Many of the world's telescopes, including NASA's Hubble Space Telescope, saw a once in 15-year event last week:a Saturn ring plane crossing. The rare event occurs when the alignment of the Sun, Saturn, and Earth cause the rings to seemingly disappear for about 24 minutes, allowing astronomers to look for new moons and other features that are normally difficult to see when the rings are illuminated. It's somewhat like rotating a phonograph record 90° and viewing it edge-on, explains Ellis Miner, NASA science manager for the spacescraft Cassini. In the next year, on August 10, 1995, and February 11, 1996, the planes will cross again to complete the cycle. Thirteen of Saturn's 18 known moons were discovered during past ring plane crossings. Learning more about Saturn's moons and rings will help astronomers guide Cassini through its observations when it flies in October of 1997. Particles larger than a millimeter would be able to penetrate the spacecraft at the speed it will be flying, Miner says.