Francisco (moon)

Francisco, also called Uranus XXII and previously known as S/2001 U 3, is one of the smallest moons of the planet Uranus. It was found on August 13, 2001, by scientists John J. Kavelaars, Matthew J. Holman, Dan Milisavljevic, and Tommy Grav using a big telescope in Chile. They used the 4.0-meter Víctor M. Blanco Telescope at the Cerro Tololo Observatory to spot it.

Francisco orbits Uranus in the opposite direction of most of the planet's other moons. It stays about 4.3 million kilometers (2.7 million miles) away from Uranus and takes around 267 days—almost three-quarters of a year—to go around the planet once. Scientists think Francisco might be about 22 kilometers (14 miles) wide, but there is still a lot they do not know about its size and other features. The moon was named after a character named Francisco, who is a lord in William Shakespeare’s play The Tempest.

Discovery

Francisco was discovered on 13 August 2001 by a group of astronomers: John J. Kavelaars, Matthew J. Holman, Dan Milisavljevic, and Tommy Grav. They were looking for distant moons of Uranus using the 4.0-meter Víctor M. Blanco Telescope at Cerro Tololo Observatory in Chile. They found Francisco and three other moons of Uranus—Trinculo, Ferdinand, and Margaret—by using a special computer method.

To learn more about Francisco’s path around Uranus, the team kept watching it from different telescopes, including the 200-inch Hale Telescope at Palomar Observatory and the 8.2-meter Very Large Telescope at Paranal Observatory. These observations continued until 5 September 2002. The discovery of Francisco was announced by the Minor Planet Center on 6 October 2003.

Name

When Francisco was found, it was called S/2001 U 3. Later, on 29 December 2005, it was named Uranus XXII. The moon is named after a character from William Shakespeare’s play, The Tempest. In the story, Francisco is a lord who gets shipwrecked with King Alonso and others.

Orbit

Francisco is an irregular moon of Uranus. These moons travel in wide, stretched-out paths around the planet. They are different from Uranus's closer moons, which have more circular paths. Scientists think moons like Francisco may have been asteroids pulled into Uranus's orbit long ago.

Francisco orbits Uranus in the opposite direction from Uranus's travel around the Sun. This is called a retrograde orbit. It is the closest known irregular moon to Uranus. Its path changes a little over time, but scientists can still describe its average path over many years.

Physical characteristics

Francisco is a very faint moon. It can only be seen using special cameras on large telescopes. Scientists think its surface is dark and made of frozen water and minerals, like some other unusual moons. They guess Francisco’s size is between 11 to 17 kilometers (7 to 11 miles) across, but some think it might be a bit larger, around 22 kilometers (14 miles).

Exploration

Francisco has not been seen up close by a space probe, but Voyager 2 passed by it in 1986. Scientists hope to learn more about Francisco and other moons of Uranus with a planned mission called the Uranus Orbiter and Probe. This mission will watch how the moons' brightness changes to learn about their shapes and how they spin. However, Francisco's orbit is different, so the mission might not get very close to it.