After a dive into Saturn, Cassini spacecraft melts into history
After 13 years revolutionizing our understanding of the solar system, NASA’s Cassini spacecraft melted during its final, fatal partial orbit into the upper reaches of Saturn.
For about a minute, running on half a hair dryer’s worth of power, the orbiter-cum-probe beamed direct measures of the planet’s atmosphere, along with final probes of its gravity and magnetic field, to mission control at the Jet Propulsion Laboratory (JPL) in Pasadena, California. Then, finally, its thrusters, designed for the vacuum of space, could no longer counter Saturn’s turbulence to keep its antenna in line with Earth.
“The signal from the spacecraft is gone,” said Earl Maize, Cassini’s project manager. “And in 45 seconds, so will be the spacecraft.” The spacecraft’s aluminum and carbon mylar then melted into the folds of Saturn’s elemental abyss. Cassini had delivered 30 more seconds of data than expected. Until their vaporizing end, all systems were nominal.
The spacecraft’s demise, necessitated by dwindling fuel and a need to protect two of Saturn’s 62 moons from potential microbial contamination from Earth, has brought forth a global outpouring of sentiment. Images from its 12 instruments, spread on currents of social media unimaginable when the mission was conceived in the 1980s, decorate dorms and desktops. It had become typical to know, in fine detail, on any given day, the weather on Saturn. Those days are no more.
On large screens mounted on
Beckman Mall at the California Institute of Technology in Pasadena, in
a sleepy predawn haze, the mission’s extended web of researchers watched the signal fade out. Many have known each other for decades, their families growing up together at science meetings spread across the globe. “My scientific life is tied to this spacecraft, to this mission,” says Luciano Iess, a planetary scientist at the Sapienza University of Rome who has led Cassini’s radio experiment since 1990. But though nostalgia and sentiment ruled those few hours, many are eager to get back to the work at hand. “What I can tell you is many of our models are too simple or just out and out wrong,” says Linda Spilker, Cassini’s project scientist at JPL. There is much more to do.
Cassini’s final 22 orbits have already made clear that scientists’ understanding of Saturn’s atmosphere and interior need rethinking. Insights into the planet’s magnetic field, mass, rings, and mysterious interior are still to come, some collected by the eight instruments that remained operating during its final descent. This research will be the culmination of a campaign that went better than many could dream, says JPL’s Dennis Matson, who served as the project scientist on Cassini from its conception until 2010. “Before we got to Saturn, I didn’t expect it—I thought it’d be like Galileo at
Jupiter,” he says. “I didn’t expect it to be a paradigm reset of everything.”
Science is not Cassini’s only legacy. It serves as the model for collaboration among disciplines and nations in planetary science, with its instrument teams featuring balanced rosters of U.S. and European scientists. JPL’s upcoming $2 billion Europa Clipper mission will borrow its orbital innovations to dodge Jupiter’s fierce radiation, and Cassini’s scientists are migrating to missions proposed or underway to Jupiter’s moons; the ice giants Neptune and Uranus; or back to Saturn’s moons, this time armed with new tools to search for life. There are lesser known legacies, too: In 2003, Cassini provided the best validation of general relativity. Tools developed in the 1990s to swap money among Cassini’s scientific instrument teams went on to run emissions trading to reduce smog in southern California. The list is long.
But most of all, Cassini will be remembered for the moons. Thanks to
the spacecraft, along with its long-lost probe, Huygens, two of Saturn’s moons, Enceladus and Titan, have now become prime candidates in the search for life. Cassini revealed Enceladus, once thought small, cold, and dead, to have more than 100 plumes of hydrogen-rich liquid water erupting from cracks in its icy crust, possible evidence of hydrothermal vents in its buried sea—a habitat similar to Earth. In one stroke, it has expanded the boundaries of the habitable zone, both for our solar system and exoplanets beyond. Meanwhile, on hazy Titan, Huygens and Cassini found an Earthlike landscape of rivers and lakes filled with liquid methane—the only liquid bodies found on the surface of moon or planet in the solar system, except Earth.
Cassini had become a model of reliability as it swung from orbit to orbit around Saturn, in a path that resembled a complex ball of yarn designed to maximize its lifetime, its shifts governed by Titan’s gravity. The spacecraft’s longevity has not just been luck; from the start, its engineers shorn it of moving parts prone to failure. And although some instruments on its 2-ton, bus-sized body have faced trouble—one of its two magnetometers failed soon after it arrived at Saturn, necessitating intricate rolls to calibrate it—most have metronomically continued as Saturn’s seasons shifted from spring to summer solstice during its 29.5-year orbit.