To Open The Sky
The Front Pages of Christopher P. Winter
The Cassini Mission to Saturn
A personal view
A Brief History of Cassini
In the years following the Apollo series of manned lunar missions, cancelled in 1972, NASA increased its emphasis on robotic probes. The result was a string of high-profile, highly successful probes whose journeys outward from the sun yielded a treasure trove of information. These far-ranging missions — with well-known names like Pioneer, Viking, Voyager, Galileo — transformed the dimly-seen planets and moons there into worlds full of new and complex puzzles.
The most ambitious of these was Voyager. It was originally conceived as a "Grand Tour" to visit all the outer planets (the four gas-giants plus Pluto) using a pair of almost-identical spacecraft. One would have been sent to Jupiter, Saturn and Pluto; the other would have visited Jupiter, Uranus and Neptune. Voyager was later scaled back. The twin spacecraft were designed to probe Jupiter and Saturn only. This redundancy lowered the risk of total failure — which was significant since the environment of Jupiter was a harsh one. These two robots were launched in 1977 on their two-year transits to Jupiter. Their paths were chosen to boost them on toward Saturn so that, if they survived, they might return some science data from the ringed planet as well. In fact, they did survive, and returned spectacular pictures along with much scientific data. Voyager 1 gave us good data on Saturn's moon Titan, a high-priority objective. This cleared the way for Voyager 2 to be targeted to reach Uranus and then Neptune, where it finally arrived in August 1989, 12 years after the start of its epic journey. While not the first use of gravity assist (or "slingshot") to get a spacecraft where rocket power alone can't take it, the flight of Voyager 2 is certainly the grandest example so far of what can be achieved by this means.
Named after the 16th century Italian scientist who first identified the larger moons of Jupiter, the Galileo mission would take the next logical step for science by going into orbit around the banded giant to study its mysterious moons for two years. There was talk of having two spacecraft built, as had been done with Voyager. In Galileo's case, the idea was that the second, if not needed as a spare, could later be sent to Saturn, funds permitting. But this was never done.
What became Cassini was thus a totally independent development. Planning for the actual Cassini mission began in 1982 as one idea proposed by a joint U.S.-European working group to foster scientific cooperation. Approval from the European Space Agency (ESA) came in 1987, and NASA selected it as one role for the standardized Mariner Mark 2 spacecraft it was then designing for outer-planets missions. (The other was the Comet Rendezvous/Asteroid Flyby mission, or CRAF.) Congress granted funding in 1989. Then, in 1992, it imposed a funding cap on the Mariner Mark 2 program, effectively killing CRAF. The cap also brought cutbacks in Cassini. No science would be done en route, as had been planned. Also, the instrument platforms were to be fixed in place rather than gimballed as originally planned. This meant that to point a camera, for example, the whole spacecraft would have to be turned. The technique worked well for Voyager 2 when its camera mount jammed, but in general it reduces the amount of scientific data obtained. NASA Administrator Dan Goldin almost cancelled Cassini in 1994; however, the multinational involvement kept it alive. Thus, this approximately $3 billion mission, sometimes called the last of the dinosaurs, was launched without a hitch on 15 October 1997. It will arrive at Saturn in 2004, after a 2 billion mile journey. There it is expected to operate for another four years.