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Samstag, 22. April 2017 - 07:20 Uhr

Raumfahrt - Countdown beginnt für Cassini Grand Finale Around Saturn

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30.04.2016

Artist’s concept of Cassini’s final orbits between the Saturn’s innermost rings and the planet’s cloud tops. This set of orbits will consist the last leg of Cassini’s mission, called “The Grand Finale,” which will culminate with a plunge on Saturn’s atmosphere in September 2017. Image Credit: Image Credit: NASA/JPL
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It has become something of a hackneyed phrase, but in the case of NASA’s Cassini spacecraft it is rather fitting: an epic mission of exploration of Saturn that has single-handedly changed our view of the ringed planet, its moons, and their potential habitability, yet like all good things it must come to an end. Having nearly completed two full decades in space, Cassini has now entered its final 18 months around Saturn on what has been a tremendously successful and productive mission, full of unexpected and ground-breaking discoveries. Last week the mission’s science team officially began the one-year countdown toward the start of Cassini’s “Grand Finale,” which will culminate with an end-of-mission daring plunge on Saturn’s cloud tops on Sept. 15, 2017.
Launched on October 1997, Cassini undertook a seven-year journey of 3.5 billion km which included two flybys of Venus, one of Earth, and one of Jupiter, before finally arriving and entering orbit around Saturn in July 2004, becoming humanity’s first ever robotic spacecraft to do so. Since that time, Cassini has literally made history with every covered mile of space around the ringed giant planet and its assortage of fascinating moons, while slowly uncovering many of their long-held secrets and returning hundreds of thousands of images of unparalleled beauty. While the mission’s overall science results are too many to mention, highlights include the dispatch of the European-built Huygens probe which made humanity’s historic first soft-landing on the surface of the moon Titan, the ground-breaking discovery of water ice geysers erupting from Enceladus, the discovery of seas and lakes of liquid ethane and methane on Titan, as well as the detailed study of Saturn’s atmosphere and rings, just to name a few.
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A diagram showing the orbits that Cassini will follow around Saturn during the Grand Finale. The green-colored orbits represent the first phase of the Grand Finale when Cassini will reach as close as 10,000 km from Saturn’s outermost visible ring. The blue-colored orbits represent the second phase when Cassini will be positioned inside Saturn’s rings during closest approach to the planet. Image Credit: NASA/JPL
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Having completed its initial four-year primary mission in 2008, Cassini was given the go-ahead by NASA for an extended mission called the Cassini Equinox Mission and a third and final extension in 2010 called the Cassini Solstice Mission. The names for these extended missions weren’t arbitrary. Since one Saturnian year equals 29.4 Earth years, Cassini has been able to study the planet and its moons essentially through half of its orbit around the Sun, allowing planetary scientists to have a first close-up view of how the change of seasons affects the climate and atmospheric circulation on both Saturn and its largest moon Titan.
Yet, now more than a decade after it entered orbit around the ringed giant, Cassini has spent almost all of its onboard fuel which allowed it to maneuver through the Saturn system by way of hundreds of close flybys of its largest moon Titan. Cassini’s mission planners had been preparing for the inevitable end ever since the conclusion of the spacecraft’s primary mission in 2008, while evaluating several scenarios as to the exact way with wich Cassini would make its farewell. Some of the options that were considered for Cassini’s end of mission by ground teams included an escape from Saturn toward Uranus or Neptune, an escape toward a heliocentric orbit, or an aerobraking and eventual placement in a stable orbit around Titan. All of these options were evaluated against certain factors like the time needed for the completion of each scenario, the delta-v required for the orbital changes, the fuel that would be available, as well as the best overall science return for each option. In the end, the only options that satisfied all of the required criteria were either an impact on one of Saturn’s icy satellites, or an impact on the giant planet itself. In the best interests of planetary protection, Cassini’s science team eventually chose the latter, in order to prevent the biological contamination of Saturn’s moons from any terrestrial microorganisms that could have been carried from Earth onboard the spacecraft.
To that end and after shifting through a list of names that were submitted by more than 2,000 members of the public, the mission’s science team chose to appropriately name the final leg of Cassini’s trek around Saturn “The Grand Finale.” The latter is comprised of two parts. The first one, which will begin this year on Nov. 30 following the spacecraft’s penultimate Titan flyby, consists of a set of 20 elongated polar orbits around the planet which will bring Cassini within 10,000 km of Saturn’s F ring (the planet’s outermost discrete ring). This will be followed by The Grand Finale’s second phase beginning on April 22, 2017, when Cassini will use its final Titan flyby in order to change its orbital orientation and execute a daring loop that will bring it just 3,800 km above Saturn’s cloud tops, thus positioning it inside the planet’s entire ring system! From this vantage point, Cassini will complete a total of 22 highly elliptical polar orbits around Saturn before finally plunging onto the gas giant’s atmosphere on Sept. 15, 2017, putting an end to its spectacular 20-year mission.
One of the reasons that this scenario was chosen for Cassini’s end of mission was that it would return the most science compared to all the other options that had been evaluated. In fact, for Cassini’s science team, The Grand Finale represents an entire new mission on its own, which hadn’t even been considered as a possibility when the spacecraft was still on the drawing board back in the 1980s. One of the mysteries that have remained unresolved to this day involve the composition of Saturn’s internal structure as well as the planet’s exact rotation rate, which has not been measured to a great precision. Cassini’s position inside the main ring system during the second phase of The Grand Finale will allow the spacecraft to make very detail measurements of Saturn’s gravity and magnetic fields which will help scientists to better answer the remaining questions regarding the planet’s interior and overall rotation. Furthermore, a different spacecraft, Juno, which is scheduled to arrive on Jupiter this summer, will be making similar measurements regarding Jupiter’s interior at the same time, which will provide planetary scientists with great insights as to the inner workings of the Solar System’s gas giant planet’s during the same point in time. “We’ll have a much better understanding of how the planet works,” says Dr. Jonathan Fortney, a professor of astronomy at the University of California, Santa Cruz, who is part of the science team that has been selected by NASA to coordinate Cassini’s Grand Finale mission. “We really don’t know what their interiors are like. What’s great is that, in the space of a year, we’ll have once-in-a-lifetime data sets for both Jupiter and Saturn.”
One other important set of science results expected to come during Cassini’s Grand Finale is the exact mass determination of Saturn’s ring system. For the entire duration of its mission to date, the spacecraft has operated solely outside of Saturn’s rings which has limited its ability to determine their mass, since it had to take into account the gravitational effects of the giant planet on the ring system. By positioning itself inside the rings, Cassini will be able to differentiate between these gravitational effects and the mass of the rings themselves. In addition, Cassini will have an unprecedented close-up view at the rings’ overall structure and composition, which might help scientists shed more light on their age and determine whether they have formed late in the planet’s history or have the same age as Saturn itself.
NASA has already set officially the countdown clock ticking toward the start of Cassini’s Grand Finale, as announced late last week by Ron Baalke, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, Calif., with a post on his Twitter account. This countdown marks the final 18 months of life for Cassini, which for many has been the epitome and highlight of NASA’s Planetary Science Division for the last two decades.
Both Cassini and Juno will end their missions around the same time, with the latter scheduled to make a final plunge onto Jupiter’s cloud tops in February 2018. After that, and save for the Europa Clipper mission which is still in the conceptual stage, no other missions toward the outer Solar System exist in the NASA pipeline for the foreseeable future. This unfortunate reality, which is a result of cuts in the space agency’s planetary science budgets in recent years, means that we can probably not expect to see another dedicated probe toward the outer planets for the next couple of decades. This doesn’t mean that there hasn’t been a lack of proposals from the planetary science community, including mission concepts for the exploration of the ice giants Uranus and Neptune, both of which have been largely neglected ever since the Voyager 2 fly bys of the 1980s. Yet, as things stand at the moment, with the coming conclusion of the Cassini mission in 2017 and Juno in early 2018, the outer Solar System will likely remain out of reach for at least the next couple of decades.
Quelle: AS
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Update: 5.04.2017
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NASA’s Cassini Mission Prepares for 'Grand Finale' at Saturn

This illustration shows NASA’s Cassini spacecraft above Saturn's northern hemisphere prior to one of its 22 grand finale dives.
This illustration shows NASA’s Cassini spacecraft above Saturn's northern hemisphere prior to one of its 22 grand finale dives.
Credits: NASA/JPL-Caltech

NASA's Cassini spacecraft, in orbit around Saturn since 2004, is about to begin the final chapter of its remarkable story. On Wednesday, April 26, the spacecraft will make the first in a series of dives through the 1,500-mile-wide (2,400-kilometer) gap between Saturn and its rings as part of the mission’s grand finale.

 

"No spacecraft has ever gone through the unique region that we'll attempt to boldly cross 22 times," said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. "What we learn from Cassini’s daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve. This is truly discovery in action to the very end."

 

During its time at Saturn, Cassini has made numerous dramatic discoveries, including a global ocean that showed indications of hydrothermal activity within the icy moon Enceladus, and liquid methane seas on its moon Titan.

 

Now 20 years since launching from Earth, and after 13 years orbiting the ringed planet, Cassini is running low on fuel. In 2010, NASA decided to end the mission with a purposeful plunge into Saturn this year in order to protect and preserve the planet's moons for future exploration – especially the potentially habitable Enceladus.

 

But the beginning of the end for Cassini is, in many ways, like a whole new mission. Using expertise gained over the mission's many years, Cassini engineers designed a flight plan that will maximize the scientific value of sending the spacecraft toward its fateful plunge into the planet on Sept. 15. As it ticks off its terminal orbits during the next five months, the mission will rack up an impressive list of scientific achievements.

 

"This planned conclusion for Cassini's journey was far and away the preferred choice for the mission's scientists," said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. "Cassini will make some of its most extraordinary observations at the end of its long life."

 

The mission team hopes to gain powerful insights into the planet's internal structure and the origins of the rings, obtain the first-ever sampling of Saturn's atmosphere and particles coming from the main rings, and capture the closest-ever views of Saturn's clouds and inner rings. The team currently is making final checks on the list of commands the robotic probe will follow to carry out its science observations, called a sequence, as it begins the finale. That sequence is scheduled to be uploaded to the spacecraft on Tuesday, April 11.

 

Cassini will transition to its grand finale orbits, with a last close flyby of Saturn's giant moon Titan, on Saturday, April 22. As it has many times over the course of the mission, Titan's gravity will bend Cassini's flight path. Cassini's orbit then will shrink so that instead of making its closest approach to Saturn just outside the rings, it will begin passing between the planet and the inner edge of its rings.

 

"Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft. But we're also being cautious by using our large antenna as a shield on the first pass, as we determine whether it's safe to expose the science instruments to that environment on future passes," said Earl Maize, Cassini project manager at JPL. "Certainly there are some unknowns, but that's one of the reasons we're doing this kind of daring exploration at the end of the mission."

 

In mid-September, following a distant encounter with Titan, the spacecraft's path will be bent so that it dives into the planet. When Cassini makes its final plunge into Saturn's atmosphere on Sept. 15, it will send data from several instruments – most notably, data on the atmosphere's composition – until its signal is lost.

 

"Cassini's grand finale is so much more than a final plunge," said Spilker. "It's a thrilling final chapter for our intrepid spacecraft, and so scientifically rich that it was the clear and obvious choice for how to end the mission."

Quelle: NASA

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Cassini mission’s final performance a fiery plunge to Saturn


NASA has revealed plans to send the Cassini spacecraft into a death spiral,allowing it to capture new data and images of Saturn and its rings, writes Richard A. Lovett.


An artist’s impression of Cassini breaking up in Saturn’s atmosphere.
NASA/JPL

NASA’s 20-year-old Cassini mission is about to make a final orbit correction that will cause it to crash into Saturn and burn up in September. 

Before it meets its fiery demise, however, the long-lived spacecraft, which left Earth in 1997 and first entered Saturn orbit in 2004, will make a series of 22 close flybys, diving within 2,000 km of the giant planet’s cloud tops. Its final passes, in fact, would be so close that it would be able to sample the outer fringes of the planet’s atmosphere, Cassini project manager Earl Maize said at a press conference on April 4 in Pasadena, California. 

This Grand Finale, as NASA is dubbing it, begins on April 22 when the spacecraft makes a flyby of Titan, using that moon’s gravity to turn toward Saturn. There it will dive through the gap between the innermost of Saturn’s rings and the giant planet’s atmosphere. 

At each of these close approaches, the spacecraft will be traveling 122,000 km/h – fast enough that hitting anything more substantial than a speck of dust could damage it irreparably. “We would never take a flagship mission on that kind of course at any other time in the mission except when it’s about to end,” Maize said. 

Cassini’s mission is about to end one way or another, because the spacecraft is on the verge of running out of maneouvring fuel. Project leaders decided years ago that they did not want to leave the spacecraft drifting without maneouvring power, for fear it might eventually crash into one of Saturn’s moons, potentially contaminating it with microbes that had hitchhiked all the way from Earth. 

The Grand Finale does more than simply dump the spacecraft safely into Saturn’s atmosphere, where conditions are unsuitable for Earth life. It would also produce some exciting science, said Linda Spilker, the mission’s project scientist: “I would not be surprised if some of the discoveries will be the best we’ve obtained.” 

One goal is to measure the mass of the rings more accurately than has previously been possible. That can help determine how old they are because ring material is being constantly eroded away by micro-meteorite bombardment from outside the ring system. Massive rings survive longer than less-massive ones, and thus might also be older. 

It would also be possible, Spilker said, to determine what the rings are made of by studying the impacts of harmless, smoke-sized dust particles with Cassini’s cosmic dust analyser. “We know they are 99% water ice,” she said, “but we’re not sure about the other 1%. What is it? Iron, silicates, organics, a mix of all three, or something else we’ve not thought of?” 

 

There will also be the best-yet close-up views of Saturn’s poles, which have giant hurricanes and a mysterious hexagonal feature Spilker called a “six-sided jet stream, two Earth diameters across”. 

Some of the passes will even come close enough to measure trace constituents in Saturn’s upper atmosphere. 

Measurements of Saturn’s gravity field will also allow the scientists to peel back its atmosphere and determine the size of its rocky core. 

“The Grand Finale is like a brand new mission,” Spilker said. 

Even on the final dive into the atmosphere, the spacecraft will continue to return data as it fights to keep its antenna pointing in the right direction. Not that it will be able to do this for long, because this is a flight for which it was never designed. Eventually, Maize said, it will lose contact, break up and vaporise. 

There is, of course, a risk that Cassini will hit something big enough to damage it. Maize put the odds of successfully completing all 22 passes at 98.8%, based on the best models of the density and size of dust particles in the not-entirely-empty gap. “Our most dire models put us at 97%,” he said. 

“If we get surprised, we have a bunch of contingency plans,” he added. Even if the spacecraft is knocked entirely out of commission, it is not at risk of hitting a moon, because from the moment it lines up for its upcoming Titan flyby, its orbit is determined by the laws of physics. “Cassini will still end up as planned, but we’ll get less science,” Maize said. 

When the end comes, it’s going to be an emotional moment for the project team. “I’ve worked on it for almost three decades,” Spilker said of the mission. “My oldest daughter started kindergarten when I started working on Cassini. Now she’s married and has a daughter of her own. It’s really going to be hard to say goodbye to this plucky, capable spacecraft that has returned all of this great science.” 

Cassini had also made its mark in the planning for other missions, added Jim Green, director of NASA’s Planetary Science Division. Just to start with, he said, the upcoming Europa Clipper mission (scheduled for launch in the 2020s) would use an approach similar to that which Cassini used to study Titan. Rather than orbiting Europa, the Europa Clipper will spend as little time as possible in Jupiter’s dangerous radiation belts by instead making repeated flybys of Europa. 

“We’re taking a page out of Cassini’s book,” Green said. 

Quelle: COSMOS

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Update: 6.04.2017

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NASA will destroy a $3.26 billion Saturn probe this summer to protect an alien water world

cassini huygens mission spacecraft probe nasa jpl caltech PIA21438

An illustration of the Cassini spacecraft over Saturn's north pole with its hexagon-shaped storm.NASA/JPL-Calte

 

 

 
 

 

  • 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.

enceladus

False-color image showing plumes erupting from Enceladus' surface.NASA/JPL/Space Science Institute

 

 

"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.

Titan

Skye Gould/Business Insider

 

 

"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

saturn interior layers metallic hydrogen rocky core nasa jpl caltech

An illustration of Saturn's internal structure.NASA/JPL-Caltech

 

 

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."

saturn hexagonal storm north pole cassini nasa jpl caltech jason major

Saturn's hexagon-shaped jet stream at the planet's north pole.NASA/JPL-Caltech/Space Science Institute; Jason Major/Lights in the Dark

 

 

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.

cassini spacecraft clean room wokrers nasa jpl caltech KSC 97PC1111 orig

The Cassini spacecraft being prepared for flight in 1997.NASA

 

 

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."

Quelle: BI

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Update: 10.04.2017

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Plateaus Up Close

Saturn's rings

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.

Quelle: NASA

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Update: 15.04.2017

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Cassini finds final ingredient for alien life in Enceladus’s sea

 

Enceladus: home to alien life, we hope
Home to alien life, we hope

NASA

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.”

Quelle: NewScientist

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Update: 22.04.2017

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The Grand Finale: Cassini prepares for final mission

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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.

10:44 GMT will also mark Cassini’s entry into Saturn’s atmosphere

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.

Saturn-Mond Raumfahrt Cassini The Grand Finale: Cassini prepares for final mission Cassini Grand Finale Around Saturn 

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