital distance from Uranus is 4.28 e6km, with an average orbital period of 267 d. Francisco is the innermost known irregular moon of Uranus. 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. 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. Francisco's orbital elements fluctuate over time due to perturbations: its semi-major axis varies by ±700 km, eccentricity varies by ±0.05, and inclination varies by ±2°. Compared to other Uranian irregular moons, Francisco's eccentricity variations are significant, although they occur over relatively slow periods of several thousand years. Francisco's orbit exhibits nodal and apsidal precession with average periods over 10,000 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.

File:Animation of Sycorax orbit around Uranus.gif|Animation of Francisco's orbit (cyan) around Uranus.

File:Irregular moon orbits a vs. i.jpg|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, so it could only be observed with long-exposure imaging by large-aperture telescopes. 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. Francisco is expected to have a dark surface composed of water ice, hydrated silicates, and organic compounds, similar to other irregular moons. Assuming a geometric albedo range of 0.04–0.10 that is typical for most irregular moons, Francisco should have a diameter between 11–17 km. Scott Sheppard, who has discovered many irregular moons around other giant planets, estimates Francisco's diameter to be 22 km.

Exploration

Francisco has not been imaged up close by a space probe, although Voyager 2 did pass within 2.4 e6km of Francisco during its flyby of Uranus in 1986. 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. The UOP may not be able to do a close flyby of Francisco because the moon does not orbit near the ecliptic plane.

Notes

References

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|url-status = dead |archive-url = https://web.archive.org/web/20110706190829/http://salish.dao.nrc.ca/~kavelaar/Uranus/faint/ |archive-date = 2011-07-06}}

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|url-status = dead |archive-url = https://web.archive.org/web/20240229115731/https://elib.dlr.de/196827/2/8187.pdf |archive-date = 2024-02-29}}

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References

1. Benjamin Smith (1903) ''The Century Dictionary and Cyclopedia''