- The Cassini spacecraft, which launched toward Saturn in 1997, is running low on fuel.
- To avoid accidentally crashing into and contaminating a nearby moon that may harbor alien life, NASA is going to destroy the robot.
- But before Cassini perishes, it will fly between Saturn and its rings and record as much new data as possible.
For nearly three decades, researchers have worked to design, build, launch, and operate an unprecedented mission to explore Saturn.
Called Cassini-Huygens — or Cassini, for short — the golden nuclear-powered spacecraft launched in October 1997, fell into orbit around the gas giant in July 2004, and has been documenting the planet and its dizzying variety of moons ever since.
But all good things must come to an end. And for NASA's $3.26 billionprobe, that day is Friday, September 15, 2017.
During a press conference held by the US space agency on April 4, researchers explained why they're killing off their cherished spacecraft with what they call the "Grand Finale." The maneuver will use up the fleeting reserves of Cassini's fuel and put the robot on a collision course with Saturn.
"Cassini's own discoveries were its demise," said Earl Maize, an engineer at NASA's Jet Propulsion Laboratory (JPL) who manages the Cassini mission.
Maize was referring to a warm, saltwater oceanthat Cassini found hiding beneath the icy crust of Enceladus, a large moon of Saturn that spews water into space. NASA's probe flew through these curtain-like jets of vapor and ice in October 2015, "tasted" the material, and indirectly discovered the subsurface ocean's composition — and it's one that may support alien life.
"We cannot risk an inadvertent contact with that pristine body," Maize said. "Cassini has got to be put safely away. And since we wanted to stay at Saturn, the only choice was to destroy it in some controlled fashion."
But Maize and a collaboration of researchers from 19 nations aren't going to let their plucky probe go down without a fight.
They plan to squeeze every last byte of data they can from the robot, right up until Cassini turns into a brilliant radioactive comet above the swirling storms of Saturn.
'We're going in and we're not coming out'
Long before Cassini began orbiting Saturn in 2004, mission managers carefully plotted out its orbits to squeeze in as many flybys of the gas giant planet, its moons, and its expansive icy rings as possible.
Their goal: Get lots of chances to record unprecedented new images, gravitational data, and magnetic readings without putting the spacecraft into harm's way or burning up too much of its limited propellant.
But after 13 years of operation at nearly 1 billion miles (1.45 billion kilometers) away from Earth, Cassini's tank is running close to empty.
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"We're coming to the end. As it runs out of fuel, the things it can do are quite limited — until we decided on a new approach," Jim Green, the leader of NASA's planetary science program, said during the press conference.
NASA could have propelled Cassini to some other planet — perhaps Uranus or Neptune. In 2010, however, mission managers decided to keep it around Saturn, reasoning they could squeeze more science out of the mission there. But this effectively doomed the spacecraft to a fiery death.
Cassini's death spiral will officially begin on April 22, 2017. That's when it will, for the last time, fly by Titan: an icy moon of Saturn that's bigger than our own, has a thick atmosphere, seas of liquid methane, and even rain.
Titan's gravity will slingshot Cassini over Saturn, above the planet's atmosphere, and — on April 26 — through a narrow void between the planet and the innermost edge of its rings.
"That last 'kiss goodbye' will put Cassini into Saturn," Maize said. "This is a roller-coaster ride. We're going in, and we are not coming out — it's a one-way trip."
Cassini's science-packed finale
The void between Saturn and its rings is about 1,200 miles wide, or roughly the distance from northern Washington state to the southern tip of California.
"As we're skimming close to the planet, we'll have the best views ever of the poles of the planet," Linda Spilker, a Cassini project scientist and a planetary scientist at NASA JPL, said during the press briefing. "We'll see the giant hurricanes at the north and south poles."
During its final orbits above Saturn, Cassini will get its closest-ever views of the hexagon-shaped feature of Saturn's north pole, which Spilker said is "two Earth diameters across" yet poorly understood.
"Perhaps by getting close with Cassini, we'll answer the question, 'What keeps the hexagon there in this particular shape?'" she said.
Spilker said Cassini will also photograph the auroras of Saturn's poles, measure how massive the planet's rings are, sample the icy material they're made of, and even probe deep below its layers of thick clouds.
Sensitive magnetic and gravitational measurements that Cassini couldn't make before may also answer lingering questions about the internal structure of Saturn, including how big its rocky core is, plus how fast a shell of metallic hydrogen around it spins.
"How fast is Saturn rotating?" Spilker asked. "If there's just a slight tilt to the magnetic field, then it will wobble around and give us the length of a day."
Hours before it takes its final plunge on September 15, 2017, Cassini will beam back its last batch of images — then prepare for the end.
The fiery end of a longtime robotic friend
Cassini is a 2.78-ton robot with delicate instruments that was not designed to ram into icy ring material at 70,000 mph. It also wasn't made to plunge into the thick atmosphere of a gas giant and live to tell the tale.
Nevertheless, scientists behind the mission say they are going to do their best to protect its instruments from damage and keep the data flowing until the moment it dies.
They'll do this primarily by using the cone-shaped primary antenna as a shield for its camera and other important parts.
"If we get surprised, well, we've got a bunch of contingency plans ... We'll milk the best out of this," Maize said. He added that even if icy bits take out Cassini's ability to talk to Earth, the spacecraft "will still finish out exactly where we planned, but we'll have a little less science than we hoped for."
When Cassini begins its final plunge, it will use its last propellant to fight atmospheric drag and keep the antenna pointed at Earth. During that time, it will sniff Saturn's atmosphere as it descends into the gases, broadcasting its readings of the gases' composition in real time back to satellite dishes on Earth.
But the measurements won't last long.
"It will break apart, it will melt, it will vaporize, and it will become a very part of the planet it left Earth 20 years ago to explore," Maize said.
While members of the Cassini team said they're looking forward to the Grand Finale, they weren't without remorse.
"It's really going to be hard to say goodbye ... to this plucky, capable little spacecraft that has returned all of this great science," Spilker said. "We've flown together a long time."
Plateaus Up Close
Saturn’s C ring isn’t uniformly bright. Instead, about a dozen regions of the ring stand out as noticeably brighter than the rest of the ring, while about half a dozen regions are devoid of ring material. Scientists call the bright regions “plateaus” and the devoid regions “gaps.”
Scientists have determined that the plateaus are relatively bright because they have higher particle density and reflect more light, but researchers haven’t solved the trickier puzzle of how the plateaus are created and maintained.
This view looks toward the sunlit side of the rings from about 62 degrees above the ring plane. The image was taken Jan. 9, 2017 in green light with the Cassini spacecraft’s narrow-angle camera.
Cassini obtained the image while approximately 194,000 miles (312,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 67 degrees. Image scale is 1.2 miles (2 kilometers) per pixel.
Enceladus is ripe for life. In one final pass through the icy moon’s liquid plumes, NASA’s Cassini spacecraft found molecular hydrogen, which indicates favourable conditions for life in Enceladus’s subsurface sea.
For over a decade, Cassini has been exploring Saturn and its moons, sending back the best pictures and measurements we’ve ever had of the system. It dropped off the Huygens probe at hazy Titan, scrutinised the structure of Saturn’s rings, and revealed that Enceladus was much stranger than anyone expected.
Enceladus’s south pole has strange, warm fractures, and plumes of liquid water coming from an internal ocean many believed was impossible in such a small, cold world. The plumes also contain enticing compounds like organics and carbon dioxide, all necessary for life as we know it on Earth.
Those things represent tantalising hints of habitability. But there was no evidence for an energy source to feed potential life, until now. In extreme environments on Earth, hydrogen can play that role.
“What was missing to complete the story of habitability was an energy source,” says Chris McKay at NASA’s Ames Research Center in California. “This completes that story.”
Candy for microbes
Cassini did detect hydrogen in early trips through the plumes, but there was no way to determine if it came from the moon itself or from inside the instrument. When particles from the plumes entered the spacecraft’s Ion and Neutral Mass Spectrometer (INMS), they interacted with its titanium walls, producing the same sort of hydrogen as hydrothermal processes would produce under Enceladus’s ocean.
“We didn’t know we were going to do this experiment when we launched Cassini,” says Hunter Waite at the Southwest Research Institute (SwRI) in Texas. So to look for hydrogen, Waite and his team had to put the INMS instrument in a new mode that measured the molecules without allowing them to touch the walls.
Finally, they found the molecular hydrogen they were looking for – and a lot of it. Their findings indicated that there was too much hydrogen to be stored in tiny Enceladus’s ice shell or ocean. That means it must be continuously produced there, probably by hydrothermal reactions similar to those that occur near hot vents at the bottom of Earth’s oceans.
Near those vents on Earth, there is life. Some of Earth’s oldest microorganisms, called methanogens, are often found near hydrothermal vents where, deprived of light and oxygen, they convert hydrogen and carbon dioxide to methane.
“If you were to take methanogens from Earth’s ocean and transport them to Enceladus, they would have all the food they need,” says Waite. “This is like candy for microbes.” If Earth microbes could exist on Enceladus, maybe it could have homegrown life, too.
Between its liquid water, organic molecules, and hydrogen, Enceladus is looking more and more like our best bet for finding extraterrestrial life. “If we’re looking for life in the solar system, then Enceladus has a lot of potential to be the place that we could find it,” says Kelly Miller at SwRI, who was part of the team that discovered Enceladus’ molecular hydrogen.
Signs of life?
Showing Enceladus is habitable is one thing, finding life is quite another.
“Just because a place is suitable for life doesn’t mean that life is present, because we don’t understand the origin of life at all,” McKay says.
Some believe that life is inevitable, given the right conditions. Others think that it is rare and requires a great deal of luck. Right now, our sample of definitely habitable worlds has only one: Earth. But pairing observations of Enceladus with our own planet could help astrobiologists figure out the likelihood of life existing elsewhere in the universe.
“The message is in the molecules,” says Christopher Glein, another member of Waite’s group at SwRI. “We just have to keep measuring the molecules in that plume, and that’s going to tell us about what we cannot see.”
We won’t have any more molecules from Enceladus’s plumes for a long time, though. Cassini is running low on fuel, and if it were to crash into Enceladus it might destroy any extraterrestrial ecosystem living there. To protect potential life on Saturn’s ocean moons, we have to destroy the only tool we have to find it. The spacecraft will crashinto Saturn on 15 September.
Even if an Enceladus mission is selected in NASA’s next round of New Frontiers funding, to be announced in 2019, it wouldn’t reach the Saturn system until the late 2020s or early 2030s.
“To address whether there is life, we’ll have to go back,” McKay says. “Two decades can go by pretty fast.”
The Grand Finale: Cassini prepares for final mission
Nearly 20 years after launch and almost 13 years at the majestic ringed planet, the Cassini spacecraft is about to enter the final phase of its historic mission. The Grand Finale of Cassini’s mission will begin Sunday morning, setting up a series of close-proximity ops to the planet as Cassini dives between the innermost edge of Saturn’s rings and the planet itself to prepare for atmospheric entry into Saturn and the end of its mission on 15 September 2017.
The Grand Finale:
The grand finale to Cassini’s mission is set to begin at 03:46 GMT on 23 April – 23:46 EDT on 22 April) when the spacecraft reaches aposaturnium, the farthest point in its orbit of Saturn – which will mark the commencement of the first of the final 22 orbits of the craft.
The commencement of the grand finale will be aided days earlier by the spacecraft’s 126th and final close flyby of Saturn’s moon Titan on 22 April at 06:08 GMT (02:08 EDT).
The encounter with Titan will allow Cassini to gather the last bits of information possible about the atmosphered moon while simultaneously using Titan’s gravity to alter its trajectory to “leap over the planet’s icy rings.”
This trajectory shift will bring Cassini to its first Grand Finale ringplane crossing as it dives between the inner-most ring and the planet – the first time any spacecraft has attempted such a feat – on 26 April at 09:00 GMT (05:00 EDT).
This will be the first of 22 weekly ring crossings for Cassini in the final five months of its mission.
During this first ringplane crossing, Cassini’s orientation will allow the craft’s High Gain Antenna to act as a shield to protect the instruments and the spacecraft from possible ring particle impacts.
Moreover, the trajectory of each of the 22 ring crossing orbits is not identical, as each is specifically designed to allow Cassini to investigate different aspects of the planet and its rings as the final weeks of scientific data are collected.
Overall, the grand finale to Cassini’s mission carries unique mission objectives, including: detailed mapping of Saturn’s gravity and magnetic fields to reveal how the planet is arranged internally and to potentially solve the mystery of how fast Saturn is rotating; to improve knowledge of how much material is in Saturn’s rings and to help better understand and reveal their origins; to sample the icy ring particles being funneled into the atmosphere by Saturn’s magnetic field; and to take close-up images of Saturn’s rings and upper atmospheric clouds.
To successfully complete these mission objectives, the final 22 orbits will see Cassini’s distance from Saturn differ greatly, with some orbits allowing the spacecraft to skim the outer edges of the atmosphere while others will take the spacecraft farther out to skirt the very inner-most sections of the ring system.
The farthest into one of the rings Cassini will go will occur on the 6th ring crossing on 28 May. During this passing, Cassini will sample Saturn’s innermost ring, the D-ring, while being shielded by its high-gain antenna.
This will be followed by a second close venture into the D-ring on 4 June that will again allow Cassini to sample the D-ring material.
A third venture into the D-ring will then occur on 29 June followed by the fourth and final ring dive on 6 July.
On 14 August, Cassini will begin the first of five dives into Saturn’s atmosphere, sampling the gas giant’s atmosphere for the first time in history.
The first atmospheric sampling dip will occur on the 18th orbit of the grand finale mission.
Orbit 19 will see another dip into Saturn’s atmosphere, while orbit 20, on 27 August, will see the third and lowest of the dips into the atmosphere.
Orbits 21 in orbit 22 on 2 September and 9 September, respectively, will see the fourth and fifth dips into the atmosphere.
After this, on 11 September, Cassini will perform a final, distant flyby of Titan that will give the spacecraft just enough of a gravitational nudge – what’s being called the “goodbye kiss” – to send Cassini into its 293rd and final orbit of Saturn.
Cassini will reach its final aposaturnium on 12 September at 05:37 GMT – a moment that will mark the start of Cassini’s plunge into Saturn’s atmosphere.
On 15 September at 10:44 GMT (06:44 EDT), Cassini’s thrusters will fire to maintain attitude control for a roughly 60 second burn that will enable the final transmission of expected mission data back to Earth.
The spacecraft, during its fiery death plunge, will be commanded to continue relaying telemetry back to Earth until the spacecraft’s destruction.
Based on atmospheric entry parameters, the last signal from Cassini is expected to be transmitted from the spacecraft on 15 September 2017 at 10:45 GMT (06:45 EDT).
The signal is expected to arrive at Earth through the Deep Space Network 1 hour 23 minutes later at 12:08 GMT (08:08 EDT).
Cassini – 20 years on:
It was a mission slated to last only four years.
Following a flawless launch from the Cape Canaveral Air Force Station aboard a Titan IV (401) B rocket, Cassini spent seven years performing multiple gravity assist maneuvers and flybys of various planets before entering orbit of Saturn on 1 July 2004.
Even its arrival at Saturn was daring.
To insert Cassini into the proper orbit, mission controllers had to fly the vehicle through the gap between the planet’s F- and G- rings, something that caused a bit of concern due to the relatively unknown nature of how debris-free the ring-gap was.
Despite the fears, Cassini threaded the needle perfectly and entered orbit of the ringed planet for a mission that, at that point, was scheduled to end on 30 June 2008.
On 14 January 2005, the Huygens lander successfully entered Titan’s atmosphere, performing a 2.5 hour descent via parachute and subsequent landing on the surface of the moon.
The Huygens landing remains to this day the most-distant landing of a human-built craft and the only landing thus far attempted in the outer solar system.
Huygens functioned for 90 minutes after landing, returning images and scientific data of the landing site near the Xanadu region.
While Huygens touched down on land, the possibility of a methane lake landing was accounted for in its design.
On 15 April 2008, Cassini received the 27 month mission extension for 60 additional orbits of Saturn, 21 close flybys of Titan, and seven close encounters with Enceladus.
The extended mission was named the Cassini Equinox Mission and allowed the spacecraft to observe Saturn’s transition through Equinox, which occurred in August 2009.
The first mission extension came to an end in August 2010 and was succeeded by a second and final extension – a seven year mission extension out to the maximum operating time for Cassini based on its RTG (Radioisotope Thermoelectric Generator) power source.
The mission was subsequently named the Cassini Solstice Mission as it would allow Cassini to monitor Saturn’s transition through the solstice, which will occur on 24 May 2017.
The solstice mission allowed 155 more orbits of Saturn, 54 additional flybys of Titan, and 11 more close encounters with Enceladus.
Throughout the course of its nine and a half years of mission extensions, Cassini has helped unlock startling revelations about Saturn, Titan, and Enceladus.
Chief among Cassini’s discoveries regarding Enceladus was last week’s ground breaking announcement of the discovery of possible life-supporting hydrothermal vents on the surface of Enceladus’ subterranean salt-water ocean.