Cassini Can Unveil Saturn’s Secrets

A controversy has erupted over NASA’s plans to launch the Cassini spacecraft Oct. 13, the science mission of the decade for NASA, the European Space Agency and the Italian Space Agency.

Cassini’s seven-year interplanetary cruise will carry it inward in the solar system toward Venus and then back past Earth as it spirals out toward its final destination, Saturn, which it will orbit and where it will conduct scientific research for four years.

The dispute involves Cassini’s “electric batteries,” which produce power from plutonium dioxide. Critics claim that a launch vehicle failure or a collision with Earth when the spacecraft flies past on its way to Saturn presents a risk of widespread plutonium distribution around the Earth.

Will the mission’s benefits provide an appropriate counterbalance to the risk? The Saturnian system has many unique features that make it a prime target for a deep space exploration mission, the most prominent being its magnificent rings, discovered by Galileo, and its diverse satellites discovered since Galileo’s time. One of these satellites, Titan, is larger than our own moon and the planet Mercury. Like Earth, it has an atmosphere of mostly nitrogen gas and a significant amount of methane, which played an important role in the atmosphere of the early Earth. While no one should harbor illusions that life on Titan will be found by Cassini, the results of its scientific investigations will help understand the primordial Earth.

Saturn’s other satellites are also unusual in that each moon’s surface has a unique brightness and color. The individuality of each moon, combined with Saturn’s extensive ring system, suggest that the Saturnian system was subjected to a major disruption relatively recently in cosmogenic time. This disruption might have occurred 1 billion years ago; however it may have happened after humans had evolved.

Interest in Saturn is due in part to the suggestion that the system is in recovery from a recent traumatic event, probably a collision with a massive comet. In 1994, the world watched in fascination as pieces of the comet Shoemaker-Levy 9 smashed into Jupiter. The recovery of the Jovian system from the impacts happened on time scales of years. The hypothesized impact that upset Saturn was far larger but such events are common over the 5-billion-year lifetime of the solar system. For example, geologic investigations suggest that 65 million years ago, a massive asteroid or comet struck Earth. In the ecological havoc that ensued, a majority of species on our planet’s surface, most notably the dinosaurs, were extinguished. Today, we must prevent such an event from taking us by surprise.

The results of the Cassini investigations into the Saturnian system ultimately will factor into our own understanding of the changing environment on Earth.

As for the risks associated with the plutonium generators, the Department of Energy and NASA have done extensive studies. Similar electrical power supplies have been used on Voyager, the grand tour of the outer solar system; Ulysses, the solar polar orbiter; and Galileo, the Jupiter orbiter and probe mission. NASA must go through the standard environmental review on any proposed mission. These reviews have been conducted in the open, with every phase of development validated by the required peer review and multiagency reviews and additional, self-imposed independent reviews.

NASA calculates that there is less than one chance in a million that an accident could occur when Cassini “flies by” Earth in 1999. The expected radiation dosage a person might receive should such an accident occur is 1 millirem (unit of radioactivity). Each of us absorb 300 millirem of natural radioactivity annually from the Earth and from cosmic rays. People who live at high altitudes or those who fly a lot receive an even higher dose.

Over the next 50 years, it is estimated that 1 billion people will die of cancer. In a theoretical worst-case Cassini accident, it has been imputed that 120 to 1,200 deaths could be added to this number.

These estimates are the product of extensive verification and review. Those who criticize Cassini have been invited to comment on these environmental impact reports. However, these groups have yet to submit any peer-reviewed, logical arguments to support their claims. There is a tragic irony here. In the next year, humans are in greater danger of suffering a much larger loss of life from catastrophic impact by a comet or asteroid than from a Cassini mishap. Yet learning how planetary systems respond to catastrophic upsets would be one of the many byproducts of a successful Cassini mission. The adverse effects from plutonium dispersal would be so small that it would be a formidable challenge to measure them. Cassini is well worth the risk.