July 13, 2024

I, Science

The science magazine of Imperial College

When Cassini was being designed in the 1980s, an innovative plutonium energy source was developed to sustain it's long journey. A source that would and still does carry considerable ethical considerations.

The 20-year journey of the Cassini spacecraft will come to an end on the 15 September when NASA crash the craft in to Saturn’s atmosphere. The spacecraft has drawn upon its 33kg plutonium energy source for the entirety of its mission which have created questions regarding the potential nuclear pollution implications for Saturn.

When engineers began designing Cassini in the 1980s, it quickly became clear that its ambitious itinerary was going to require a self-sustaining energy source that could operate at great distances from the Sun. In order to power the craft by solar energy, arrays would have to be so large to pick up what little sunlight there is around Saturn, that the craft would simply be too heavy to fly. Thus, radioisotope thermoelectric generators (RTGs) that utilize heat energy from natural radioactive decay of plutonium were chosen to provide electric power to Cassini.

These RTGs included 33 kg of plutonium 238, whose half-life is 88 years. We all know that nuclear energy has the potential to be incredibly dangerous, so designing for safety was essential. First, the fuel was put in to several independent modular units. Second, the RTGs were sealed with layers of protective materials chosen for their outstandingly resilient attributes, including high temperature stability and high strength. Third, the fuel of plutonium dioxide was used as a more chemically stable form to ensure that the it could not produce a chemical reaction easily.

The safety of the fuel became particularly important, when, as part of Cassini’s journey, the spacecraft had to execute a slingshot around Earth. This raised considerable concerns regarding the potential outcome if the craft accidentally reentered Earth’s atmosphere. Under such a scenario it was estimated that the plutonium would cause the death of five thousand people from cancer. As part of NASA’s status as a publicly funded government organisation, removing risk to human life is particularly important. As such, engineers further reinforced the housings with graphite, which would ensure that in the event of an accidental reentry, the plutonium would remain contained.

But now concerns have turned to Saturn. Might there be an unforeseen risk of nuclear pollution to the planet?

The decision to crash Cassini in to Saturn’s atmosphere originates from NASA’s desire to safeguard a number of satellites in Saturn’s system that may have appropriate environments to sustain life. A diverse collection of Earth microbes hitched a ride on Cassini’s surface when the craft was launched, and the concern is that if the craft is allowed to float freely in Saturn’s system, it may eventually crash in to one of the potentially habitable satellites, and pollute them with these microbes. However, it is believed that Saturn’s environment is too harsh to sustain life, and that there is therefore no risk of organic contamination from Cassini.

So far, the possible implications to Saturn of plutonium contamination have not raised any significant concerns, but the reality is that scientists are mostly flying blind in trying to determine what such outcomes might be. These considerations are not, of course, limited to the Cassini mission. All spaceflight carries a continual chain of such conflicts, where decisions are made to further our knowledge and reach, with information that is mostly unavailable, and impacts that are impossible to determine.

Perhaps, one day, we will reach these distant neighbours, but in the meantime, it is essential that we focus on reducing our footprint whilst we explore space.

Chengyue Liu, Jian Yu, Yueqin Li, Makoto Hibino – Centre for Academic English, Imperial College London

Banner image: Plutonium238, g0d4th