Themisto also known as , is a small prograde irregular satellite

Irregular satellite

In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

 of Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. It was discovered in 1975, lost, and then rediscovered in 2000.

Discovery and naming

Themisto was first discovered by Charles T. Kowal

Charles T. Kowal

Charles Thomas Kowal was an American astronomer.He discovered two moons of Jupiter: Leda in 1974 and Themisto in 1975, although the latter was lost and not rediscovered until 2000....

 and Elizabeth Roemer

Elizabeth Roemer

Elizabeth Roemer is an American astronomer whose research interests center on comets and asteroids.She discovered the asteroids 1930 Lucifer and 1983 Bok...

 on September 30, 1975, reported on October 3, 1975, and designated . However, not enough observations were made to establish an orbit and it was subsequently lost. (See also Lost asteroids

Lost asteroids

Lost asteroids or lost planets are asteroids that observers lose track of after discovering. Many early lost asteroids were rediscovered in the 1980s and 1990s, but a number of asteroids and other types of small Solar System bodies continue to be lost...

.)

Themisto appeared as a footnote in astronomy textbooks into the 1980s. Then, in 2000, a seemingly new satellite was discovered by Scott S. Sheppard

Scott S. Sheppard

Scott S. Sheppard is an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Science. Starting as a graduate student at the Institute for Astronomy at the University of Hawaii, he was credited with the discovery of many small moons of Jupiter, Saturn, Uranus, and...

, David C. Jewitt

David C. Jewitt

David C. Jewitt is a professor of astronomy formerly at the University of Hawaii Institute for Astronomy, now at UCLA. He was born in 1958 in England, and is a 1979 graduate of the University of London. Jewitt received an M.Sc. and a Ph.D. in astronomy at the California Institute of Technology in...

, Yanga R. Fernández

Yanga R. Fernández

Yanga Roland Fernández is a Canadian-American astronomer at the University of Central Florida.Together with Scott S. Sheppard, he co-discovered the Carme group, a group of moons of the planet Jupiter....

 and Eugene A. Magnier, and was designated . It was soon confirmed that this was the same as the 1975 object. The Sheppard et al. announcement was immediately correlated with an August 6, 2000 observation by the team of Brett J. Gladman

Brett J. Gladman

Brett J. Gladman is a Canadian astronomer and a full professor at the University of British Columbia's Department of Physics and Astronomy in Vancouver, British Columbia. He holds the Canada Research Chair in Planetary Astronomy.-Career:...

, John J. Kavelaars

John J. Kavelaars

John J. Kavelaars, better known as JJ Kavelaars, is a Canadian astronomer who was part of a team that discovered several moons of Jupiter, Saturn, Uranus, and Neptune....

, Jean-Marc Petit, Hans Scholl

Hans Scholl (astronomer)

Hans Scholl, born 1942, is a German astronomer working at the Côte d'Azur Observatory in Nice, France. In 1999, he was part of a team that discovered the satellites Prospero, Setebos and Stephano of the planet Uranus.-External links:*-References:...

, Matthew J. Holman

Matthew J. Holman

Matthew J. Holman is a Smithsonian Astrophysicist and lecturer at Harvard University. Holman studied at MIT, where he received his bachelor's degree in mathematics in 1989 and his PhD in planetary science in 1994....

, Brian G. Marsden

Brian G. Marsden

Brian Geoffrey Marsden was a British astronomer born in Cambridge, England, and educated at The Perse School in Cambridge, New College, Oxford and Yale University. Dr...

, Philip D. Nicholson and Joseph A. Burns

Joseph A. Burns

Joseph Burns is a professor at Cornell University with a dual appointment in the Department of Theoretical and Applied Mechanics and the Astronomy department. His primary area of research is dynamics in planetary sciences.- Professional biography :...

 — an observation that was reported to the Minor Planet Center but not published as an IAU Circular (IAUC).

In October 2002 it was officially named after Themisto

Themisto

In Greek mythology, Themisto , daughter of Hypseus, was the third and last wife of Athamas. According to Apollodorus, she had five children by him: Leucon, Erythrius, Schoeneus, Ptous, and Porphyrion. In other sources there were but two: Sphincius and Orchomenus, or else Schoeneus and Leucon...

, daughter of the river god Inachus

Inachus

In Greek mythology, Inachus was a king of Argos after whom a river was called Inachus River, the modern Panitsa that drains the western margin of the Argive plain...

 by Zeus

Zeus

In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...

 (Jupiter) in Greek mythology

Greek mythology

Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

.

Characteristics

Themisto's orbit is unusual: unlike most of Jupiter's moons, which orbit in distinct groups, Themisto orbits alone. The moon is located midway between the Galilean moons

Galilean moons

The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

 and the first group of prograde irregular moons, called the Himalia group

Himalia group

The Himalia group is a group of prograde irregular satellites of Jupiter that follow similar orbits to Himalia and are thought to have a common origin.The known members of the group are :* Leda...

.

Themisto is about 8 kilometers (5 miles) in diameter (assuming an albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 of 0.04).

External links

• David Jewitt's pages
• Jupiter's Known Satellites (by Scott S. Sheppard
  Scott S. Sheppard
  Scott S. Sheppard is an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Science. Starting as a graduate student at the Institute for Astronomy at the University of Hawaii, he was credited with the discovery of many small moons of Jupiter, Saturn, Uranus, and...
  
  )