dimensions of the Solar System. In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division is a region between the A ring and B Ring. In 1787, Pierre-Simon Laplace proved that a uniform solid ring would be unstable and suggested that the rings were composed of a large number of solid ringlets. In 1859, James Clerk Maxwell demonstrated that a nonuniform solid ring, solid ringlets or a continuous fluid ring would also not be stable, indicating that the ring must be composed of numerous small particles, all independently orbiting Saturn. Later, Sofia Kovalevskaya also found that Saturn's rings cannot be liquid ring-shaped bodies. Spectroscopic studies of the rings which were carried out independently in 1895 by James Keeler of the Allegheny Observatory and by Aristarkh Belopolsky of the Pulkovo Observatory showed that Maxwell's analysis was correct. Four robotic spacecraft have observed Saturn's rings from the vicinity of the planet. Pioneer 11s closest approach to Saturn occurred in September 1979 at a distance of . Pioneer 11 was responsible for the discovery of the F ring. Voyager 1s closest approach occurred in November 1980 at a distance of . A failed photopolarimeter prevented Voyager 1 from observing Saturn's rings at the planned resolution; nevertheless, images from the spacecraft provided unprecedented detail of the ring system and revealed the existence of the G ring. Voyager 2s closest approach occurred in August 1981 at a distance of . Voyager 2s working photopolarimeter allowed it to observe the ring system at higher resolution than Voyager 1, and to thereby discover many previously unseen ringlets. Cassini spacecraft entered into orbit around Saturn in July 2004. Cassini images of the rings are the most detailed to-date, and are responsible for the discovery of yet more ringlets. The rings are named alphabetically in the order they were discovered (A and B in 1675 by Giovanni Domenico Cassini, C in 1850 by William Cranch Bond and his son George Phillips Bond, D in 1933 by Nikolai P. Barabachov and B. Semejkin, E in 1967 by Walter A. Feibelman, F in 1979 by Pioneer 11, and G in 1980 by Voyager 1). The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A.