Protesters at the Cape Canaveral Air Force Station before Cassini's launch in October 1997Chris O'Meara / AP

The Cassini mission to Saturn—which ends with a fiery plunge into the ringed planet’s atmosphere Friday morning—has been a triumph of exploration. Yielding a rich trove of scientific discoveries, images, and data about the worlds that compose the Saturnian system, Cassini has deepened our understanding of planetary evolution and the possible environs for life elsewhere in the universe.

But before it even left the ground, the mission had to survive many challenges. One of these arose out of a fear that Cassini could cause an environmental disaster on Earth. Because of Cassini’s plutonium power source, antinuclear activists in the late 1990s sought to prevent its launch through protests, direct actions, and lawsuits. Unexpectedly, environmentalism and planetary science—two fields committed to expanding awareness of life and its global context—found themselves at odds.

This created cognitive dissonance for people invested in both space exploration and environmental stewardship—myself included. Ever since the environmental movement was sparked by photos of the whole Earth taken by astronauts onboard Apollo Lunar Modules, I’ve seen planetary exploration as an extension of a reverence and care for Earth. Good stewardship requires understanding planetary functioning, and breakthroughs in comprehending environmental problems can come from expanding our knowledge beyond this sphere. Insights into the greenhouse effect, the ozone hole, and acid rain, for instance, can come from exploring the atmosphere of Venus.

But I also understood the protesters’ fear of contamination and the mistrust of government assurances. I’d been politically active ever since my parents wheeled me in a stroller in a “ban the bomb” march in Boston in 1963.

The protesters were rightfully alert to the potential dangers of Cassini’s plutonium fuel. Plutonium is one of the most dangerous substances on Earth, and Cassini carried 72 pounds of plutonium oxide. But they were also misled by outrageously exaggerated and physically absurd claims that the launch would endanger Earth’s entire biosphere, threatening to give cancer to the entire human population, or even manifesting a secret plot to militarize space. You could theoretically kill everyone on Earth with 72 pounds of plutonium, just as you could in theory wipe out the human race with a single automobile, if you lined everyone up and ran them all over. In reality, a credible worst-case scenario, in which all this material was vaporized and rained down on Florida, could have potentially killed thousands of people and contaminated a wide area.

NASA has never denied that plutonium is deadly stuff. Rather, they assume the worst, and take extraordinary measures to ensure the safety of nuclear materials during launch, encasing the plutonium fuel in iridium, graphite, ceramic, and aluminum, and hardening the containers to the point where, even in a catastrophic launch explosion where nothing else survives, they would be unscathed. In fact, these power sources—which are not nuclear reactors, but simply heat sources for generating electricity, with no moving parts—have survived launch failures. They are so robust that after one rocket crashed, its power supply was plucked from the bottom of the ocean and used onboard another spacecraft.

Cassini lifted off flawlessly in October 1997, then completed a flyby of Earth in late 1999 to gravitationally slingshot itself out toward Jupiter, where it would be further ricocheted toward its destination at Saturn. Since it arrived in 2004, Cassini has performed exquisitely. It has uncovered the mesmerizing patterns in Saturn’s rings and the planet’s stormy atmosphere. It has explored a zoo of small moons with bizarre shapes and markings, requiring us to expand our notions of planetary physics and geology.

One night in 2005, working late in my office at Southwest Research Institute in Boulder, my colleague John Spencer me called into his office. He had been analyzing some infrared data from Saturn’s icy moon Enceladus, and something was wrong. He was finding an intense hot spot centered exactly on the planet’s south pole, in a region where a curious “tiger-stripe” pattern of fissures had been photographed on the surface. This was the first hint of one of Cassini’s most wonderful and momentous finds: Enceladus is spewing water out into space from its heated and fractured southern tip. Because of the now-confirmed existence of liquid water there, and its accessibility to future spacecraft, the moon is one of the most enticing places to revisit in a more direct search for life.

Cassini also revealed Saturn’s largest moon, Titan, to be a complex, shifting world where giant dune fields of organic molecules blow in nitrogen winds over icy plains, and where organic chemistry mirrors precursors to life on Earth. Titan has rivers and lakes of liquid methane and ethane, methane weather systems of clouds and storms that mirror Earth’s hydrologic cycle, and seasonal cycles that rival Earth’s in complexity. Titan, too, is a world that could harbor life, and another place we’ll surely return to, perhaps to explore with airships or boats. In the meantime Titan has already helped us see climate in new ways. Repeated measurements of the radiation, chemistry, and particles in that strange atmosphere have added to the body of understanding with which we predict and respond to the changing climate of our own planet.

After all these years and discoveries, the discord surrounding Titan’s launch now seems like ancient history. Fittingly, Cassini’s demise on Friday will be carried out as one final act of environmental responsibility. The spacecraft is being deliberately crashed into Saturn due to the possibility, however slim, of living creatures on Enceladus and Titan. NASA, and all the other spacefaring nations of the world, have agreed to a set of “planetary-protection” principles, aimed at preventing the accidental contamination of another habitable world with organisms from Earth. Once Cassini runs out of fuel, if it is still in orbit, it could conceivably crash into, and contaminate, one of the moons. To prevent that, it is being hurtled into Saturn, on purpose, before the fuel runs out, when we can still control its movements.

What are the chances that earthly organisms not only have somehow survived Cassini’s journey and all its years in Saturn’s punishing radiation belts, but also would survive and thrive in the alien environs of one of those moons, potentially threatening indigenous organisms? Extremely low, to say the least. Perhaps absurdly low. But we cannot say that they are zero. And the risk of making a mistake in this arena is incalculable. So Cassini must be sacrificed. (It’s worth noting that plutonium is a naturally occurring element, so Cassini’s crash won’t introduce anything to the chemistry of the planet that isn’t already there.)

Cassini’s fiery swan dive is an expression of applied environmental ethics, orchestrated out of concern for the environmental protection of Saturn’s potentially habitable moons. As the spacecraft plummets, it will be returning new data on the composition of the planet’s upper atmosphere, a machine proxy for our human curiosity, exploring with its last electronic breaths. Then, glowing with the heat of entry and buffeted by the increasing atmospheric pressure, it will go silent, begin to vaporize, and fall to pieces. As it comes apart, among the last parts to remain intact will be those containers of plutonium, encased in iridium, hardened to survive reentry. Eventually, as they descend into the crushing depths where pressures far exceed any conditions on Earth, these too will disintegrate and merge into the strange hyperbaric liquid soup of the Saturnian depths.

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