The moon of Jupiter – Cyllene (XLVIII) – was previously temporarily designated as S/2003 J13. It was discovered in April 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).
    Cyllene is one of six new satellites discovered since the →Voyager-2 Jupiter system fly-by which occured 24 years ago (1979). With this moon, the astronomer discovered also: Kore, S/2003 J15, S/2003 J16, S/2003 J17 and S/2003 J18. 
    The moon is in elliptical retrograde orbit (→eccentricity e = 0.4115) with a →semimajor axis a = 23,809,000 km. At pericenter (closest to the planet) Cyllene is separated from the Jupiter a distance of q = 14,011,597 km. At apocenter (furthest from the planet) this moon is separated from the Jupiter a distance of Q = 33,606,404 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 microscopic mass: ~1.4987×10¹³ kg – it equals 490 times less than a mountain like Giewont!), and in future will possibly return to heliocentric orbit.

    The orbital period is over 2,1 earthly years, which means that the average speed of its celestial motion is only 0°28'43'' 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 23.2^m (almost 7.6 million times fainter than the faintest stars visible to naked eye). For comparison: Jupiter's brightness is about –2.7^m, so Cyllene is shining fainter by about 25.9^m. The corresponding difference in brightness is almost 23 billion times!