Space · Moons

Themisto

A moon of Jupiter — Discovered twice — found in 1975, lost, then rediscovered in 2000.

All moons → · All planets →

Quick facts

Parent planet

Jupiter

Diameter (mean)

8 km

Mass

6.9 × 10¹⁴ kg
9.4e-09 Moon masses

Mean orbital radius

7,507,000 km

Orbital period

130 Earth days

Discovery year

1975, 2000

Discoverer

Charles Kowal & Elizabeth Roemer (1975); rediscovered 2000

Naming origin

Greek nymph, mother of three sons by Zeus

Surface conditions

Themisto orbits alone between the Galilean moons and the Himalia group at 7.5 million km from Jupiter, occupying a unique orbital niche. Originally found in 1975, the moon was lost (no follow-up observations established a reliable orbit) and was rediscovered in 2000 from the Mauna Kea Observatory.

Missions and observations

Every Jupiter-system mission has had an opportunity to image or characterize Themisto. The list below is the Jupiter-system mission catalog; specific Themisto encounters are documented in mission archives.

Mission	Year at Jupiter	Status
Pioneer 10 NASA	1973	Completed
Pioneer 11 NASA	1974	Completed
Voyager 1 NASA	1979	Completed
Voyager 2 NASA	1979	Completed
Ulysses NASA/ESA	1992	Completed
Galileo NASA	1995	Completed
Cassini-Huygens NASA/ESA/ASI	2000	Completed
New Horizons NASA	2007	Completed
Juno NASA	2016	Active
Europa Clipper NASA	2030	On the way
JUICE ESA	2031	On the way

Naming etymology

Themisto was a daughter of Inachus and lover of Zeus, mother of Ister. The IAU adopted the name in 2002 after the moon's rediscovery secured its orbit.

Methodology & sources

Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.