The moon of Jupiter – S/2003 J12 – was discovered in March 2003 by S.S. Sheppard. This discovery was achieved by 8.3-m Subaru telescope atop Mauna Kea (Hawaii). You can read additional details in The International Astronomical Union Circular (→IAU).
    S/2003 J12 is one of four new satellites Jupiter's new satellites discovered since the →Voyager-2 upiter system fly-by which occured 24 years ago (1979). With this moon, the astronomer discovered also: S/2003 J9, S/2003 J10 and Kallichore. 
    The moon is in elliptical retrograde orbit (→eccentricity e = 0.4920) with a →semimajor axis a = 17,833,000 km. At pericenter (closest to the planet) S/2003 J12 is separated from the Jupiter a distance of q = 9,059,164 km. At apocenter (furthest from the planet) this moon is separated from the Jupiter a distance of Q = 26,606,836 km. In future these parameters may vary due to a large orbital distance from Jupiter and the orbital motion being disturbed by the Sun and other factors. Perhaps it is a captured asteroid (with a insignificant mass: ~1.4987×10¹² kg  – it equals 4903 times less than a mountain like Giewont!), and in future will possibly return to heliocentric orbit.

    The orbital period is 1.3 earthly years, which means that the average speed of its celestial motion is only 0°44'06'' per day. When this motion was detected and compared to the changes in Jupiter's position, this body was classified as a true planetary satellite (and not a distant celestial body with its position projected near the planet).
    Other obstacle in detecting this moon earlier was its brightness of only 23.9^m (over 14.5 million times fainter than the faintest stars visible to naked eye). For comparison: Jupiter's brightness is about –2.7^m, so S/2003 J12 is shining fainter by about 26.6^m. The corresponding difference in brightness is almost 44 billion times!