SOME three centuries ago, French-Italian astronomer Giovanni Domenico Cassini discovered a gap in Saturn's rings to which he lent his name. NASA has borrowed his name again for a mission to Saturn that would advance planetary exploration into the next century.
The Cassini project would send a spacecraft to survey Saturn, its rings, and several moons. To be launched in 1996, it would begin this research in 2002. Late last year, the European Space Agency (ESA) agreed to join the Cassini program and to supply a probe to drop on the moon Titan.
This is one of several missions sponsored by the United States and Soviet Union that, if approved, will make the 1990s a busy decade for planetary science.
The Soviets have identified Mars as their main target. Their mission to the Martian moon Phobos, now is nearing that objective, is the first phase of their program. They plan to follow this with a Mars orbiter that would send an instrumented balloon into the atmosphere, deploy small surface weather stations, and probe the soil with laser penetrators. That mission, in turn, would help pick a landing site for a 1996 mission with a rover robot explorer.
Meanwhile, the Soviets also expect to resume exploration of Earth's moon. A briefing for visiting French space officials last month included plans for a 1992 lunar orbiter. This would provide site selection data for a possible 1996 lunar sample-return mission - a feat the Soviets have not repeated since their successful sampling missions two decades ago.
The National Aeronautics and Space Administration is also interested in Mars. Its 1992 Mars Orbiter mission - already postponed several years - is to make an extensive surface-materials study.
The US and Soviets are continuing to discuss possible joint efforts in Mars exploration. NASA has agreed to use its Mars Orbiter data and other information to help the Soviets pick sites for their 1996 rover. The two are also talking about a joint project involving both roving robots and the return of geological samples to Earth.
But Cassini is the main new planetary mission NASA now seeks. ESA space scientist J.-P. Lebreton has called it ``the next logical step in detailed systematic exploration'' of the outer planets. He called the Saturn moon, Titan, as an ``especially interesting target.'' It has a largely nitrogen atmosphere with traces of hydrocarbons in it and a surface density 60 percent higher than the atmosphere on Earth. An ocean of liquid ethane with absorbed methane in it may underlie this atmosphere.
Scientists would like to probe this intriguing moon, where chemical reactions akin to those associated with the rise of life on Earth may occur.
ESA has picked Cassini participation as its next medium-priced space science project. If NASA can get authorization for the spacecraft and launch rocket, ESA will supply the Titan probe for about $260 million.
NASA hopes to get that authorization in fiscal 1990 - the first budget of the Bush administration. It is twinning Cassini with the Comet Rendezvous Asteroid Flyby (CRAF) mission in this request. Using similar versions of the same basic spacecraft and thus sharing some development and hardware costs, NASA would expect the missions to cost $850 million to $950 million (in 1988 dollars) as a joint project. On the other hand, it estimates that the total cost could run from $1.1 billion to $1.3 billion, if they are started separately.
It's difficult to foresee whether or not NASA will get a go-ahead for Cassini/CRAF (or Cassini alone), given budget-cutting pressure in Washington. But if it can start this ambitious new project, the agency will be taking its first step into the planetary science of the 21st century.