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

by FEDOR KOSSAKOVSKI

NASA’s Cassini probe ILLUSTRATION/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.”


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