The Cassini space probe—the largest interplanetary probe ever launched, weighing 6.3 tons, and extending 22 feet in length—was a joint NASA, European Space Agency (ESA), and Italian Space Agency (ASI) mission to study Saturn and its rings, moons, and magnetic environment. Launched on October 17, 1997, atop a Titan IV rocket from Cape Canaveral, Florida, it required three radioisotope thermoelectric generators (RTG) with 72 lbs of plutonium 238 to power a wide array of scientific instruments at Saturn.
This use of nuclear power in space prompted protests from American antinuclear activitists. In addition, Cassini required gravity assist to reach Saturn in 6.7 years. It followed a Venus-Venus-Earth-Jupiter Gravity Assist (VVEJGA) trajectory that energized the antinuclear community as had nothing since the Galileo launch.
Cassini’s three RTGs and 117 lightweight radioisotope heater units (RHU) provided the necessary electrical power to operate its 19 instruments and maintain the temperatures of critical components and the Huygens probe that was destined for deployment by parachute onto the surface of Titan, Saturn’s largest moon. Those three RTGs provided 888 W of electrical power at mission beginning, but would still generate 596 W after 16 years of operation.
As always, Cassini’s RTGs were tested extensively to ensure that they could withstand any conceived destructive force associated with the flight. Also, as had been the practice for many years independent safety analyses by General Electric, Lockheed Martin, and other technical organizations considered possible results from pre-launch fires and explosions, launch accidents, and spacecraft crashes and uncontrolled reentry. Three major reports resulted from those efforts, with the final prepared one year in advance of the projected launch.
This material, along with additional studies by the Department of Energy and NASA, went to an independent Interagency Nuclear Safety Review Panel responsible for judging whether or not to recommend a decision in favor or launch to the President of the United States. As a GAO audit of the Cassini mission documented:
The processes used by NASA to assess the safety and environmental risks associated with the Cassini mission reflected the extensive analysis and evaluation requirements established in federal laws, regulations, and executive branch policies. For example, DOE designed and tested the RTGs to withstand likely accidents while preventing or minimizing the release of the RTG’s plutonium dioxide fuel, and a DOE administrative order required the agency to estimate the safety risks associated with the RTGs used for the Cassini mission. Also, federal regulations implementing the National Environmental Policy Act of 1969 required NASA to assess the environmental and public health impacts of potential accidents during the Cassini mission that could cause plutonium dioxide to be released from the spacecraft’s RTGs or heater units. In addition, a directive issued by the Executive Office of the President requires an ad hoc interagency Nuclear Safety Review Panel. This panel is supported by technical experts from NASA, other federal agencies, national laboratories, and academia to review the nuclear safety analyses prepared for the Cassini mission. After completion of the interagency review process, NASA requested and was given nuclear launch safety approval by the Office of Science and Technology Policy, within the Office of the President, to launch the Cassini spacecraft.
This detailed and involved process led to the conclusion that while risk could not be eliminated entirely that the chances of any breech of the plutonium-238 container was exceptionally low. The estimated health effect of an accident was that over a 50-year period not one more person would die of cancer caused by radiation exposure than if there were no accident. These analyses also found that during Cassini’s Earth encounter there was less than a one in a million chance that the vehicle would accidentally reenter Earth’s atmosphere.
None of this review convinced some in the antinuclear community and it mobilized to prohibit the Cassini launch. The well-organized STOP CASSINI! campaign rested its opposition on the claim that NASA’s technical risk assessment omitted, neglected, or underestimated the welfare of the public as a whole. Accepting that NASA had fulfilled the letter of law, this protest asserted that the government as a whole had to be redirected away from the use of nuclear power or weaponry in any form whatsoever.
Sociologist Jürgen Habermas has suggested that when the “instrumental rationality” of the bureaucratic state intrudes too precipitously into the “lifeworld” of its citizenry, they rise up in some form to correct its course or to cast it off altogether. The “lifeworld” is evident in the ways in which language creates the contexts of interpretations of everyday circumstances, decisions, and actions. He argues that the “lifeworld” is “represented by a culturally transmitted and linguistically organized stock of interpretive patterns.” The STOP CASSINI! campaign represented an effort to exile nuclear material from the “lifeworld” of modern America, as their expressions of discontent demonstrated, and they could obtain no resolution from the “instrumental rationality” residing in the state. They took direct action and justified it without a tinge of conscience as necessary for the greater good.
Opponents of Cassini organized a rally of about 1,500 participants at Cape Canaveral, Florida, in May 1997 with several prominent disarmament leaders speaking. They received publicity from CNN and the NBC local affiliate, as well as print journalists and radio stations. They argued for greater involvement in choosing the technologies used on spacecraft, specifically nuclear power. They tried to sensitize the public to dangers from the use of nuclear power for space exploration, and addressed not only environmental risks but also the motives behind the reason for using nuclear power. One protester commented:
The military has made an unholy alliance with NASA in its quest for space domination. Now people-power and a commitment to compassion and conscience must be brought into an area where it is not wanted and where it is lacking. There must be resistance to the U.S. push to weaponize and nuclearize space…a renegade government spending massive amounts of money to weaponize and nuclearize space, and at the same time saying that no money is available for schools and other social needs. This issue is not about losing our democracy—we have lost it.”
The STOP CASSINI! protest received news reporting from many of the major U.S. news outlets, and the Internet buzzed with discussion of its efforts to end the Cassini mission. It deserved credit for gaining the attention of several members of Congress, who demanded additional analysis from NASA and the Department of Energy.
When Cassini launched safely on October 14, 1997, the media gave credit to the protesters for forcing NASA to reconsider its use of nuclear power in space and to undertake more extensive testing and verification of systems. A vigil outside the main gate of KennedySpaceCenter by the STOP CASSINI! campaign was peaceful. It had raised important questions about this technology and its meaning for society. As one scholar noted, NASA responded poorly to this challenge in terms of public communication. It believed that more information would resolve the crisis, but there is little reason to believe that this would be the case as the protest had more to do with ideology and values than with assessments of information.
In the end the Cassini mission has been conducted with stunning success. Cassini is the first spacecraft to orbit Saturn, beginning July 1, 2004, and to send a probe (Huygens) to the surface of Saturn’s moon Titan on January 15, 2005. But even before its Saturnian encounter, the Cassini mission advanced science by finding individual storm cells of upwelling bright-white clouds in dark “belts” in Jupiter’s atmosphere, and by conducting a radio signal experiment on October 10, 2003, that supported Einstein’s theory of general relativity. At Saturn, Cassini has discovered three new moons (Methone, Pallene and Polydeuces) and observed water ice geysers erupting from the south pole of the moon Enceladus. Cassini demonstrated that icy moons orbiting gas giant planets are potential refuges of life, and attractive destinations for a new era of robotic planetary exploration.
Fortunately, there were no health repurcussions from the nuclear power system aboard the spacecraft. As these types of missions continue great care must always be taken to ensure safety.