{"id":14809,"date":"2017-03-11T21:28:49","date_gmt":"2017-03-11T13:28:49","guid":{"rendered":"https:\/\/wp-productionenv-bjg9h2g2bgg5b8aa.southeastasia-01.azurewebsites.net\/news\/perils-and-discoveries-lie-ahead-for-long-lived-saturn-orbiter\/"},"modified":"2017-03-11T21:28:49","modified_gmt":"2017-03-11T13:28:49","slug":"perils-and-discoveries-lie-ahead-for-long-lived-saturn-orbiter","status":"publish","type":"post","link":"https:\/\/starpath.global\/news\/perils-and-discoveries-lie-ahead-for-long-lived-saturn-orbiter\/","title":{"rendered":"Perils and discoveries lie ahead for long-lived Saturn orbiter"},"content":{"rendered":"
\"\"
This view shows Saturn\u2019s northern hemisphere in 2016, as that part of the planet nears its northern hemisphere summer solstice in May 2017. Saturn\u2019s year is nearly 30 Earth years long, and during its long time there, Cassini has observed winter and spring in the north, and summer and fall in the south. Credit: NASA\/JPL-Caltech\/Space Science Institute<\/figcaption><\/figure>\n

Scientists are bracing for a blitz of discoveries in the last six months of Cassini\u2019s mission at Saturn, when the plutonium-powered robot will repeatedly ram through an unexplored gap in the planet\u2019s famous rings, then make a destructive plunge into the atmosphere in September.<\/p>\n

The dramatic last act has been in the works since 2010, when NASA formally approved the plan, using flybys of Saturn\u2019s moon Titan and periodic thruster burns to reshape Cassini\u2019s orbit around the ringed planet.<\/p>\n

Cassini\u2019s mission will end with a Sept. 15 dive into Saturn\u2019s hydrogen-helium atmosphere after a series of 22 close-in week-long orbits passing between the planet\u2019s innermost icy ring and its cloud tops. The robotic spacecraft will set up for the mission\u2019s last phase \u2014 dubbed the \u201cgrand finale\u201d \u2014 with a flyby of Saturn\u2019s moon Titan on April 22, followed by the first dip through the ring gap around four days later.<\/p>\n

\u201cIn many ways, the grand finale for Cassini is like a brand new mission,\u201d said Linda Spilker, Cassini\u2019s project scientist at the Jet Propulsion Laboratory in Pasadena, California. \u201cWe\u2019re going to probe Saturn\u2019s interior, measure the magnetic field, look for the magnetic dynamo, and try and figure out why is there is so little, or perhaps no, tilt between the magnetic field axis and the spin axis of Saturn. What\u2019s going on there?\u201d<\/p>\n

The docket of scientific investigations planned from April through September runs deep, but Cassini first has to survive the journey inside the rings.<\/p>\n

\u201cJust the feat of navigating and engineering our way through the gap between the rings and the planet, that in and of itself I consider an engineering triumph,\u201d said Earl Maize, Cassini\u2019s project manager at JPL.<\/p>\n

It\u2019s a story played out in meeting rooms, memos and on presentation slides among scientists and engineers working on many space missions. Scientists hungry for new revelations push for more data, while engineers warn of risks and dangers that could overtax a spacecraft or instrument.<\/p>\n

Cassini\u2019s daring last act has members of the mission\u2019s team struggling with balancing the same dilemma: More science data or less risk?<\/p>\n

But the calculation has changed with Cassini, which is in the last six months of a 13-year odyssey around Saturn. While managers say they want to avoid doing anything foolish with the spacecraft, the mission\u2019s shortened time horizon has officials willing to take more risks.<\/p>\n

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The spacecraft will make its first passage through the 1,500-mile-wide (2,400-kilometer) gap between Saturn\u2019s rings and atmosphere with its dish-shaped high-gain antenna pointing forward, blocking the orbiter\u2019s sensitive electronics, computer and scientific sensors from collisions with ice and dust that may populate the region.<\/p>\n

No spacecraft has ever passed through the gap, and although images do not show any signs of dust or ice in Cassini\u2019s path, officials cannot be sure of the threat. Cassini will be moving so fast that a smash-up with a tiny grain could cause catastrophic damage.<\/p>\n

\u201cThe innermost ring is called the D ring, and it sort of just slowly fades away into areas we can\u2019t see,\u201d Maize said. \u201cWe\u2019re going into the area where we cannot see. We have really good models of the rings, and we believe we\u2019re going to be safe, but nevertheless, there are going to be five instances where we\u2019re going to hide behind the high-gain antenna as we go through the rings just because we\u2019re kind of close.\u201d<\/p>\n

The high-gain antenna will be in its so-called \u201cram\u201d position, pointed in Cassini\u2019s direction of travel, on the first trip through the ring gap, giving ground controllers a chance to assess how much ice and dust is actually there.<\/p>\n

Cassini will fly through the gap at slightly different locations on each orbit. On four passages from May through July, the spacecraft will be closer to the D ring, and engineers will pivot Cassini to again put its antenna in the ram position on those orbits.<\/p>\n

\u201cThere is a possibility, and it\u2019s higher than we normally accept, of dust collision,\u201d Maize said in an interview with Spaceflight Now. \u201cWe\u2019re going to try to be careful, but at the same time there is that possibility.<\/p>\n

\u201cIf we get surprised, and we have way more dust than we thought, then we will probably hide behind the high-gain antenna much more frequently,\u201d Maize said.<\/p>\n

But officials are wary of big changes.<\/p>\n

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Spaceflight Now members can read a transcript of our interview with Cassini project manager Earl Maize. Become a member today and support our coverage.<\/h6>\n
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<\/h5>\n

Cassini\u2019s flight plan is uploaded to the spacecraft in 10-week chunks, and scientific observations for this summer\u2019s grand finale campaign are already planned in detail.<\/p>\n

\u201cThe science has all been carefully integrated and coordinated between all the instruments, and if we start to move when we\u2019re hiding behind the high-gain (antenna) and when we\u2019re not, then that can be quite disruptive,\u201d Maize said.<\/p>\n

Another hazard awaits Cassini the other edge of the gap, where the top layers of Saturn\u2019s atmosphere will tug on the orbiter. The slight aerodynamic forces could be too strong for Cassini\u2019s reaction wheels, a set of spinning masses designed to keep the craft pointed with momentum.<\/p>\n

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This chart shows where Cassini will fly relative to Saturn\u2019s atmosphere (represented at the bottom) and the innermost D ring (represented at top) on 22 orbits from April through September. The chart also notes on which orbits Cassini\u2019s high-gain antenna will fly in the \u201cram\u201d position to block incoming ice and dust particles, and when Cassini will rely on its rocket thrusters for pointing close to Saturn\u2019s atmosphere. Credit: Linda Spilker\/NASA\/JPL-Caltech<\/figcaption><\/figure>\n

For the mission\u2019s final five trips through the ring gap, ground controllers will activate the probe\u2019s rocket thrusters, burning hydrazine to keep the aerodynamic forces from putting Cassini in a tumble.<\/p>\n

The Cassini project, first conceived in the 1980s, has cost nearly $4 billion from start to finish. Cassini launched in October 1997 from Cape Canaveral aboard a Titan 4 rocket, flew by Venus and Jupiter, and reached Saturn in July 2004, becoming the first space probe to slip into orbit there.<\/p>\n

The orbiter dropped a European probe named Huygens to land on the surface of Titan, Saturn\u2019s largest moon, in January 2005. Since then, Cassini has circled Saturn more than 260 times, collecting detailed imagery of Saturn\u2019s atmosphere and mysterious hexagonal polar vortex, explored its rings in minute detail, and observed 49 of Saturn\u2019s 62 known moons with close and long-range flybys.<\/p>\n

Cassini was originally scheduled to collect data for four years after arriving in orbit around Saturn, but NASA extended the mission as the probe discovered that the planet and its moons demanded further study.<\/p>\n

Titan harbors several Earth-like features, like a thick atmosphere, rivers, lakes and rain, but the liquid on Titan\u2019s surface is not water. It\u2019s a mix of ethane, methane and other hydrocarbons.<\/p>\n

Saturn\u2019s 313-mile-diameter (504-kilometer) moon Enceladus has a global ocean of water buried under ice \u2014 a finding made by scientists using Cassini. Eruptions at Enceladus\u2019s south pole spray gas, dust, and organic material into space, and Cassini has sampled the jets in a series of flybys.<\/p>\n

The build-up of knowledge has been incremental, with each of Cassini\u2019s hundreds of encounters with Saturn\u2019s moons adding another piece of the puzzle. Meanwhile, other NASA missions like the Curiosity rover and New Horizons made headlines when they landed on Mars and unveiled the face of Pluto for the first time.<\/p>\n

\u201cWe always think we ought to be on the front page every day,\u201d Maize said of Cassini\u2019s legacy. \u201cI think that it has gotten its due in the scientific community. It\u2019s a discovery machine.\u201d<\/p>\n

He cited NASA\u2019s decision last year to ask for proposals for new missions to Saturn focusing on Titan and Enceladus. The space agency currently has no confirmed mission to Saturn after Cassini.<\/p>\n

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Three of Saturn\u2019s moons \u2014 Tethys, Enceladus and Mimas (top right to bottom left) \u2014 are captured with Saturn\u2019s rings in this group photo from NASA\u2019s Cassini spacecraft captured Dec. 3, 2015. Credit: NASA\/JPL-Caltech\/Space Science Institute<\/figcaption><\/figure>\n

\u201cThe fact that they\u2019ve actually created an Ocean Worlds program, and are allowing new missions to be proposed to Titan and Enceladus, that\u2019s on us,\u201d Maize said. \u201cThose are Cassini discoveries that opened up this whole new set of horizons, that not only are there a few ocean worlds, but there may be many, and they don\u2019t have to be big. Look at Enceladus!\u201d<\/p>\n

The moon Dione may also have an underground ocean, and the rest of Saturn\u2019s motley crew of moons have their own stories.<\/p>\n

There\u2019s Hyperion, which rotates unpredictably, is less dense than water, and looks like a sponge or a wasp\u2019s nest. Mimas, the closest of the major moons to Saturn, likely consists almost entirely of water ice, and its surface is scarred with a giant crater, earning it the moniker of the \u201cDeath Star.\u201d<\/p>\n

Two small saucer-shaped moons, Pan and Atlas, have ridges along their equators. Scientists believe the objects, each about the width of a large city, accumulate dust and ice grains as they orbit Saturn near the planet\u2019s rings.<\/p>\n

Cassini is currently getting some of its best views of Saturn\u2019s smaller moons.<\/p>\n

The spacecraft swung into an orbit in November that grazes the outer edge of Saturn\u2019s rings, setting up for the Titan encounter in April, when Cassini will cross inside the rings. The \u201cring-grazing\u201d orbit has yielded detailed views of the ring structure, as well as Saturn\u2019s numerous moons that carve out lanes between the individual rings.<\/p>\n

NASA released images Thursday revealing the distinct shape of Pan, drawing comparisons to ravioli or a walnut. In January, Cassini captured dazzling views of the 5-mile-wide (8-kilometer-wide) Daphnis, which plows through a 26-mile (42-kilometer) gap between Saturn\u2019s rings, its weak gravity making waves in the neighboring ring layers.<\/p>\n

In the next month, Cassini will closely observe several intriguing features inside Saturn\u2019s rings nicknamed propellers. Scientists believe the disturbances, named for famous aviators, are created by tiny unseen moonlets as small as 300 feet, or 100 meters, embedded in the rings. The spacecraft will collect some of the mission\u2019s best images of the propellers in the coming weeks.<\/p>\n

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This raw, unprocessed image of Saturn\u2019s moon Pan was taken on March 7, 2017, by NASA\u2019s Cassini spacecraft. Credit: NASA\/JPL-Caltech\/Space Science Institute<\/figcaption><\/figure>\n

Saturn\u2019s polar aurora, the dust environment around the rings, and long-range imaging of the moons Tethys and Enceladus are also on tap. Cassini will get its closest view ever of Atlas, the saucer-shaped twin to Pan, and take a picture from inside Saturn\u2019s shadow with the planet and rings backlit by the sun, allowing scientists to produce a mosaic of the rings\u2019 fainter components.<\/p>\n

Then comes the mission\u2019s last encounter with Titan on April 22. The moon\u2019s gravity will slingshot Cassini closer to Saturn than any spacecraft in history, into an egg-shaped orbit with a high point outside the rings and a low point threading between the rings and Saturn\u2019s cloud tops.<\/p>\n

Researchers are eager for Saturn\u2019s close-up, even if the mission\u2019s end will be a \u201cpoignant moment,\u201d Maize said.<\/p>\n

He said most members of the Cassini team \u201cthink that they\u2019ve landed on one of the best missions that NASA has ever flown.\u201d<\/p>\n

\u201cIt\u2019s a passing and the end of an era \u2014 a great era \u2014 it\u2019s been a great ride, and I think the the team is all deservedly very proud of their accomplishments,\u201d Maize said. \u201cIt\u2019s like with any good thing that has to come to an end, you don\u2019t want it to, but we understand why.\u201d<\/p>\n

Cassini has tripled the duration of its planned stay at Saturn, and is now running low on fuel.<\/p>\n

\u201cIt\u2019s over 19 years since launch, and we\u2019ve been at Saturn over 12,\u201d Maize said. \u201cThe spacecraft is showing its age, in some cases.\u201d<\/p>\n

One instrument, the Cassini Plasma Spectrometer, stopped working in 2012, and the spacecraft is running on a backup set of rocket thrusters.<\/p>\n

\u201cOur reaction wheels, which we use to fine tune our attitude control, are cranky but still functioning. It\u2019s kind of like my knees in the morning,\u201d Maize joked.<\/p>\n

But most of the spacecraft\u2019s systems are still healthy.<\/p>\n

\u201cGiven its age and the amount of stress we\u2019ve put it through, it\u2019s performing remarkably well,\u201d Maize said.<\/p>\n

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The Cassini spacecraft and Huygens probe during launch preparations inside Kennedy Space Center\u2019s Payload Hazardous Servicing Facility in 1997. Credit: NASA\/KSC<\/figcaption><\/figure>\n

So why send Cassini on a suicide mission?<\/p>\n

Officials worry that if Cassini died before falling into Saturn, the spacecraft could plow into Titan or Enceladus, polluting the moons with toxic rocket fuel, metal alloys and potentially microbes carried from Earth.<\/p>\n

\u201cIn a certain sense, Cassini has been a victim of its own success,\u201d Maize said. \u201cWe found these prebiotic worlds, which almost mandate that we can\u2019t contaminate them, so we\u2019ve got to do something sensible with the spacecraft.\u201d<\/p>\n

A wreck with Cassini could throw any future discovery of life on those moons into doubt.<\/p>\n

\u201cThe inside of Cassini is room temperature,\u201d Maize said. \u201cWe\u2019ve got electronics in there that are running right around 75 degrees Fahrenheit. For a hardy microbe, that\u2019s just as comfortable as can be, so you really don\u2019t want to leave that around Saturn.\u201d<\/p>\n

Navigators plotted this summer\u2019s novel trajectory inside the rings nearly a decade ago, and NASA settled on the audacious plan after considering colliding Cassini with one of Saturn\u2019s smaller, less habitable moons or dispatching the craft to fly by Uranus, Neptune, Jupiter, or an enigmatic Centaur object, a cross between an asteroid and a comet.<\/p>\n

\u201cThe cruise to Uranus was something like 30 years for a fast flyby,\u201d Spilker recalled.<\/p>\n

A joint study by officials at JPL and engineers at Purdue University in 2009 identified a way to send Cassini through the rings with a push from Titan\u2019s gravity.<\/p>\n

\u201cIt was really a no-brainer at that point,\u201d Spilker said. \u201cThe chance to go into that gap, not only for ring scientists but for the Saturn scientists, was just too much to pass up.\u201d<\/p>\n

\u201cSaturn still remains so compelling that we chose to use our last ounce of fuel in the spacecraft to explore that system,\u201d Maize said.<\/p>\n

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Artist\u2019s concept of Cassini\u2019s ring-grazing orbits, in gray, and grand finale orbits, in blue. Credit: NASA\/JPL-Caltech<\/figcaption><\/figure>\n

NASA considered steering the Pioneer 11 flyby probe through a gap between two parts of Saturn\u2019s rings in 1979. The agency again thought about guiding the Voyager probes through the so-called Cassini Division in the rings in the 1980s, but managers opted for a farther flyby out of safety concerns.<\/p>\n

Cassini will go even closer to Saturn than proposed on the Pioneer 11 and Voyager missions.<\/p>\n

Maize said there is a small chance Cassini could run out of rocket fuel before Sept. 15, but its reaction wheels could keep the craft pointed to complete the bulk of this summer\u2019s planned science campaign.<\/p>\n

But once Cassini jumps inside the rings next month, its trajectory will naturally fall into Saturn in September, even if the spacecraft fails, runs out of fuel, or crashes into an unexpected icy debris cloud.<\/p>\n

\u201cThere\u2019s little chance of us actually running out of gas and sputtering to a halt,\u201d Maize said. \u201cIt\u2019s just how we\u2019re going to get there.\u201d<\/p>\n

Spilker said scientists will measure Saturn\u2019s gravity field better than ever before by analyzing radio signals passed between Cassini and Earth to see how much they are distorted by the planet\u2019s gravity.<\/p>\n

\u201cWe hope to measure the size of the rocky core in Saturn,\u201d Spilker said Feb. 22 in a presentation to NASA\u2019s Outer Planets Assessment Group. \u201cAnd it\u2019s this rocky core that attracted material that eventually formed Saturn. We\u2019ll look at the interior also to try to measure the internal rotation rate.\u201d<\/p>\n

Cassini will be close enough to Saturn to map its gravity field with the precision to determine how deep winds penetrate inside the planet\u2019s atmosphere.<\/p>\n

\u201cThey could be anywhere from 300 to 3,000 kilometers (186 to 1,860 miles) in depth, and irregularities in the gravity field will provide the depth for those winds,\u201d Spilker said.<\/p>\n

Cassini\u2019s grand finale orbits are similar to the elliptical laps made by NASA\u2019s Juno spacecraft now exploring Jupiter. Spilker said information on Saturn\u2019s interior structure learned in the coming months will be compared to data on Jupiter obtained by Juno.<\/p>\n

Once the orbiter jumps inside the rings, scientists will be able to separate the total mass of the material inside the rings and of Saturn itself. Spilker said the uncertainty in the rings\u2019 mass will be reduced to around 5 percent, yielding crucial clues about their origins.<\/p>\n

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The colorful globe of Saturn\u2019s largest moon, Titan, passes in front of the planet and its rings in this true color snapshot from NASA\u2019s Cassini spacecraft. Credit: NASA\/JPL-Caltech\/Space Science Institute<\/figcaption><\/figure>\n

\u201cThat will tell us if the rings are less massive,\u201d Spilker said. \u201cThere are some indications that might be true, (in which case) they\u2019re young rings, formed from perhaps the breakup of a moon or a comet that came too close to Saturn.<\/p>\n

\u201cIf they\u2019re more massive, then there is a possibility that they could have formed at the same time as Saturn \u2014 it\u2019s not a given \u2014 but they could have been massive enough to survive the micrometeoroid bombardment to still be there until this day,\u201d Spilker said.<\/p>\n

Cassini will also sample the plasma hiding between Saturn and its rings, probing the planet\u2019s weak radiation field.<\/p>\n

If there are any microscopic ring particles in Cassini\u2019s flight path, the spacecraft\u2019s Cosmic Dust Analyzer will scoop up ice grains and directly measure their composition.<\/p>\n

\u201cWe know the rings are 99 percent water ice,\u201d Spilker said. \u201cBut what\u2019s the other 1 percent or so non-icy constituent? Iron? Silicates? Organics? Tholins? A mix? We\u2019ll get a chance to measure that directly.\u201d<\/p>\n

In the mission\u2019s last five passes in August and September, Cassini will be low enough to skim the atmosphere, telling the ground team about the molecules that make up the outer rarefied layers of Saturn itself.<\/p>\n

\u201cOn the very final orbit, we\u2019re deep enough that we\u2019ll actually be holding the high-gain antenna pointed toward the Earth for as long as we can,\u201d Spilker said.<\/p>\n

Cassini\u2019s mass spectrometer will be gathering \u201cin situ\u201d data on the conditions inside the atmosphere and piping the readings back to Earth in real-time \u2014 but with a nearly 90-minute lag due to Saturn\u2019s distance \u2014 rather than storing the measurements on recorders for playback later.<\/p>\n

\u201cCassini will be delivering science data down to its last seconds of life,\u201d Maize said.<\/p>\n

The orbiter\u2019s antenna can downlink information at about 140 kilobits per second. At that speed, it takes 10-to-20 seconds to transmit an image, Maize said, limiting the possibility for a final picture during the plunge.<\/p>\n

\u201cThe pointing isn\u2019t quite right for images anyway, although we\u2019re still toying with the idea of maybe one more,\u201d Maize said. \u201cWhy not? If we can rake the camera across the rings while we\u2019re going in, it will be spectacular.\u201d<\/p>\n

The spacecraft\u2019s control thrusters will be feverishly firing to keep the probe stable as long as possible as thicker streams of air tug on Cassini.<\/p>\n

Cassini will fall into Saturn at a speed of around 78,000 mph, or 35 kilometers per second.<\/p>\n

\u201cAs we\u2019re sampling Saturn\u2019s atmosphere, as long as Cassini can continue to point at the Earth, we will be sending back science data,\u201d Maize said. \u201cWhat happens is that the atmosphere will eventually push it to the point where it can\u2019t maintain its pointing with the antenna, and it\u2019ll probably be crushed a few tens of seconds later.\u201d<\/p>\n

Email the author.<\/i><\/b><\/p>\n

Follow Stephen Clark on Twitter: @StephenClark1.<\/strong><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

This view shows Saturn\u2019s northern hemisphere in 2016, as that part of the planet nears its northern hemisphere summer solstice in May 2017. Saturn\u2019s year is nearly 30 Earth years long, and during its long time there, Cassini has observed winter and spring in the north, and summer and fall in the south. Credit: NASA\/JPL-Caltech\/Space […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":"","_links_to":"","_links_to_target":""},"categories":[2],"tags":[2394,1183,1561,1562],"class_list":["post-14809","post","type-post","status-publish","format-standard","hentry","category-news","tag-cassini","tag-jet-propulsion-laboratory","tag-planetary-science","tag-saturn"],"acf":[],"_links":{"self":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/14809"}],"collection":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/comments?post=14809"}],"version-history":[{"count":0,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/posts\/14809\/revisions"}],"wp:attachment":[{"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/media?parent=14809"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/categories?post=14809"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/starpath.global\/blog\/wp-json\/wp\/v2\/tags?post=14809"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}