The Planetary Report

November/December 1983

From Our Member Magazine

In 1983, Carl Sagan Urged NASA to Send a Mission to Saturn and Titan

Editor's note: Long before the Cassini-Hugygens spacecraft launched in 1997 to explore Saturn and Titan, The Planetary Society urged NASA to make the mission a reality. The below exchange between Planetary Society President Carl Sagan and NASA Administrator James Beggs in 1983 was printed in the November/December 1983 edition of The Planetary Report

To: Dr. James Beggs, Administrator, National Aeronautics and Space Administration

Dear Jim:

The Planetary Society is concerned that one of the most promising objectives for future planetary exploration is being given insufficient priority by NASA. Titan, the largest moon of Saturn, turns out, in the light of Voyager and subsequent discoveries, to be an extraordinary place.

It is the only moon in the solar system with a substantial atmosphere. Composed mainly of N2, and with a surface pressure of 1.6 bars, its atmosphere is most like Earth’s of any in the solar system. Recent evidence provides an at least plausible case for extensive surface oceans of methane and ethane, something like liquefied natural gas. This would make Titan the only other object in the solar system which, so far as we know, has a surface ocean.

A large number of more complex organic molecules are detected or suspected in the gas phase. Titan almost certainly has a dense cloud layer of condensed methanol and also has a red aerosol haze layer composed of very complex organic molecules; recent laboratory simulations suggest that building blocks of proteins and nucleic acids are present in the Titan clouds. The amount of complex organics that has been produced and sedimented out of the atmosphere over geological time may be considerably in excess of 100 meters, encrusting the land and accumulating as submarine deposits. These complex organic solids—produced from an N2/CH4 atmosphere—probably closely resemble the organic molecules that were produced in the primitive atmosphere of the Earth and which led, four billion years ago on our planet, to the origin of life. (The surface temperature is, of course, very low; and there is no evidence for liquid water, so the conditions are hardly identical to those during the early history of the Earth. But there is no place in the solar system which seems more similar to the primitive Earth.)

For all these reasons, Titan constitutes a unique exploratory objective, embracing a wide range of scientific disciplines, the promise of dramatic new discoveries, in missions likely to command—unlike a number of alternatives being proposed—major public and congressional enthusiasm and support.

Typical instruments for a first mission would include a radar reflectometer to map the distribution of land and ocean, and an entry mass spectrometer or GC/MS able to make in situ measurements of organic chemistry at preselected altitudes. Since at least several percent of ambient sunlight appears to penetrate the clouds, this or a subsequent mission could also incorporate photometric and imaging systems. If such a mission were coupled with a Saturn orbiter, able to follow up some of the many tantalizing questions raised by the Voyager exploration of Saturn and its rings and other moons, that would, of course, make a still more attractive mission.

Recent estimates suggest that a Galileo-type configuration, combining a Saturn orbiter with a Titan entry probe, could be achievable for three hundred to four hundred million dollars, rather than the 750 to 1000 million dollars that was estimated a few years ago. There is considerable congressional staff interest in such a mission—both because of the enormous public appeal of a Titan entry probe, and because such a mission is consistent with the recommendations of the Solar System Exploration Committee. SSEC has recommended that “resources be made available in FY 1985 to preserve the option of building a spare Galileo orbiter for a Saturn orbiter mission, either alone or as part of an international collaborative project, that also would send a probe into the atmosphere of Titan.” While international cooperation in such a mission is, or course, desirable, present indications are that the European Space Agency has yet to find a way to consider a 1989-91 Saturn/Titan mission. We are concerned that any delay of such a mission beyond that time frame would have serious adverse implications for the U.S. planetary program. On the other hand, a major commitment for such a Titan mission, as the centerpiece of essentially the SSEC recommendations, could produce a real renaissance in the now sluggish U.S. planetary program.

We think that a 1989 or 1990 launch of a Galileo class Titan mission would be enormously cost­-effective, and consistent with SSEC guidelines; it would be extremely meritorious for sciences ranging from prebiological organic chemistry to oceanography; and it would command enthusiastic public support. We very much hope that NASA will be able to make an early commitment to such a mission.

Cordially, CARL SAGAN


As man continues to reach out to explore the unknown and to search for his origins, the idea of exploring Titan poses a truly interesting and exciting challenge. This has perhaps best been summarized by the Solar System Exploration Committee Report which states: “We hope to study the chemical processes on Titan today that are analogous to those that led to the origin of life on Earth.”

NASA feels strongly that within the context of a balanced planetary program, we must return to Titan. To this end NASA and the European Space Agency have agreed to study a possible joint exploration of Titan. 

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The Planetary Report • November/December

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