Naiad . Neptune III - 1989N6 . Naiad, the last satellite discovered, is about 54 kilometers (33 miles) in diameter and orbits Neptune every 7 hours and 6 minutes about 23,200 kilometers (14,400 miles) above the clouds. ... Naiad circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane. ... Discovered by . ... Mean density (gm/cm^3) . Mean distance from Neptune (km) . ... Rotational period (days) . Orbital period (days) . ...