This Friday, NASA 's Cassini spacecraft will come to a similar end. At 6:31 A.M. , Eastern Daylight Time, after two decades of flight and thirteen years of spectacular discovery around Saturn, and with the blessing of its controllers at the Jet Propulsion Laboratory, in Pasadena, California, Cassini will touch the gas giant's dense atmosphere and be drawn into it. The end will come quickly. Cassini, travelling at seventy-six thousand miles per hour, will continue to relay data for a minute or two, until it begins to tumble; then its high-gain antenna will aim away from Earth and its signal will cease. In a matter of minutes or even seconds, the mounting pressure of the atmosphere will reduce Cassini to particles, long before the spacecraft reaches the surface of Saturn, if Saturn even has a surface. "It will be a plunge, not a crash," Joan Stupik, one of Cassini's guidance-and-control engineers, told me earlier this week.