Thebe is a small moon of the planet Jupiter, which was first discovered by Voyager 1 in 1979. One of Jupiter’s relatively thin rings, known as the Thebe ring, consists of dust lost from the surface of this moon. It orbits just over 200,000 kilometres from Jupiter, or about one-half the distance from the Earth to the Moon.
There is a long history of Jupiter’s extremely large moon system (currently 63, the largest number in the solar system). The four largest of its moons – Ganymede, Europa, Io, and Callisto – were first seen by Renaissance astronomer Galilei and was part of the inspiration for a more modern Sun-centered or heliocentric (as compared to geocentric, or Earth-centered) view of the universe. The many small moons around Jupiter, however, attracted substantially less attention. Thebe was not discovered until the first relatively close approach to Jupiter by a space probe – Voyager 1 – was made in 1979. Subsequently, its sister probe Voyager 2 also managed to image the moon. The Galileo probe also was able to image Thebe during its lengthy study of Jupiter and the Jovian moons.
Thebe is a very small moon, with a radius of about 30 miles. In contrast, Earth’s moon has a diameter of nearly 1100 miles, and the largest of the Jovian moons, and Jupiter’s largest moon, Ganymede (which is also the largest moon in the solar system), has an intimidating radius of about 1600 miles. Thebe simply could not have been discovered with past generations of telescopes because of its great distance from Earth as well as its comparatively puny size.
This moon is small enough that it has not been compressed by its gravity into spherical shape. However, images by space probes show that it is dark red, may have an icy layer underneath its surface, and that it has suffered from several massive impact craters. The Zethus crater, the largest, is 25 miles across. It is the moon’s most distinguishing feature, and the only surface landmark named (as of yet) by astronomers.
Like all moons orbiting at such comparatively short distances from Jupiter, Thebe is gravitationally locked, meaning that (like Earth’s Moon) its period of rotation (one Thebe “day”) lasts the same amount of time as its orbital period around Jupiter, or about 16 hours.
NASA’s next space probe bound for Jupiter, Juno, is scheduled for launch in 2011 and will probably take new images of Thebe several years after that. However, as a small inner moon with no prospect of harbouring life, Thebe is not currently a major target for space research. Astronomers are much more interested in its larger siblings, particularly Europa, which is believed to harbour a massive underground water ocean and perhaps even primitive life within that ocean.