As revealed by the Voyager cameras, the rings extend outward from the upper atmosphere to a distance of 53 000 kilometers above the cloud tops, 1.8 R_J from the center of the planet. The main rings, however, are much narrower, spanning from 47 000 to 53 000 kilometers above Jupiter. There are two main rings, a 5000-kilometer-wide segment, and a brighter, outer 800-kilometer segment. The thickness of the rings is unknown, except that it is certainly less than 30 kilometers, and probably under 1 kilometer. [Illustration: Structure within the ring can be seen in the best Voyager 2 images, taken about 27 hours after closest approach to Jupiter. This enlarged portion of a wide-angle picture taken with a clear filter shows a bright core about 800 kilometers across with a dimmer region a few thousand kilometers across on the inside, and a narrow dim region on the outside. [260-674]] The rings of Jupiter are quite tenuous, which explains why they are invisible from Earth [although they have been detected from Earth since Voyager]. Seen face on, the brightest part of the ring blocks less than one part in ten thousand of the light passing through, making it essentially transparent. In fact, the ring does not even offer much resistance to a spacecraft; Pioneer 11 traversed the ring in 1974 with no obvious ill consequences. Apparently the individual particles that make up the ring are widely dispersed. They can be seen only when the rings are viewed nearly edge-on, or toward the Sun, where they show up well in forward scattered light. It is this extra brilliance when backlit that created the excellent photos taken by Voyager 2 from inside the shadow of Jupiter. The individual ring particles are probably dark, rocky fragments that are very small—essentially dust grains. They move around Jupiter in individual orbits, circling the planet in 5 - 7 hours. Scientists postulate that such orbits are not stable and that the particles fall slowly in toward Jupiter. Apparently the rings are constantly renewed from some source, which may be the satellite Adrastea (J14), discovered by Voyager 2. There has also been speculation that Adrastea may influence the ring structure by sweeping particles out of the ring. At present the rings of Jupiter remain mysterious. They are clearly very different from the rings of Saturn and Uranus, and reaching an understanding of their origins and dynamics presents many challenges to planetary scientists.