On September 15, a meteor will burst through the cloud tops of Saturn’s atmosphere, burning bright and breaking apart into hundreds of pieces. From Saturn’s surface, this would appear as a beautiful cosmic event, like shooting stars that arc across the Earth’s night sky. But this meteor won’t be a piece of rock jostled loose from an asteroid. It will be the Cassini spacecraft in its final moments of life.
Jupiter saw a similar tail of fire streak through its atmosphere back in 2003, when the Galileo probe turned to face the planet, fired its thrusters, and sped into Jupiter at 108,000 miles per hour. More than a year earlier, a team of people at NASA’s Jet Propulsion Laboratory had decided they would kill the spacecraft by throwing it into into the giant planet. It’s a decision not to be taken lightly, especially when these missions cost billions of dollars and can take decades of planning. Every mission has a team that plans the deaths of these spacecraft; some have planned both the demise of Galileo and Cassini, and they likely won’t stop there. They have good reason to kill these robotic explorers.
When Galileo was sent to investigate the inner workings of the Jovian System, it was meant to be a major flagship mission. But then the probe made a monumental discovery: The thick crust of ice that blankets Jupiter’s lacerated moon Europa hides a vast ocean of salt water, more than what covers Earth.
“The discovery of water on Europa was really what sealed Galileo’s fate,” says Rosaly Lopes, a team scientist on Galileo and member of the Cassini Flight Project. NASA’s Office of Planetary Protection prohibits the contamination of potentially habitable bodies in the solar system with microbes from Earth, so with Europa now in that category, Galileo had to disappear to avoid ever crashing into the watery moon. The probe “de-orbited” into Jupiter on September 21, 2003, never to be heard from again.
The kind of skill set required for creating and managing deep-space planetary missions is hard to come by, so many of the people at NASA work together again and again over the course of their careers. Many current team leaders got their start on the Voyager probe fresh out of graduate school, moving to higher positions over time. But Voyager is still roaming the skies, en route for another solar system. For some team members, Cassini will be their first experience in a spacecraft kill.
On July 3, 2014, more than 20 scientists sat at an oblong mahogany conference table with Jim Green, the director of planetary science at NASA’s headquarters in Washington, D.C. It was the day that Cassini’s fate would become law. When the spacecraft launched for Saturn in 1997, little was known about the planet’s system and moons. Two of the largest, Titan and Enceladus, were flagged for their interesting surface features, but it wasn’t until Cassini arrived at Saturn and spotted a plume of water spewing from Enceladus that things got complicated. Titan, too, was found to be covered in lakes and seas composed of methane and ethane.
Linda Spilker, the lead scientist on Cassini, knew her team had to preserve the integrity of the potentially habitable moons. “We don’t want to go back to Enceladus and find there’s microbes that we put there,” she says. As a spacecraft runs out of fuel, as Cassini now has, each orbit increases the chance of losing control and impacting a moon. That’s why they need to be destroyed.
It can take years of bureaucratic legwork to destroy a spacecraft. For Cassini, this began at JPL in 2009. Spilker and her team started with science meetings, checks on the fuel-tank levels, orbital-mechanic calculations, and a wish list of scientific objectives, some only achievable in a death orbit. Suggestions from those meetings morphed into formal letters, eventually making their way to NASA HQ, where they were sent up the ladder for a final vote and a signature on a paper.
The death of Galileo taught the team a lot about planning the end of Cassini. Galileo was the first robotic probe to collide with a gas giant; Cassini will be the second. Cassini’s final marching orders outlined a grand finale of 22 orbits of Saturn. The final five would begin to skim the upper atmosphere, scooping the clouds with its instruments. These orbits would yield unprecedented scientific results. Spilker and her team had always wanted to fly through the planet’s rings, but it was too dangerous. Now was their chance to try something deadly.
Back when deciding to destroy Galileo, “it wasn’t easy to come to consensus,” says Torrence Johnson, the Galileo project scientist and a Cassini science-team member. “You had the analysis from the orbits, what the probabilities were, and what the scientific benefits of the things were. We thought, well, let’s just go out in a blaze of glory.” Throwing a billion-dollar science instrument into a giant ball of gaseous planet could allow them to do a close flyby by of Jupiter’s moon Io or get data from deep in the Jupiter magnetosphere.
Destruction of this kind is likely to be used again for the Juno mission currently orbiting Jupiter. “All of these missions are connected, and they build on each other,” says Jonathan Lunine, a Cassini science team member and now a co-investigator on the Juno mission.
Still, it’s extremely difficult for the team to let go of the spacecraft. Some members refer to the probes as their children. Many spent upward of 30 years on one mission. “It’s like in the death of a loved one—you look back and you think about all the good memories, the times you’ve shared together, went on vacation together, grandchildren,” says Spilker. “I think of it more like planning perhaps a wake.”
When it was time for Galileo’s end, the mission’s team issued a timeline of commands, the way they will do for Cassini and Juno. At 9:25 a.m., “the intensity of radiation interference [reached] a point where even a bright star like Vega can no longer reliably be seen by the attitude-control star scanner. The software [was] told to expect to see no more stars.” Three hours later, Galileo disappeared over the horizon of Jupiter. At 12:49 p.m., the signal was lost.
Cassini’s death will not pass without a vigil-like ritual. On September 15, thousands will gather at JPL and wait until the early morning hours for Cassini’s final orbit to begin. They’ll share stories of a lifetime of work, as that effort culminates in fireworks 800 million miles away.
Mission control will watch over the downlink for the last bits of Cassini’s data. It’s not known exactly how long it will take for Cassini to perish once its metal begins to melt in the clouds of Saturn, but it will be quick. In these final moments, like a dying person whispering their last words into the ears of loved ones, Cassini will pour out information as it dutifully descends lower and lower into Saturn.
When humans die, they release a final breath. The ancient Egyptians called this last exhale a “wind” that arrives to carry away a person’s soul to the afterlife. For Cassini, the impact from falling backward through the hydrogen-thick atmosphere will tear away its parts—first the large sections, then the instruments, until the antenna pointing toward Earth sends back one final beep. This message will breeze past Jupiter and Mars, through the solar plasma pushing toward deep space, and finally run into Earth, where it will be collected in the antennas of the Deep Space Network.
Cassini’s team will be there to feel the wind that’s traveled from the depths of the solar system. The probe’s final whispers will be sought-after science, whatever is discovered in those last moments. Then just like that, the grasp we’ve had on Cassini for 13 years will end. There will be no more commands to send, nothing else to say. The only thing to do will be to listen and to say goodbye.
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