Cassini's Death March
« Thread started on: Apr 12th, 2017, 1:44pm »
Saturn Spacecraft Begins Science Swan-song
NASA’s Cassini probe will go where no spacecraft has gone before — between a planet and its rings.
Alexandra Witze 12 April 2017
Scientists still debate the age and origin of Saturn’s rings, but the Cassini craft’s final months promise to tell us more about their provenance. (NASA/JPL)
After 13 years exploring Saturn and its moons, NASA’s Cassini spacecraft has just 5 months left to live. But it will go out with a scientific bang.
On 22 April, Cassini will slingshot past Titan, Saturn’s largest moon, for the last time. Four days later, the probe will hurtle into the unexplored region between the giant planet and its rings. Cassini will thread that 2,400-kilometre-wide gap 22 times before its kamikaze dive into Saturn’s atmosphere on 15 September.
This unprecedented journey promises to yield fresh discoveries for the venerable spacecraft. “It will be like a whole new mission,” says Linda Spilker, Cassini’s project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “There are fundamental new scientific measurements to make.”
Those include the first direct tastes of particles in Saturn’s rings, and of its upper atmosphere; the best measurements yet of the planet’s magnetic and gravitational fields, which could answer long-standing questions such as how fast the planet rotates and how old its rings are; and the sharpest look yet at enigmatic ripples in the rings.
It all begins with the spacecraft’s final fly-by of Titan, the 127th such close encounter. Cassini will scan the moon’s methane lakes one last time, looking for waves, bubbles or other phenomena roiling the surface. Earlier fly-bys have revealed changes in the lakes over time, and the final pass is the last chance to look for seasonal shifts, says Sarah Hörst, a planetary scientist at Johns Hopkins University in Baltimore, Maryland.
Titan’s gravitational pull will fling Cassini into its ‘grand finale’ orbits, plunging between Saturn’s innermost ring and the planet’s cloud tops (see ‘Cassini: the final frontier’). The spacecraft will turn its main antenna forward, to act as a protective shield against any errant ring particles as it whizzes along at 110,000 kilometres per hour.
Since November, the probe has been climbing higher relative to Saturn’s equatorial plane, providing a new vantage point on the planet’s outer rings. The upcoming inner dives will also reveal spectacular new details, says Carolyn Porco, a planetary scientist at the University of California, Berkeley, who leads the mission’s imaging team.
High-resolution photographs will capture the mysterious propeller-shaped gaps that ripple through some of the rings, probably formed by unseen moonlets. “The rings really are changing before our eyes,” says Jeffrey Cuzzi, a planetary scientist at NASA’s Ames Research Center in Moffett Field, California.
Cassini’s remote-sensing instruments will get their closest look yet at the rings, on sides both lit and unlit by the Sun. Measurements will show how the chemical make-up of the ring particles varies from place to place — information that is crucial for researchers who are trying to tease out which compounds pollute the rings’ otherwise pure ice.
And scientists might finally unravel the rings’ biggest mystery — how old they are and how they formed. Between May and July, Cassini will make its most precise measurements of Saturn’s gravitational field; by tracking the spacecraft’s motion as it flies between the planet and the rings, mission scientists expect to improve their calculations of the mass of the rings by an order of magnitude. A relatively high mass would suggest that the rings were ancient, perhaps formed by a big moon ripped apart billions of years ago. Lighter-weight rings would suggest a more recent formation, perhaps from a visiting comet that disintegrated.
Other fundamental measurements will tackle the giant planet itself. On the grand-finale orbits, Cassini’s magnetometer will measure Saturn’s magnetic field close to the planet. There, it is roughly ten times stronger — and more complex and scientifically interesting — than in areas already probed, says Marcia Burton, a planetary scientist at JPL.
Those data should shed light on long-standing mysteries such as the depth of Saturn’s metallic hydrogen core — which powers its magnetic field — and how quickly the planet rotates. Observations by the Voyager spacecraft in the 1980s suggested that one rotation takes just under 11 hours. But the numbers are different when measured in the northern and southern hemispheres, which hints that something more complicated is going on. “It is hard to imagine how the grand-finale orbits could not lead to a huge improvement in our understanding of Saturn’s magnetic field,” Burton says.
On 15 September, with its tanks almost out of fuel, mission controllers will steer Cassini directly into Saturn. But the craft will still radio back observations of the gases that make up Saturn’s atmosphere. “Even in its final moments, Cassini will be doing groundbreaking science,” says Hörst.
By Mike Wall, Space.com Senior Writer April 27, 2017
NASA's Cassini spacecraft has survived its first plunge through the narrow gap between Saturn's cloud tops and the giant planet's innermost rings, a region that no probe had ever explored before.
The space agency's Deep Space Network Goldstone Complex in California picked up Cassini's signal at 11:56 p.m. PDT yesterday (April 26; 2:56 a.m. EDT and 0656 GMT today, April 27) — nearly a full day after the historic dive took place. Data began coming in from the probe 5 minutes after contact was established, NASA officials said.
"In the grandest tradition of exploration, NASA's Cassini spacecraft has once again blazed a trail, showing us new wonders and demonstrating where our curiosity can take us if we dare," Jim Green, director of NASA's Planetary Science Division, in Washington, D.C., said in a statement.
Re: Cassini's Death March
« Reply #8 on: May 2nd, 2017, 11:43am »
Cassini finds ‘big empty’ near Saturn
By Deborah Byrd in Space May 2, 2017
It’s not totally empty, but the space between Saturn and its rings is much emptier than scientists expected. Cassini will make its 2nd dive through this gap at 3:38 p.m. EDT (19:38 UTC) today.
There’s much less dust between Saturn and its inner rings than expected, said NASA engineers, after last week’s historic dive through this gap by the Cassini spacecraft. Astronomers have been contemplating this maneuver by a spacecraft for decades, since the two Voyager spacecraft passed Saturn in the early 1980s. The fear was that a spacecraft might encounter debris that would suddenly end its mission! But Cassini – which is running out of fuel after orbiting Saturn since 2004 – not only passed through the gap successfully but also found it surprising debris-free. Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California said:
The region between the rings and Saturn is ‘the big empty,’ apparently. Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.
Cassini will make its second dive through the gap today (May 2, 2017) at 12:38 p.m. PDT (3:38 p.m. EDT, 19:38 UTC; translate UTC to your time zone)
With information from the first dive in hand, the Cassini team will now move forward with its preferred plan of science observations. NASA said:
A dustier environment in the gap might have meant the spacecraft’s saucer-shaped main antenna would be needed as a shield during most future dives through the ring plane. This would have forced changes to how and when Cassini’s instruments would be able to make observations. Fortunately, it appears that the “plan B” option is no longer needed. (There are 21 dives remaining. Four of them pass through the innermost fringes of Saturn’s rings, necessitating that the antenna be used as a shield on those orbits.)
Based on images from Cassini, models of the ring particle environment in the approximately 1,200-mile-wide (2,000-kilometer-wide) region between Saturn and its rings suggested the area would not have large particles that would pose a danger to the spacecraft.
But because no spacecraft had ever passed through the region before, Cassini engineers oriented the spacecraft so that its 13-foot-wide (4-meter-wide) antenna pointed in the direction of oncoming ring particles, shielding its delicate instruments as a protective measure during its April 26 dive.
The video below represents data collected by Cassini’s Radio and Plasma Wave Science instrument, as it crossed through the gap between Saturn and its rings on April 26. The instrument is able to record ring particles striking the spacecraft in its data. In the data from this dive, there is virtually no detectable peak in pops and cracks that represent ring particles striking the spacecraft. The lack of discernible pops and cracks indicates the region is largely free of small particles. William Kurth, RPWS team lead at the University of Iowa, Iowa City said:
It was a bit disorienting — we weren’t hearing what we expected to hear. I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.
The team’s analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (about 1 micron across).
Today’s ring crossing will occur in a region very close to where Cassini passed on last week’s dive. Prior to today’s crossing, Cassini’s cameras have been looking closely at the rings; in addition, the spacecraft was rotated (or “rolled”) faster than engineers have ever allowed it to before, in order to calibrate the magnetometer.
As with the first finale dive, Cassini will be out of contact during closest approach to Saturn, and is scheduled to transmit data from this dive on May 3.
Bottom line: During its April 26, 2017 dive between Saturn and its rings – its first of 22 dives in its Grand Finale this year – the Cassini spacecraft found a relatively dust-free region. Scientists are calling it The Big Empty.
Re: Cassini's Death March
« Reply #9 on: May 28th, 2017, 08:31am »
It must be fun to sit around and wait......
Cassini Takes Most Dangerous Saturn Ring Dive Yet
By Sarah Lewin, Staff Writer May 28, 2017
The Cassini spacecraft completes its sixth dive between Saturn and its rings today (May 28), and this is the most dangerous dive yet. Instead of passing safely between the planet and its rings, the spacecraft is plunging straight through the inner edge of Saturn's D ring.
The spacecraft, which is a collaboration between NASA, the European Space Agency and the Italian Space Agency, will turn its broad, high-gain antenna dish to rest in front of it as a shield during the crossing, for the first time since its very first ring dive, which occurred in April, NASA officials said. (For that dive, researchers didn't know whether the area between the rings and Saturn would be clear of debris.)
This dive is the first of four paths through the planet's faint innermost ring, and it's the deepest Cassini will go into the dusty loop, according to NASA's Grand Finale Orbit Guide. The spacecraft will make the crossing at 10:22 a.m. EDT (1422 GMT), but researchers don't expect to hear back from it until 11:29 p.m. EDT (0329 on May 29 GMT) once it is able to turn and reestablish contact with Earth.
Researchers have identified the 6-minute period when Cassini is most likely to collide with ring particles. During this time, the spacecraft's Radio and Plasma Wave Science instrument will be poised to detect the plasma clouds released when particles hit the antenna dish. The instrument sticks out past the dish, letting it take stock of particles while the rest of the craft is protected, the guide said.
During this orbit around Saturn, Cassini has been taking photos of the edge of the planet's A and F rings, as well as the space between, to investigate the rings' structure and how their particles interact. It also used the Radar instrument to scan all the way from the A ring's outer edge through the C ring, in the first of a three-part radio-wave experiment.
Cassini's next dive will also take it through the edge of the D ring, although not as far, and it will have two more opportunities after that to get firsthand experience of the planet's ephemeral innermost ring.
After Cassini completes all 22 ring dives, it will begin the final phase of its Grand Finale mission: plummeting into Saturn's atmosphere on Sept. 15 in its most dangerous (and fatal) dive of all. The spacecraft will send data back until it loses contact and burns up; the maneuver will protect Saturn's moons from any further contamination by Earth microbes and collect invaluable details about the planet's atmosphere in its last moments.
But in the meantime, Cassini has lots of science to do.
Re: Cassini's Death March
« Reply #10 on: Jul 27th, 2017, 4:31pm »
Has Cassini Found A Universal Driver For Prebiotic Chemistry At Titan ?
July 26, 2017
The international Cassini-Huygens mission has made a surprising detection of a molecule that is instrumental in the production of complex organics within the hazy atmosphere of Saturn’s moon Titan.
Titan boasts a thick nitrogen and methane atmosphere with some of the most complex chemistry seen in the Solar System. It is even thought to mimic the atmosphere of early Earth, before the build-up of oxygen. As such, Titan can be seen as a planet-scale laboratory that can be studied to understand the chemical reactions that may have led to life on Earth, and that could be occurring on planets around other stars.
In Titan’s upper atmosphere, nitrogen and methane are exposed to energy from sunlight and energetic particles in Saturn’s magnetosphere. These energy sources drive reactions involving nitrogen, hydrogen and carbon, which lead to more complicated prebiotic compounds.
These large molecules drift down towards the lower atmosphere, forming a thick haze of organic aerosols, and are thought to eventually reach the surface. But the process by which simple molecules in the upper atmosphere are transformed into the complex organic haze at lower altitudes is complicated and difficult to determine.
Chemistry in Titan’s atmosphere
One surprising outcome of the Cassini mission was the discovery of a particular type of negatively charged molecule at Titan. Negatively charged species – or ‘anions’ – were not something scientists expected to find, because they are highly reactive and should not last long in Titan’s atmosphere before combining with other materials. Their detection is completely reshaping current understanding of the hazy moon’s atmosphere.
Re: Cassini's Death March
« Reply #12 on: Sep 5th, 2017, 11:02am »
Cassini: Milestones before the final plunge
By Eleanor Imster in Space August 31, 2017
The 20-year-old Cassini mission will end September 15 with a dramatic plunge into planet Saturn. Here are the milestones for Cassini’s final days.
The Cassini spacecraft’s fateful plunge into planet Saturn on September 15, 2017 is a foregone conclusion, said NASA, as an April 22 gravitational kick from Saturn’s moon Titan placed the two-and-a-half ton vehicle on its path for impending destruction. Yet several mission milestones have to occur over the coming two-plus weeks to prepare the vehicle for one last burst of trailblazing science.
Linda Spilker, Cassini project scientist from NASA’s Jet Propulsion Laboratory in Pasadena, California. She said in a statement: "The Cassini mission has been packed full of scientific firsts, and our unique planetary revelations will continue to the very end of the mission as Cassini becomes Saturn’s first planetary probe, sampling Saturn’s atmosphere up until the last second. We’ll be sending data in near real time as we rush headlong into the atmosphere – it’s truly a first-of-its-kind event at Saturn."
The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn’s upper atmosphere. But on the way down, before contact is lost, eight of Cassini’s 12 science instruments will be operating. On the day before the plunge, Cassini will make detailed, high-resolution observations of Saturn’s auroras, temperature, and the vortices at the planet’s poles. Cassini’s imaging camera will be off during this final descent, having taken a last look at the Saturn system the previous day (September 14).
In its final week, Cassini will pass several milestones en route to its Saturn plunge. (Times below are predicted and may change slightly; see https://go.nasa.gov/2wbaCBT for updated times.)
September 9 Cassini will make the last of 22 passes between Saturn itself and its rings – closest approach is 1,044 miles (1,680 kilometers) above the clouds tops. September 11 Cassini will make a distant flyby of Saturn’s largest moon, Titan. Even though the spacecraft will be at 73,974 miles (119,049 kilometers) away, the gravitational influence of the moon will slow down the spacecraft slightly as it speeds past. A few days later, instead of passing through the outermost fringes of Saturn’s atmosphere, Cassini will dive in too deep to survive the friction and heating. September 14 Cassini’s imaging cameras take their last look around the Saturn system, sending back pictures of moons Titan and Enceladus, the hexagon-shaped jet stream around the planet’s north pole, and features in the rings. September 14 (5:45 p.m. EDT / 21:45 UTC) Cassini turns its antenna to point at Earth, begins a communications link that will continue until end of mission, and sends back its final images and other data collected along the way.
September 15 (4:37 a.m. EDT / 8:37 UTC) The “final plunge” begins. The spacecraft starts a 5-minute roll to position its instruments for optimal sampling of the atmosphere, transmitting data in near real time from now to end of mission.
September 15 (7:53 a.m. EDT / 11:53 UTC) Cassini enters Saturn’s atmosphere. Its thrusters fire at 10 percent of their capacity to maintain directional stability, enabling the spacecraft’s high-gain antenna to remain pointed at Earth and allowing continued transmission of data.
September 15 (7:54 a.m. EDT / 11:54 UTC) Cassini’s thrusters are at 100 percent of capacity. Atmospheric forces overwhelm the thrusters’ capacity to maintain control of the spacecraft’s orientation, and the high-gain antenna loses its lock on Earth. At this moment, expected to occur about 940 miles (1,510 kilometers) above Saturn’s cloud tops, communication from the spacecraft will cease, and Cassini’s mission of exploration will have concluded. The spacecraft will break up like a meteor moments later. Find out more about the Cassini mission here:https://www.nasa.gov/mission_pages/cassini/main/index.html
"Since its launch in 1997, the findings of the Cassini mission have revolutionized our understanding of Saturn, its complex rings, the amazing assortment of moons and the planet’s dynamic magnetic environment. The most distant planetary orbiter ever launched, Cassini started making astonishing discoveries immediately upon arrival and continues today. Icy jets shoot from the tiny moon Enceladus, providing samples of an underground ocean with evidence of hydrothermal activity. Titan’s hydrocarbon lakes and seas are dominated by liquid ethane and methane, and complex pre-biotic chemicals form in the atmosphere and rain to the surface. Three-dimensional structures tower above Saturn’s rings, and a giant Saturn storm circled the entire planet for most of a year. Cassini’s findings at Saturn have also buttressed scientists’ understanding of processes involved in the formation of planets." Bottom line: The NASA Cassini spacecraft’s mission-ending dive into the atmosphere of Saturn is September 15, 2017. Mission milestones 2+ weeks ahead.
Re: Cassini's Death March
« Reply #14 on: Sep 14th, 2017, 10:36am »
Cassini in final approach to Saturn
By EarthSky in Space September 13, 2017
Click the links to see the last raw images gathered by Cassini prior to Friday’s plunge into Saturn, and to follow the mission’s fiery end online.
NASA’s Cassini spacecraft is on final approach to Saturn, following confirmation by mission navigators that it is on course to dive into the planet’s atmosphere on Friday, September 15, 2017.
Cassini is ending its 13-year tour of the Saturn system with an intentional plunge into the planet to ensure Saturn’s moons – in particular Enceladus, with its subsurface ocean and signs of hydrothermal activity – remain pristine for future exploration. The spacecraft’s fateful dive is the final beat in the mission’s Grand Finale, 22 weekly dives, which began in late April, through the gap between Saturn and its rings. No spacecraft has ever ventured so close to the planet before. The mission’s final calculations predict loss of contact with the Cassini spacecraft will take place on September 15 at 7:55 a.m. EDT (11:55 UTC; translate to your time zone). Cassini will enter Saturn’s atmosphere approximately one minute earlier, at an altitude of about 1,190 miles (1,915 km) above the planet’s estimated cloud tops (the altitude where the air pressure is 1-bar, equivalent to sea level on Earth).
For the next couple of days, as Saturn looms ever larger, Cassini expects to take a last look around the Saturn system, snapping a few final images of the planet, features in its rings, and the moons Enceladus and Titan. The final set of views from Cassini’s imaging cameras is scheduled to be taken and transmitted to Earth on Thursday, September 14. If all goes as planned, images will be posted to the Cassini mission website beginning around 11 p.m. EDT (03:00 UTC on September 15). The unprocessed images will be available at:
Live mission commentary and video from JPL Mission Control will air on NASA Television from 7 to 8:30 a.m. EDT (11 to 12:30 UTC; translate to your time zone) on September 15. A post-mission news briefing from JPL is currently scheduled for 9:30 a.m. EDT (13:30 UTC), also on NASA TV.
During its dive into the atmosphere, the spacecraft’s speed will be approximately 70,000 miles (113,000 km) per hour. The final plunge will take place on the day side of Saturn, near local noon, with the spacecraft entering the atmosphere around 10 degrees north latitude.
When Cassini first begins to encounter Saturn’s atmosphere, the spacecraft’s attitude control thrusters will begin firing in short bursts to work against the thin gas and keep Cassini’s saucer-shaped high-gain antenna pointed at Earth to relay the mission’s precious final data. As the atmosphere thickens, the thrusters will be forced to ramp up their activity, going from 10 percent of their capacity to 100 percent in the span of about a minute. Once they are firing at full capacity, the thrusters can do no more to keep Cassini stably pointed, and the spacecraft will begin to tumble.
When the antenna points just a few fractions of a degree away from Earth, communications will be severed permanently. The predicted altitude for loss of signal is approximately 930 miles (1,500 kilometers) above Saturn’s cloud tops. From that point, the spacecraft will begin to burn up like a meteor. Within about 30 seconds following loss of signal, the spacecraft will begin to come apart; within a couple of minutes, all remnants of the spacecraft are expected to be completely consumed in the atmosphere of Saturn.
Due to the travel time for radio signals from Saturn, which changes as both Earth and the ringed planet travel around the Sun, events currently take place there 86 minutes before they are observed on Earth. This means that, although the spacecraft will begin to tumble and go out of communication at 6:31 a.m. EDT (3:31 a.m. PDT) at Saturn, the signal from that event will not be received at Earth until 86 minutes later.
Earl Maize, Cassini project manager at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, said:
"The spacecraft’s final signal will be like an echo. It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone. Even though we’ll know that, at Saturn, Cassini has already met its fate, its mission isn’t truly over for us on Earth as long as we’re still receiving its signal."
Cassini’s last transmissions will be received by antennas at NASA’s Deep Space Network complex in Canberra, Australia.
Cassini is set to make groundbreaking scientific observations of Saturn, using eight of its 12 science instruments. All of the mission’s magnetosphere and plasma science instruments, plus the spacecraft’s radio science system, and its infrared and ultraviolet spectrometers will collect data during the final plunge.
Chief among the observations being made as Cassini dives into Saturn are those of the Ion and Neutral Mass Spectrometer (INMS). The instrument will directly sample the composition and structure of the atmosphere, which cannot be done from orbit. The spacecraft will be oriented so that INMS is pointed in the direction of motion, to allow it the best possible access to oncoming atmospheric gases.
Bottom line: Click here for links to the last few raw images gathered by Cassini prior to its plunge into Saturn on September 15, 2017, and to learn how to follow the mission’s end online.