After 13 years revolutionizing our understanding of the solar system, NASA’s Cassini spacecraft melted this morning 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 Earth-like 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.
A model of reliability
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.
While Enceladus and Titan stole the show, Cassini, on its way to taking a total of more than 450,000 images, documented the intricacies of Saturn and its orbiting rings and moonlets. It captured a surprise storm on Saturn’s otherwise placid taupe surface, arriving nearly a decade earlier than expected. It showed the persistent, hexagonal jet stream on Saturn’s north pole, first hinted at by Voyager. And its probing of waves in the planet’s rings allowed a window into Saturn’s interior and history.
But many revelations have been delayed until Cassini’s final orbits, which NASA dubbed its Grand Finale. Starting in April, the spacecraft began plunging once a week between Saturn and its rings, its closest look at each. The mysteries started early; during its first orbit, Cassini wielded its large radio antenna like a shield to protect itself from possible ring particles. But there were almost none in the gap, a puzzling absence. “We’re all still struggling to understand that,” says Matt Hedman, a planetary scientist at the University of Idaho in Moscow.
Perhaps most important were six runs maximized for Cassini’s radio science experiment. As the spacecraft plunged through these orbits, a radio telescope in Argentina, run by the European Space Agency, NASA’s partner on the mission, listened for tiny Doppler shifts in Cassini’s signal. Previously, Saturn’s rings and planet were inextricably bound up in this measure. But by threading the needle, Cassini has allowed scientists to untangle them, telling them about the mass, and therefore age, of the rings—and uncloaking Saturn’s interior.
Scientists are not yet ready to settle the long debate over the rings’ age; a definitive statement on whether they’re 100 million or billions of years old could come later this year. But the finale has already made clear that the interior and exterior of Saturn rotate at different rates, with a significant difference in speeds, Iess says. A similar pattern is seen in the sun, but NASA’s Juno spacecraft, now orbiting Jupiter, has found only a marginal difference in rotation in its gas giant. Something more complex is going on at Saturn, adds Jonathan Fortney, a planetary scientist at the University of California, Santa Cruz; it appears one template does not govern gas giants. “Saturn is not a small version of Jupiter,” he says. “The planets are distinct and unique.”
Measuring the magnetic field
The final orbits also saw Cassini take its finest measures of Saturn’s magnetic field. Lacking any fixed surface features to measure, scientists still don’t know whether a Saturn day lasts 10.6 or 10.8 hours. Regular bursts of radio waves that made it possible to know Jupiter’s day have proved irregular on Saturn. This close look could resolve the day length and another mystery: why it appears that the axis of Saturn’s magnetic field is perfectly aligned with its rotation; scientists had thought a slight offset between the two is necessary to maintain such a field, as seen on Earth and other planets.
More answers await: By combining their gravity and magnetic measures, Michele Dougherty, a space physicist at Imperial College London, also expects to reveal new information about the composition and mass of Saturn’s core, along with some sense of how the heavier elements distribute among the gas giant’s stew of hydrogen and helium. But this will take time, which had been sparse when orbits were coming every week. That’s why, beyond pride and sadness, she says, “There is also a little bit of relief. Because we’re all exhausted now.”
As it neared its end, Cassini also made close study of Saturn’s rings, which are largely made up of water ice—including, for the first time, the capture of a few stray, submicron particles. “We have measured the ring directly,” says Nicolas Altobelli, Cassini’s project scientist at the European Space Agency in Madrid. But once Cassini dipped its toes into Saturn’s atmosphere during its final orbits, the ring grains disappeared, surprising his team. Altobelli isn’t sure whether this absence is due to an unknown atmospheric dynamic—or his instrument’s inability to operate outside the vacuum it was designed for.
Cassini also captured the closest images of the rings, down to clumps of the particles. These pictures have revealed distinct, hidden structures in rings that otherwise have the same density and composition, with stark differences in reflectivity. “It’s like how an ice cube has a different texture from a snowball,” says Matthew Tiscareno, a ring specialist at the SETI Institute in Mountain View, California. There’s no moon or other force they know of that could cause such bands. “There are hints it may have to do with the structure in the planet,” he adds.
Like many researchers, Tiscareno has spent his scientific career so far on Cassini. But he has much time left and plans to return. So rather than staying in Pasadena for Cassini’s end today, he flew home last night to watch the proceedings with his family. After all, he needed to be fresh for meetings back at JPL next week. “We’ve got a new goal,” he says. “We are dreaming of a dedicated Saturn ring orbiter.”