Sinope was discovered on July 21, 1914, by Seth Barnes Nicholson on photographic plates taken with the Lick Observatory's 36-inch (0.9 meter) telescope.
Sinope is generally considered a member of the Pasiphae group, a family of Jovian satellites which have similar orbits and are therefore thought to have a common origin. However, there is some uncertainty about whether Sinope belongs in this group or not.
Most or all of the Pasiphae satellites are thought to have begun as a single asteroid that, after being captured by Jupiter's gravity, suffered a collision which broke off a number of pieces. The bulk of the original asteroid survived as the moon called Pasiphae, and the other pieces became some or all of the other moons in the group.
All of the Pasiphae moons are retrograde, so they orbit Jupiter in the opposite direction from the planet's rotation. Their orbits are also eccentric (elliptical rather than circular) and highly inclined with respect to Jupiter's equatorial plane. All of these characteristics support the idea that the Pasiphae satellites began as one or more captured asteroids, rather than forming as part of the original Jupiter system.
Compared to Jupiter's other satellite groups, confidence is lower that all the moons in the Pasiphae group originated in a single collision. This is due to differences in color (varying from red to gray), and differences in orbital eccentricity and inclination among the members of the Pasiphae group. Sinope, in particular, is suspected of starting out as an independent asteroid.
Both Pasiphae and Sinope are locked in secular resonances with Jupiter, which means that Jupiter's gravity tugs at them at regular intervals in a way that can modify their orbits over time. This could account for the differences in their orbits compared to each other and to other presumed members of the Pasiphae group.
If Sinope does not belong in the Pasiphae group, then the individual moon called Pasiphae retains 99 percent of the mass of the original asteroid. If Sinope is included, Pasiphae still retains the lion's share: 87 percent of the original mass. None of the Pasiphae members is massive enough to pull itself into a sphere, so they are probably all irregularly shaped.
Sinope has a mean radius of 11.8 miles (19 kilometers), assuming an albedo of 0.04. At a mean distance of about 14.8 million miles (23.9 million kilometers) from Jupiter, the satellite takes about 759 Earth days to complete one orbit.
Sinope was named for a nymph who outsmarted a smitten Zeus (the Greek equivalent of the Roman god Jupiter). Hoping to win her favor, he offered her anything she wanted and she replied that what she wanted was to remain a virgin. Sinope outsmarted Apollo in the same way.