What do you do when your spacecraft runs out of fuel after a 20-year journey? Send it barreling into the planet it has studied for more than a decade in a final blaze of glory, of course.
The Cassini mission launched on October 15, 1997. Seven years later the spacecraft entered Saturn’s orbit. Since then it has made some incredible discoveries at the solar system’s second-largest world, including finding liquid hydrocarbon lakes on Titan, Saturn’s largest moon, as well as spying plumes of water ice and particles jetting from a subsurface ocean within another moon, Enceladus.
Now the spacecraft is nearing the end of its journey. Speaking from the Jet Propulsion Laboratory (JPL) on Tuesday, a panel of Cassini scientists discussed the mission’s dramatic conclusion.
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In November, to prepare for the end, Cassini moved closer to Saturn, tilting its orbit from nearly equatorial to circling the planet from pole to pole. Images from earlier this year showed a new, up-close look at the rings as the spacecraft glided past them. On April 22 engineers will use Titan’s gravity to tweak Cassini’s orbit for the last time—a fuel-saving maneuver that has been used throughout the mission to change the craft’s velocity by slingshotting it through the moon’s gravitational field. This time the adjustment will send Cassini zooming between the rings and the planet, a route never taken by a spacecraft before. The probe will circle the planet in this manner nearly two dozen times before spiraling into its gassy atmosphere in a dramatic final descent in September.
Researchers on the team have modeled the precise path Cassini will take, barreling at more than 110,000 kilometers per hour into a relatively narrow 1,200-mile gap. Even with careful planning it is a risky maneuver. “A piece of sand at that velocity will take out one of our instruments or, if it’s in the wrong place, could cripple our spacecraft,” said Earl Maize, Cassini’s project manager at JPL. “This is something we wouldn’t want to really try any other time. But now it is the time.”
Carolyn Porco, Cassini Imaging Team lead, who was not at the JPL conference, wrote in an e-mail we can expect some amazing views from these final orbits. “The images we've seen from the ring-grazing orbits have been the clearest looks yet at the very-small-scale behavior of the ring particles in the outer part of the rings,” she wrote. “I'm expecting to see the same powerful detail in the images of the inner part of the rings.”
In addition to capturing images Cassini will measure the rings’ mass to offer insight into their possible origin, age and particle composition. “We know that Saturn’s rings are 99 percent water ice but we’re not certain about that other 1 percent,” said Cassini Project Scientist Linda Spilker at JPL. “Could it be tiny grains of iron, silicates, organics, a mix of all three, something else we haven’t even thought of?” Cassini’s Cosmic Dust Analyzer instrument will scoop up the ring particles to find out.
During its 22 dives between the rings and Saturn Cassini will also capture the best-ever views of both giant hurricanes that swirl at the planet's poles—notably the curiously shaped hexagonal jet stream at the north pole. It will also measure the abundances of hydrogen, helium and other atmospheric gases as well as study the polar auroras.
Cassini will also measure Saturn’s gravitational and magnetic fields, giving researchers an unprecedented glimpse of the gas-giant planet’s hidden interior and revealing the size of the gas-shrouded rocky core. This is something of particular interest to Jonathan Lunine, who works with radar and other instruments on Cassini and is co-investigator on the Juno mission, which entered orbit around Jupiter last summer to delve into that mammoth planet’s depths. “Juno is getting the first gravity data for Jupiter,” Lunine says. “So the opportunity to compare the interior structures of Jupiter and Saturn is really remarkable.”
On its final dive between Saturn’s rings on September 15, 2017, Cassini’s path will send it plunging into the planet’s atmosphere. For up to three minutes the spacecraft will struggle to keep its antenna pointed toward Earth, transmitting final data before disintegrating into the planet it has explored for so long. Cassini’s self-immolation may seem poetic, but it was really a practical maneuver to remove the risk of it accidentally coming in contact with and contaminating any of the myriad Saturnian moons with biological material from Earth—especially Titan and Enceladus. Both are high on the list of potential abodes for extraterrestrial life, making them likely targets for future exploration.
The end of Cassini is bittersweet. “We have been at Saturn for 13 years,” Maize said at the JPL briefing. “You can get up in the morning, you can get a weather report, you can see what the images look like, you can get the status on the space weather [or] magnetic fields.”
This fall scientists will lose that contact with a distant planet, and with it our most robust and capable eye upon the outer solar system, which remains largely unexplored. Juno won’t last much longer in Jupiter’s intense radiation, and with the exception of New Horizons, journeying beyond Pluto into the Kuiper Belt, the outer solar system beyond Mars will go dark.
Although it could be 20 years before the next mission of this scale, Cassini left too many questions unanswered not to go back.
Planetary scientists like Lunine are already looking forward to the search for life in Enceladus’s notorious plumes, something Cassini was ill-equipped to measure. “We can send another spacecraft with today’s mass spectrometers that can determine whether there are biological molecules in the plume itself,” he says. “That’s a very exciting possibility in the next step of Saturn exploration, and it’s possible because of discoveries Cassini made.”