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A lawsuit almost stalled NASA's Cassini mission


The Cassini mission, named after the 17th century Italian-French astronomer Jean-Dominique Cassini, marks the end of an era for NASA. It is likely the final “flagship-class” mission (those costing more than $1 billion) fielded by the space agency, if NASA Administrator Charles Bolden’s claims from 2013 are still accurate. Other flagships included the vaunted Viking and Voyager missions as well as the Mars Curiosity rover and the Hubble Space telescope.

JPL engineers working on the Cassini orbiter propulsion module

The Cassini mission started in 1982 when the European Science Foundation and NASA were still kicking around the idea of conducting their own respective solo missions to Saturn. Despite an impassioned report from astronaut Sally Ride in 1986, titled NASA Leadership and America’s Future in Space, NASA and the ESA decided to go in halfsies on a joint mission.

However, by 1994, the mission’s Congressional critics had begun to question the value of such a mission. The program had already eclipsed $3.3 billion in development costs — that’s $5 billion in 2017 money, adjusted for inflation, or about half of what we spent on the new James Webb Space Telescope. It was only because the ESA was also contributing funds to the mission and NASA was able to demonstrate that technology developed for Cassini would carry over to the Mars Global Surveyor, Mars Pathfinder and the Spitzer Space Telescope projects that this one was allowed to move forward.

That forward momentum came to a sudden halt three years later and just a day after then-President Clinton approved the mission. On October 4th, 1997, 800 protesters showed up (27 of which were arrested) at Cape Canaveral Air Force Station in opposition to the Cassini launch, which was then scheduled for October 7th.

US Air Force security police form a line to thwart

Protesters outside of Cape Canaveral

The protesters were worried that, should the Titan IV rocket ferrying the orbiter into space suffer a catastrophic mishap during launch, it would vaporize the 73 pounds of Plutonium-238 that the Cassini carried and spread radioactive fallout across central Florida. The protesters were even more worried about that Cassini’s upcoming gravitational slingshot, which would use the Earth’s pull to accelerate the spacecraft into the outer solar system, could spread fallout across the globe, should Cassini accidentally re-enter orbit during the maneuver. The Green Party even went so far as to file a federal lawsuit against the government in a Hawaiian court to halt the launch.

“Winds can blow (plutonium) into Disney World, Universal City, into the citrus industry and destroy the economy of central Florida,” Michio Kaku, professor of theoretical physics at the City College of New York, told Mother Jones. He calculated that as many as a million people could be exposed to radiation if the launch went wrong.

The protesters’ issue focused on, again, the 73 pounds of Plutonium-238 aboard the Cassini orbiter. This wasn’t the first time that NASA had utilized radioactive materials as a power source for its long-endurance spacecraft — New Horizons, Galileo, and Ulysses all carried similar setups — but none had ever carried this much Pu-238 at one time before. The orbiter actually employed three radioisotope thermoelectric generators (RTGs) during its 11-year mission. They’re not for propulsion, mind you (that’s what the gravitational slingshots were for), but rather a means to power the onboard scientific instruments for the duration of the trip.

RTGs are fairly simple devices and have been used for both civilian and military applications since their development in the 1950s. They consist of a container of radioactive material with a pair of thermocouples attached to the outside. The opposite end of each thermocouple is also attached to a heat sink. As the material decays, it produces heat. The difference in temperature between the container and the heatsink enables the thermocouples to generate an electrical charge. It’s the same principle that allows camp stoves to also charge your phone.

Cassini’s instruments aren’t particularly power-hungry, drawing around 600 – 700 watts of electricity, but 11 years is a long time in the cold depths of interplanetary space. So why not just use solar panels, Cassini protesters argued, as NASA had for a bunch of other missions? The problem with that is the sheer distance between Saturn and the Sun — 888 million miles on average. NASA did actually look into outfitting Cassini with solar panels but the math simply didn’t work.

The Mars Rover does well enough with solar power, for example, but it’s six times closer to the Sun than Saturn. In order to produce the requisite wattage while in Saturn’s orbit, the Cassini would have had to sport panels the size of tennis courts. These would have had proven too bulky and too heavy to get the orbiter out of Earth’s atmosphere.

Plutonium-238 is 280 times more radioactive than Plutonium-239, the stuff we use to make nuclear weapons, and has a half life of 88 years. That makes it a potent and long-lasting power source. What’s more, the alpha particles that Pu-238 emits can’t penetrate further than a few cellular levels, so the biggest threat comes from inhaling the stuff. However, “it cannot be exploded like a bomb,” Beverly Cook of the Energy Department, told CNN. “It is an alpha emitter. Alpha radiation can be stopped by a piece of paper.”

The chances of having Cassini’s payload of nuclear material vaporize during a catastrophic engineering failure were exceedingly remote, according to NASA. “This is not a nuclear reactor. They are nuclear batteries,” Wes Huntress, associate administrator for space science at NASA, explained to PBS Newshour. “They’re not used for propulsion. It’s not a nuclear power plant. We don’t have any nuclear reactions going on. We simply use the isotope to generate heat, and from the heat we generate electricity for the spacecraft.”