Analysis: Why NASA's Cassini probe had to be destroyed

NASA's Cassini probe. Illustration by NASA/Jet Propulsion Laboratory-Caltech

Update: At 7:55 a.m. ET on September 15, 2017, NASA reports they received Cassini's final transmission.

This Friday, NASA's Cassini spacecraft will execute its final maneuver, careening into Saturn's atmosphere and melting without a trace.

"Cassini's own discoveries were its demise."

Scientists and engineers at NASA's Jet Propulsion Lab, many of them with the 20-year mission since the very start, will work feverishly to keep their beloved spacecraft alive and sending back data for as long as possible.

"Of course it's really going to be hard to say goodbye to this plucky, capable little spacecraft that has returned all this great science," said Cassini Project Scientist Linda Spilker at a NASA news conference in April.

Spilker has been there for all of Cassini's discoveries: from spotting how a hexagonal storm on Saturn's north pole changes color with the seasons to providing insights into how Saturn's rings formed. Perhaps most intriguing were Cassini's discoveries off-planet, on Saturn's moons, with the identification of methane seas on Titan and exposing the likelihood of a warm, saltwater ocean underneath Enceladus' icy surface. Both locations carry some ingredients for life, with Enceladus showing greater potential.

So, considering Cassini's scientific fruitfulness, why is the spacecraft being steered into a death spiral?

This fate seems a little odd, considering other spacecraft that have ventured to other worlds. Most, except those sent to an inhospitable locale like Venus, remain drifting in space. The Voyager 1 spacecraft, 20 years Cassini's senior, still coasts through interstellar space, beaming back data from beyond the bounds of the solar system. Perhaps a lonely existence, but an existence nonetheless.

One oft-cited reason is that the Cassini spacecraft is running out of fuel. Though true that Cassini has a measly 61 of its original 6,565 pounds of propellant, this fuel is used only for reorienting its trajectory. In theory, the Cassini team could have used the last wisps of fuel to push the spacecraft into a stable orbit around Saturn. Cassini could even have still collected data, since all of its instruments run on power from a different, much more long-lived source (more on that later).

No, the real reason Cassini must die is because of an international treaty with a rule to not contaminate potentially-habitable worlds. To comply with this rule, NASA maintains an Office of Planetary Protection in order "to preserve our ability to study other worlds as they exist in their natural states; to avoid contamination that would obscure our ability to find life elsewhere — if it exists; and to ensure that we take prudent precautions to protect Earth's biosphere in case it does."

A Cassini-captured movie of the 2,200-mile, six-sided jet stream at Saturn's north pole known as "the hexagon." Image by NASA

"Cassini's own discoveries were its demise," said Cassini Program Manager Earl Maize at the April conference. "Enceladus has got a warm, saltwater, undersea ocean, and it's got plumes coming out. We cannot risk an inadvertent contact with that pristine body."

Parking Cassini in orbit creates a risk of someday colliding with Enceladus. Such an accident could contaminate Enceladus in serious ways, perhaps the most unpleasant of which would be radioactive pollution.

All of Cassini's electronics run on radioisotope thermoelectric generators (RTGs). These highly reliable nuclear batteries produce heat through slow radioactive decay, which is converted to electricity by a device called a thermocouple. RTGs have no moving parts and, hence nothing to fix, which is perfect for a probe that's 746 million miles from the nearest mechanic.

Cassini uses three radioisotope thermoelectric generators to power its electronics. Illustration by NASA

Voyager, Pioneer, Cassini, the Curiosity rover — virtually every major mission exploring outer space — have used RTGs to generate power for their everyday activities. But these systems aren't efficient. Cassini launched with 72 pounds of plutonium-238, which generated about the same amount of power as a microwave… a pretty weak one at that. For comparison, the atomic bomb that destroyed Nagasaki had less than 14 pounds of plutonium, though it was highly enriched.

Still, slamming radioactive material onto a potentially habitable world might not be the best idea.

Cassini could also unknowingly deposit tiny Earth passengers if it crashed into Enceladus. Some microorganisms can survive the harsh vacuum of space. Researchers have found tardigrades, a microscopic aquatic animal, can live in outer space for at least 10 days. Bacteria embedded in rocks survived a year-and-a-half long stint glued to the outside of the International Space Station.

Cassini observes Enceladus for the last time, on August 28, 2017. Image by NASA/JPL-Caltech/Space Science Institute

Bringing invasive species to an isolated ecosystem could prove disastrous. Just think of the devastation suffered by Native Americans when Europeans brought smallpox and other germs to the New World. These new diseases killed roughly 90 percent of the Native American population.

But what of Huygens, the European Space Agency probe, that Cassini released onto Titan in 2005? Wouldn't that be a contamination hazard? Perhaps, but landing on Titan was considered a much more acceptable risk. "Given the fact that Titan is too cold and that there is no liquid water for life as we know it to evolve, the risk of contamination is practically non-existing," the European Space Agency stated. "The harsh environment is expected to kill microorganisms that may have hitchhiked from Earth on board the clean space probe."

Saturn, too, possesses zero indications of life, and even if there were, the spacecraft and any hitchhiking microbes would be completely destroyed in the planet's thick atmosphere.

"Cassini has got to be put safely away," Maize said at the April news conference, adamant. "The only choice was to destroy it in some controlled fashion."

1997 NewsHour report on Cassini's launch

A version of this story appeared on Miles O'Brien Productions.

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