Saturn Archives

July 6, 2017July 6, 2017 by Matt Williams

Titan’s Lakes are Nice and Calm. The Perfect Spot for a Landing

A new study has revealed that Titan's methane lakes could be calm enough for future missions to land there. Credit: bisbos.com

Ever since the Cassini orbiter and the Huygens lander provided us with the first detailed glimpse of Saturn’s moon Titan, scientists have been eager to mount new missions to this mysterious moon. Between its hydrocarbon lakes, its surface dunes, its incredibly dense atmosphere, and the possibility of it having an interior ocean, there is no shortage of things that are worthy of research.

The only question is, what form would this mission take (i.e. aerial drone, submarine, balloon, lander) and where should it set down? According to a new study led by the University of Texas at Austin, Titan’s methane lakes are very calm and do not appear to experience high waves. As such, these seas may be the ideal place for future missions to set down on the moon.

Their study, which was titled “Surface Roughness of Titan’s Hydrocarbon Seas“, appeared in the June 29th issue of the journal Earth and Planetary Science Letters. Led by Cyril Grima, a research associate at the University of Texas Institute for Geophysics (UTIG), the team behind the study sought to determine just how active the lakes are in Titan’s northern polar region are.

Titan’s three largest lakes and their surrounding areas as seen by the Cassini RADAR instrument. The researchers used the instrument to study waves on the lake surfaces. Credit: Cyril Grima/ The University of Texas at Austin

As Grima explained in a University of Texas press release, this research also shed light on the meteorological activity on Titan:

“There’s a lot of interest in one day sending probes to the lakes, and when that’s done, you want to have a safe landing, and you don’t want a lot of wind. Our study shows that because the waves aren’t very high, the winds are likely low.”

Towards this end, Grima and his colleagues examined radar data obtained by the Cassini mission during Titan’s early summer season. This consisted of measurements of Titan’s northern lakes, which included Ontario Lacus, Ligeia Mare, Punga Mare, and Kraken Mare. The largest of the three, Kraken Mars, is estimated to be larger than the Caspian Sea – i.e. 4,000,000 km² (1,544,409 mi²) vs 3,626,000 km²(1,400,000 mi²).

With the help of the Cassini RADAR Team and researchers from Cornell University, the Johns Hopkins University Applied Physics Laboratory (JHUAPL), NASA’s Jet Propulsion Laboratory (JPL) and elsewhere, the team applied a technique known as radar statistical reconnaissance. Developed by Grima, this technique relies on radar data to measure the roughness of surfaces in minute detail.

This technique has also been used to measure snow density and the surface roughness of ice in Antarctica and the Arctic. Similarly, NASA has used the technique for the sake of selecting a landing site on Mars for their Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (Insight) lander, which is scheduled to launch next year.

The left image shows a mosaic of images of Titan taken by the Cassini spacecraft in near infrared light. Titan’s polar seas are visible as sunlight glints off of them. The right image is a radar image of Kraken Mare. Credit: NASA Jet Propulsion Laboratory.

From this, Grima and his colleagues determined that waves on these lakes are quite small, reaching only 1 cm in height and 20 cm in length. These findings indicate that these lakes would be a serene enough environment that future probes could make soft landings on them and then begin the task of exploring the surface of the moon. As with all bodies, waves on Titan could be wind-driven, triggered by tidal flows, or the result of rain or debris.

As a result, these results are calling into question what scientists think about seasonal change on Titan. In the past, it was believed that summer on Titan was the beginning of moon’s windy season. But if this were the case, the results would have indicated higher waves (the result of higher winds). As Alex Hayes, an assistant professor of astronomy at Cornell University and a co-author on the study, explained:

“Cyril’s work is an independent measure of sea roughness and helps to constrain the size and nature of any wind waves. From the results, it looks like we are right near the threshold for wave generation, where patches of the sea are smooth and patches are rough.”

These results are also exciting for scientists who are hoping to plot future missions to Titan, especially by those who are hoping to see a robotic submarine sent to Titan’s to investigate its lakes for possible signs of life. Other mission concepts involve exploring Titan’s interior ocean, its surface, and its atmosphere for the sake of learning more about the moon’s environment, its organic-rich environment and probiotic chemistry.

And who knows? Maybe, just maybe, these missions will find that life in our Solar System is more exotic than we give it credit before, going beyond the carbon-based life that we are familiar with to include the methanogenic.

Further Reading: University of Texas JSG, Earth and Planetary Science Letters

June 7, 2017June 7, 2017 by David Dickinson

Summer Astronomy, Minimoon & Saturn Opposition 2017

Saturn from June 1st. Image credit and copyright: Peter on the Universe Today Flickr forum.

Saturn on June 1st, nearing opposition. Image credit and copyright: Peter on the *Universe Today* Flickr forum

Summertime astronomy leaves observers with the perennial question: when to observe? Here in Florida, for example, true astronomical darkness does not occur until 10 PM; folks further north face an even more dire situation. In Alaska, the game in late July became “on what date can you first spot a bright planet/star? around midnight.

And evening summer thunder showers don’t help. Our solution is to get up early (4 AM or so) when the roiling atmosphere has settled down a bit.

But there’s one reason to stay up late, as the planet Saturn reaches opposition next week on June 15^th and crosses into the evening sky.

Southern hemisphere observers have it best this year, as the ringed planet loiters in southern declinations for the next few years. In fact, Saturn won’t pop up over the celestial equator again until April, 2026. You’ll still be able to see Saturn from mid-northern latitudes, looking low to the south.

First, a brief rundown of the planets this summer. Mars is currently on the far side of the Sun and headed towards solar conjunction of July 26^th. Meanwhile, Mercury is headed towards greatest eastern (dusk) elongation on June 21^st. Early AM viewers, can follow Venus, which has just passed greatest elongation west of the Sun on June 3^rd, just last week. Finally, Jupiter joins Saturn in the dusk sky, high to the south at sunset and headed towards quadrature 90 degrees east of the Sun on July 6th.

Looking eastward on the evening of June 9th. Credit: Stellarium.

There’s another astronomical curiosity afoot this coming weekend: the MiniMoon for 2017. This is the Full Moon nearest to lunar apogee, a sort of antithesis of the over-hyped “SuperMoon.” Lunar apogee occurs on Thursday, June 8^th and the Full Moon occurs just 14 hours after.

2017 sees Saturn traveling from the dreaded “13^th constellation” of zodiac Ophiuchus the Serpent Bearer into Sagittarius. This also means that Saturn is headed towards bottoming out near 23 degrees southern declination next year in late 2018. Saturn truly lives up to its “father time” namesake, marking up its slow 29 year passage once around the zodiac. This struck home to us a few years back when Saturn passed Spica in the constellation Virgo, right back where I first started observing the planet as a teenager three decades before.

The path of Saturn through the last half of 2017. Credit: Starry Night Education Software.

The rings are also at their widest tilt in 2017, making for an extra photogenic view. 27 degrees wide as seen from our Earthly vantage point is as wide as Saturn’s ring system ever gets. Saturn isn’t really “tipping” back and forth as much as it’s orbiting the Sun and dipping one hemisphere towards us, and then another. In 2017, it’s the planet’s northern hemisphere time to shine.

Saturn: the changing view. Image credit and copyright: Andrew Symes (@failedprotostar)

Here’s the last/next cycle rundown:

-Rings wide open: (southern pole of Saturn tipped earthward): 2003

–Rings edge on: 2009

–Rings wide open: (northern pole of Saturn tipped earthward): 2017

-Rings edge on: 2025

-Rings wide open: (southern pole of Saturn tipped earthward): 2032

Even a small 60 mm refractor and a low power eyepiece will reveal the most glorious facet of Saturn: its glorious rings. Galileo first saw this confounding view in 1610, and sketched Saturn as a curious double-handled world. In 1655 Christaan Huygens first correctly deduced that Saturn’s rings are a flat plane, fully disconnected from the planet itself.

Crank up the magnification a bit, and the large Cassini Gap in the rings and the shadow play of the rings and the planet becomes apparent. This gives the view an amazing 3-D effect unparalleled in observational astronomy. The shadow cast by the bulk of the planet disappears behind it during opposition, then slowly starts to reemerge to one side after. Other things to watch for include the retro-reflector Seeliger Effect ( also known as opposition surge) as the planet brightens near opposition. And can you spy the bulk of the planet through the Cassini gap?

The moons of Saturn. Image credit and copyright: John Chumack

Hunting for Saturn’s moons is also a fun challenge. Saturn has more moons visible to a backyard telescope than any other planet. Titan is easiest, as the +8 magnitude moon orbits Saturn once every 16 days. In a small to medium-sized (8-inch) telescope, six moons are readily visible: Enceladus, Mimas, Rhea, Dione, Iapetus and Tethys. Large light bucket scopes 10” and larger might just also tease out the two faint +15th magnitude moons Hyperion and Phoebe.

There’s also something else special about Saturn in 2017 in the world of space flight: the venerable Cassini mission comes to an end this September. Hard to believe, this mission soon won’t be with us. Launched in 1997, Cassini arrived at Saturn in in July 2004, and has since provided us with an amazing decade plus of science. The internet and science writing online has grown up with Cassini, and it’ll be a sad moment to see it go.

All thoughts to ponder, as you check out Saturn at the eyepiece this summer.

June 5, 2017June 5, 2017 by Matt Williams

How Big is Saturn?

Beyond the Solar System’s Main Asteroid Belt lies the realm of the giants. It is here, staring with Jupiter and extending to Neptune, that the largest planets in the Solar System are located. Appropriately named “gas giants” because of their composition, these planets dwarf the rocky (terrestrial) planets of the inner Solar System many times over.

Just take a look at Saturn, the gas giant that takes its name from the Roman god of agriculture, and the second largest planet in the Solar System (behind Jupiter). In addition to its beautiful ring system and its large system of moons, this planet is renowned for its incredible size. Just how big is this planet? Well that depends on what your frame of reference is.

Diameter:

First let’s consider how large Saturn is from one end to the other – i.e. it’s diameter. The equatorial diameter of Saturn is 120,536 km ± 8 km (74,898 ± 5 mi) – or the equivalent of almost 9.5 Earths. However, as with all planets, their is a difference between the equatorial vs. the polar diameter. This difference is due to the flattening the planet experiences at the poles, which is caused by the planet’s rapid rotation.

*Like all the giant planets, Saturn is many times the size of Earth and the other rocky planets. Credit: NASA/JPL-Caltech/Space Science Institute.*

The poles are about 5,904 km closer to the center of Saturn than points on the equator. As a result, Saturn’s polar radius is about 108,728 ± 20 km (67,560 ± 12 mi) – or the equivalent of 8.5 Earths. That’s a pretty big difference, and you can actually see that Saturn looks a little squashed in pictures. Just for comparison, the equatorial diameter of Saturn is 9.4 times bigger than Earth, and it’s about 84% the diameter of Jupiter.

Volume and Surface Area:

In terms of volume and surface area, the numbers get even more impressive! For starters, the surface area of Saturn is 42.7 billion km² (16.5 billion sq miles), which works out to about 83.7 times the surface area of Earth. That’s smaller than Jupiter though, being only 68.7% of Jupiter’s surface area. Still, that is pretty astounding when put into perspective.

On the other hand, Saturn has a volume of 827.13 trillion km³ (198.44 trillion cubic miles), which effectively means you could fit Earth inside of it 763 times over and still have room enough for about twenty Moons! Again, Jupiter has it beat since Saturn has only 57.8% the volume of Jupiter. It’s big, but Jupiter is just that much bigger.

Mass and Density:

What about mass? Of course Saturn is much, much, MUCH more massive than Earth. In fact, it’s mass has been estimated to be a whopping 568,360,000 trillion trillion kg (1,253,000,000 trillion trillion lbs) – which works out to 95 times the mass of Earth. Granted, that only works out to about 30% the mass of Jupiter, but that’s still a staggering amount of matter.

*Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons*

Looking at the numbers, you may notice that this seems like a bit of a discrepancy. If you could actually fit 763 Earth-sized planets inside Saturn with room to spare, how is it that it is only 95 times Earth’s mass? The answer to that has to do with density. Since Saturn is a gas giant, its matter is distributed less densely than a rocky planet’s.

Whereas Earth has a density of 5.514 g/cm³ (or 0.1992 lb per cubic inch), Saturn’s density is only 0.687 g/cm³ (0.0248 lb/cu in). Like all gas giants, Saturn’s is made up largely of gases that exist under varying degrees of pressure. Whereas the density increases considerably the deeper one goes into Saturn’s interior, the overall density is less than that of water – 1 g/cm³ (0.0361273 lb/cu in).

Yes, Saturn is quite the behemoth. And yet, ongoing investigations into extra-solar planets are revealing that even it and its big brother Jupiter can be beaten in the size department. In fact, thanks to the Kepler mission and other exoplanet surveys, astronomers have found a plethora of “Super-Jupiters” in the cosmos, which refers to planets that are up to 80 times the mass of Jupiter.

I guess the takeaway from this is that there’s always a bigger planet out there. So watch your step and remember not to throw your weight (or mass or volume) around!

We have written many articles about Saturn for Universe Today. Here’s Ten Interesting Facts About Saturn, The Orbit of Saturn, how Long is a Year on Saturn?, What are Saturn’s Rings Made Of?, How Many Moons Does Saturn Have?, What’s the Weather Like on Saturn?, and What is the Atmosphere Like on Saturn?

If you want more information on Saturn, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We have also recorded an entire episode of Astronomy Cast all about Saturn. Listen here, Episode 59: Saturn.

Sources:

May 10, 2017May 10, 2017 by Matt Williams

Saturn’s Hexagon Will be the Star of the Cassini Finale

This illustration shows Cassini above Saturn's northern hemisphere prior to one of its 22 Grand Finale dives. Credit: NASA/JPL-Caltech

The Cassini spacecraft is nearing the end of its lifespan. This September, after spending the past twenty years in space – twelve and a half of which were dedicated to studying Saturn and its system of moons – the probe will be crash into Saturn’s atmosphere. But between now and then, the probe will be making its “Grand Finale” – the final phase of its mission where it will dive between the planet and its rings 22 times.

In addition to exploring this region of Saturn (something no other mission has done), the probe will also be using this opportunity to study Saturn’s hexagonal polar jet stream in greater detail. This persistent storm, which rages around Saturn’s northern polar region, has been a subject of interest for decades. And now that it enjoys full sunlight, Cassini will be able to directly image it with every pass it makes over Saturn’s north pole.

This persistent storm was first noticed in images sent back by the Voyager 1 and 2 missions, which flew by Saturn in 1980 and 1981, respectively. As storms go, it is extremely massive, with each side measuring about 13,800 km (8,600 mi) in length – longer than the diameter of the Earth. It also rotates with a period of 10 hours 39 minutes and 24 seconds, which is assumed to be equal to the rotation of Saturn’s interior.

*Image of Saturn’s hexagonal polar jet stream, taken by Cassini on Jan. 22nd, 2017. Credit: NASA/JPL-Caltech/Space Science Institute*

When the Cassini spacecraft arrived around Saturn in 2004 to conduct the first part of its mission, this region was in shadow. This was due to the fact that the northern hemisphere was still coming out of winter, and was hence tilted away from the Sun. However, since Saturn began its summer solstice in May of 2017, the northern polar region is now fully illuminated – at least by Saturn’s standards.

In truth, between its distance from the Sun (an average of 9.5549 AU) and its axial tilt (26.73°), the northern polar region only gets about 1% as much sunlight as Earth does. And from the perspective of the north pole, the Sun is very low in the sky. Nevertheless, the sunlight falling on the north pole at this point is enough to allow the Cassini mission to directly image the region by capturing its reflected light.

Images of the hexagonal jet stream (like the one above) will be taken by Cassini’s wide-angle camera, which uses special filters that admit wavelengths of near-infrared light. Already, Cassini has captured some impressive imagery during its first plunge between Saturn and its rings (which took place on April 26th, 2017). The rapid-fire images acquired by one of Cassini’s cameras were then stitched together to create a movie (posted below).

As you can see, the movie begins with a view of the vortex at the center of the hexagon, then heads past the outer boundary of the jet stream and continues further southward. Toward the end of the movie, the spacecraft reorients itself to direct its saucer-shaped antenna in the direction of the spacecraft’s motion, which is apparent from the way the camera frame rotates.

The images that make up this movie were captured as the Cassini spacecraft dropped in altitude from 72,400 to 6,700 km (45,000 to 4,200 miles) above Saturn’s cloud tops. As this happened, the features which the camera could resolve changed drastically – going from 8.7 km (5.4 mi) per pixel to 810 meters (0.5 mi) per pixel.

The movie was produced by Kunio Sayanagi and John Blalock – an associate of the Cassini imaging team and a graduate research assistant (respectively) at Hampton University in Virginia – who collaborated with the Cassini imaging team. And thanks to this video, new insights are already being made into the hexagonal jet stream and the mechanisms that power it.

For example, as Sayanagi indicated in a NASA press release, the video captured the boundary regions of the jet stream rather nicely, which allowed him to note an interesting fact about them. “I was surprised to see so many sharp edges along the hexagon’s outer boundary,” he said. “Something must be keeping different latitudes from mixing to maintain those edges.”

Andrew Ingersoll, a member of the Cassini imaging team based at Caltech, expressed how similar movies will result from future plunges taken as part of the Grand Finale. “The images from the first pass were great, but we were conservative with the camera settings,” he said. “We plan to make updates to our observations for a similar opportunity on June 29th that we think will result in even better views.”

Between now and the end of the mission, who knows what we might learn about this mysterious storm? The next plunge – aka. Grand Finale Dive No. 4 – will take place on Sunday, May 15th at 4:42 p.m. UTC (12:42 p.m EDT; 9:42 a.m. PDT). A total of 22 dives will be made on a weekly basis before the probe takes the final plunge – the one that will cause it to breakup in Saturn’s atmosphere – on Friday, September 15th, 2017.

For more information, consult Cassini’s Grand Finale Orbit Guide. And be sure to enjoy this video of the final phase of the probe’s mission, courtesy of NASA:

Further Reading: NASA, NASA/JPL

May 4, 2017May 5, 2017 by Matt Williams

Titan Ripe For Drone Invasion

A proposed eight-bladed drone (aka. "dragonfly") could be ideally suited for exploring Saturn's moon Titan in the coming decades. Credit: APL/Michael Carroll

With its dense and hydrocarbon-rich atmosphere, Titan has been a subject of interest for many decades. And with the success of the Cassini-Huygens mission, which began exploring Saturn and its system of moons back in 2004, there are many proposals on the table for follow-up missions that would explore the surface of Titan and its methane seas in greater depth.

The challenges that this presents have led to some rather novel ideas, ranging from balloons and landers to floating drones and submarines. But it is the proposal for a “Dragonfly” drone by researchers at NASA’s JHUAPL that seems particularly adventurous. This eight-bladed drone would be capable of vertical-takeoff and landing (VTOL), enabling it to explore both the atmosphere and the surface of Titan in the coming decades.

The mission concept was proposed by a science team led by Elizabeth Turtle, a planetary scientist from NASA’s Johns Hopkins University Applied Physics Laboratory (JHUAPL). Back in February, the concept was presented at the “Planetary Science Vision 2050 Workshop” – which took place at NASA’s headquarters in Washington, DC – and again in late March at the 48th Lunar and Planetary Science Conference in The Woodlands, Texas.

ASA’s Cassini spacecraft looks toward the night side of Saturn’s largest moon and sees sunlight scattering through the periphery of Titan’s atmosphere and forming a ring of color.
Credit: NASA/JPL-Caltech/Space Science Institute

Such a mission, as Turtle explained to Universe Today via email, is both timely and necessary. Not only would it build on many recent developments in robotic explorers (such as the Curiosity rover and the Cassini orbiter); but on Titan, there is simply no shortage of opportunities for scientific research. As she put it:

“Titan’s an ocean world with a unique twist, which is the rich and complex organic chemistry occurring in its atmosphere and on its surface. This combination makes Titan a particularly good target for studying planetary habitability. One of the big questions about the development of life is how chemical interactions led to biological processes. Titan’s been doing experiments in prebiotic chemistry for millions of years – timescales that are impossible to reproduce in the lab – and the results of these experiments are there to be collected.”

Their proposal is based in part on previous Decadal Surveys, such as the Campaign Strategy Working Group (CSWG) on Prebiotic Chemistry in the Outer Solar System. This survey emphasized that a mobile aerial vehicle (i.e an airship or a balloon) would well-suited to exploring Titan. Not only is Titan the only known body other than Earth that has a dense, nitrogen-rich atmosphere – four times as dense as Earth’s – but it’s gravity is also about 1/7th that of Earth’s.

However, balloons and airships would be unable to study Titan’s methane lakes, which are one of the most exciting draws as far as research into prebiotic chemistry goes. What’s more, an aerial vehicle would not be able to conduct in-situ chemical analysis of the surface, much like what the Mars Exploration Rovers (Spirit, Opportunity and Curiosity) have been doing on Mars.

*Artist’s concept of a Titan Aerial Daughter quadcopter and its “Mothership” balloon. Credit: NASA/STMD*

As such, Turtle and her colleagues began looking for a proposal that represented the best of both worlds – i.e. an aerial platform and a lander. This was the genesis of the Dragonfly concept.

“Several different methods have been considered for in-situ aerial exploration of Titan (helicopters, different types of balloons, airplanes),” said Turtle. “Dragonfly takes advantage of the recent developments in multi-rotor aircraft to provide aerial mobility for a lander with a sophisticated payload. Because Dragonfly would be able to travel long distances – a few tens of kilometers at a time, and up to a few hundred kilometers over the course of the mission – it would be possible to make measurements at multiple sites with very different geologic histories.”

The mission is also in keeping with concepts that Turtle and her colleagues – which includes Ralph Lorenz (also from JHUAPL), Melissa Trainer of the Goddard Space Flight Center, and Jason Barnes of University of Idaho – have been exploring for years. In the past, they proposed a mission concept that would combine a Montgolfière-style balloon with a Pathfinder-like lander. Whereas the balloon would explore Titan from a low altitude, the lander would explore the surface up close.

By the 48th Lunar and Planetary Science Conference, they had officially unveiled their “Dragonfly” concept, which called for a qaudcopter to conduct both aerial and surface studies. This four-rotor vehicle, it was argued, would be able to take advantage of Titan’s thick atmosphere and low gravity to obtain samples and determine surface compositions in multiple geological settings.

*Artist’s concept of the dragonfly being deployed to Titan and commencing its exploration mission. Credit: APL/Michael Carroll*

In its latest iteration, the Dragonfly incorporates eight rotors (two positioned at each of its four corners) to achieve and maintain flight. Much like the Curiosity and upcoming Mars 2020 rovers, the Dragonfly would be powered by a Multimission Radioisotope Thermoelectric Generator (MMRTG). This system uses the heat generated by decaying plutonium-238 to generate electricity, and can keep a robotic mission going for years.

This design, says Turtle, would offer scientists the ideal in-situ platform for studying Titan’s environment:

“Dragonfly would be able to measure compositional details of different surface materials, which would show how far organic chemistry has progressed in different environments. These measurements could also reveal chemical signatures of water-based life (like that on Earth) or even hydrocarbon-based life, if either were present on Titan. Dragonfly would also study Titan’s atmosphere, surface, and sub-surface to understand current geologic activity, how materials are transported, and the possibility of exchange of organic material between the surface and the interior water ocean.”

This concept incorporates a lot of recent advances in technology, which include modern control electronics and advances in commercial unmanned aerial vehicle (UAV) designs. On top of that, the Dragonfly would do away with chemically-powered retrorockets and could power-up between flights, giving it a potentially much longer lifespan.

The view from the beach on Titan? Image: NASA — *Artist’s impression of the view from the surface of Titan, looking over one of its methane seas. Credit: NASA*

“And now is the perfect time,” says Turtle, “because we can build on what we’ve learned from the Cassini-Huygens mission to take the next steps in Titan exploration.”

Currently, NASA’s Jet Propulsion Laboratory is developing a similar concept. Known as the Mars Helicopter “Scout”, for use on Mars, this aerial drone is expected to be launched aboard the Mars 2020 mission. In this case, the design calls for two coaxial counter-rotating rotors, which would provide the best thrust-to-weight ratio in Mars’ thin atmosphere.

This sort of VTOL platform could become the mainstay in the coming decades, wherever long-term missions that involve bodies that have atmospheres are called for. Between Mars and Titan, such aerial drones could hop from one area to the next, obtaining samples for in-situ analysis and combining surface studies with atmospheric readings at various altitudes to get a more complete picture of the planet.

Further Reading: USRA, LPI, Space

April 27, 2017May 3, 2017 by Nancy Atkinson

A Survivor’s Tale: Cassini Lives Through First Ring Dive

Artist's concept of Cassini diving between Saturn and its innermost ring. Credit: NASA/JPL-Caltech

One down, twenty-one to go! The Cassini spacecraft survived the first dive through the narrow gap between Saturn and its rings, and is now back communicating with Earth.

“No spacecraft has ever been this close to Saturn before. We could only rely on predictions, based on our experience with Saturn’s other rings, of what we thought this gap between the rings and Saturn would be like,” said Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California. “I am delighted to report that Cassini shot through the gap just as we planned and has come out the other side in excellent shape.”

It was a long day for Cassini scientists and engineers at the Jet Propulsion Laboratory while the spacecraft was out of contact for 20 hours during this first dive, signaling the beginning of the end for the mission.

Cassini, running out of fuel, is heading toward its ultimate death by crashing into Saturn on September 15, 2017. But during the next few months, Cassini will make twenty-one more passes through the gap, and in doing so, further our understanding of how giant planets, and planetary systems everywhere, form and evolve.

A raw image of Saturn’s polar vortex, taken on April 26, 2017 by the Cassini spacecraft during the first close pass between Saturn and its rings. Credit: NASA/JPL-Caltech.

Project Scientist Linda Spilker said Cassini will be able to make close up measurements of Saturn and its rings to finally help us understand the mass and internal structure of Saturn. And the images should be absolutely stunning.

Contact was lost as the ring-plane crossing started at 2 a.m. PDT (5 a.m. EDT) on April 26. NASA’s Deep Space Network Goldstone Complex in California’s Mojave Desert acquired Cassini’s signal at 11:56 p.m. PDT on April 26, 2017 (2:56 a.m. EDT on April 27) and data began flowing at 12:01 a.m. PDT (3:01 a.m. EDT) on April 27.

Cassini was programmed to collect science data while close to the planet. As a protective measure, the spacecraft used its large, dish-shaped high-gain antenna (13 feet or 4 meters across) as a deflector shield, orienting it in the direction of oncoming ring particles. This orientation put the spacecraft out of contact with Earth.

“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,” said Jim Green, director of the Planetary Science Division at NASA Headquarters.

We did it! Cassini is in contact with Earth and sending back data after a successful dive through the gap between Saturn and its rings. pic.twitter.com/cej1yO7T6a

— CassiniSaturn (@CassiniSaturn) April 27, 2017

The gap between the rings and the top of Saturn’s atmosphere is about 1,500 miles (2,000 kilometers) wide, and Cassini came within about 1,000 miles (1,600 kilometers) of Saturn’s cloud tops.

The best models for the region suggested that if there were ring particles in the area where Cassini crossed the ring plane, they would be tiny, on the scale of smoke particles. However, the spacecraft was traveling at speeds of about 77,000 mph (124,000 kph) relative to the planet, so small particles hitting a sensitive area could potentially have disabled the spacecraft.

The spacecraft is being destroyed after a successful 13 year mission at Saturn, as NASA needs to follow the protocol of planetary protection, and not allow a spacecraft with possible microbes from Earth to crash into a potentially habitable moon such as Enceladus or Titan.

Cassini’s next dive through the gap is scheduled for May 2.

You can see the Cassini raw images page here, but look for more processed images to be available soon.

Jason Major put together this animation of some of the first images from Cassini’s close pass:

Animation of images from @CassiniSaturn's close pass over #Saturn on April 26 pic.twitter.com/yklpSXJwrS

— Jason Major (@JPMajor) April 27, 2017

Source: JPL

April 25, 2017 by Matt Williams

Adieu Titan: So Long & Thanks For All The Hydrocarbons

Artist's conception of Cassini winging by Saturn's moon Titan (right) with the planet in the background. Credit: NASA/JPL-Caltech

The Cassini spacecraft has done some amazing things since it arrived in the Saturn system in 2004. In addition to providing valuable information on the gas giant and its system of rings, it has also provided us with extensive data and photographs of Saturn’s many moons. Nowhere has this been more apparent than with Saturn’s largest moon, the hydrocarbon-rich satellite known as Titan.

And with just a few hours left before Cassini makes its final plunge between Saturn and its innermost ring (something that no other spacecraft has ever done), we should all take this opportunity to say goodbye to Titan. In the past few years, it has dazzled us with its methane lakes, dense atmosphere, and potential for hosting life. And it shall be sorely missed!

Cassini’s last encounter with Titan – where it passed within 979 km (608 mi) of the moon’s surface – took place on April 21st, at 11:08 p.m. PDT (April 22nd, 2:08 a.m. EDT). The probe also used this opportunity to take some radar images of the moon’s northern polar region. While this area has been photographed before, this was the first time that radar images were acquired.

*Unprocessed image of Saturn’s moon Titan, captured by NASA’s Cassini spacecraft during its final close flyby on April 21st, 2017. Credit: NASA/JPL-Caltech/Space Science Institute*

Over the course of the next week, Cassini’s radar team hopes to pour over theses images, which provide a detailed look at the methane seas and lakes in the northern polar region. It is hoped that this data will allow scientists to shed more light on the depths and compositions of some of the small lakes in the area, as well as provide more information on the evolving surface feature known as “magic island“.

With this last pass complete (its 127th in total), Cassini is now beginning the final phase of its mission – known as the Grand Finale. This will consist of the spacecraft making a final set of 22 orbits around the ringed planet between April 26th and September 15th. The maneuver will allow Cassini to go where no other probe has gone before and get the closest look ever at Saturn’s outer rings.

The final pass over Titan was part of this maneuver, using the moon’s gravity to bend and reshape the probe’s orbit so that it would be able to pass through Saturn’s ring system – instead of passing just beyond the main rings. As Earl Maize, Cassini project manager at JPL, said in a NASA press release:

“With this flyby we’re committed to the Grand Finale. The spacecraft is now on a ballistic path, so that even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn’s atmosphere on Sept. 15 no matter what.”

*Some key numbers for Cassini’s Grand Finale and final plunge into Saturn. Credit: NASA/JPL-Caltech*

Cassini’s final pass with Titan allowed it to acquire a boost in velocity, increasing its speed by 860.5 meters per second (3098 km/h; 1,925 mph). It then reached its farthest point in its orbit around Saturn (apoapse) on April 22nd, :46 p.m. PDT (11:46 p.m. EDT). This effectively began the Grand Finale orbits, with the first dive coming on April 26th, at 02:00 a.m. PDT (05:00 a.m. EDT).

This orbit will provide Cassini with its best look to date at Saturn’s north pole, which it will be studying with both its Visible and Infrared Mapping Spectrometer (VIMS) and Composite Infrared Spectrometer (CIRS). These studies will lead to the creation of the sharpest movies to date in the near-infrared band, which will also allow the science team to study the motions of the hexagon pattern around Saturn’s north pole in more detail.

Between now and September, when the mission will end, the probe will provide information that is expected to improve our understanding of how giant planets form and evolve. Things will finally wrap on September 15th, 2017, when the probe will plunge into Saturn’s atmosphere. But even then, the probe will be sending back information until its very last seconds of operation.

Safe journeys Cassini! And so long Titan! We hope to be exploring you again someday soon, preferably with something that can float or fly around inside your dense atmosphere, or perhaps investigate your methane seas in serious depth!

In the meantime, be sure to check out this narrated, 360-degree animated video from NASA. As you can see, it simulates what a ride on the Cassini spacecraft might look like as it makes its Grand Finale:

Further Reading: NASA, Cassini – The Grand Finale

April 21, 2017April 21, 2017 by Bob King

Earth Beams From Between Saturn’s Rings in New Cassini Image

NASA’s Cassini spacecraft captured the view on April 13, 2017 at 12:41 a.m. CDT. The probe was 870 million miles (1.4 billion km) away from Earth when the image was taken. The part of Earth facing toward Cassini at the time was the southern Atlantic Ocean. Look closely to the left of Earth; that pinprick of light is the Moon. Credit: NASA/JPL Caltech

Look at us. Packed into a gleaming dot. The entire planet nothing more than a point of light between the icy rings of Saturn. The rings visible here are the A ring (top), followed by the Keeler and Encke gaps, and finally the F ring at bottom. During this observation, Cassini was looking toward the backlit rings with the sun blocked by the disk of Saturn.

Cassini first photographed Earth from Saturn in July 2013. Credit: NASA/JPL-Caltech

Seen from Saturn, Earth and the other inner solar system planets always appear close to the sun much like Venus and Mercury do from Earth. All orbit interior to Saturn; even at maximum elongation, they never get far from the Sun. Early this month, as viewed from Saturn, Earth was near maximum elongation east of the sun, thus an “evening star,” making it an ideal time to take a picture.

In this cropped view of the April 13 image, you can better see the Moon, located a short distance to the left of the Earth. Credit: NASA/JPL-Caltech

Opportunities to capture Earth from Saturn have been rare in the 13 years Cassini has spent orbiting the ringed planet. The only other photo I’m aware of was snapped on July 19, 2013. Each is a precious document with a clear message: we are all tiny, please let’s be kind to one another.

This graphic shows Cassini’s flight path during the final two phases of its mission. The 20 Ring-Grazing Orbits are in gray (completed) and the 22 Grand Finale Orbits are in blue. The final partial orbit is colored orange. The first of the Grand Finale orbits begins on April 22 at 10:46 p.m. CDT. Credit: NASA/JPL-Caltech/Space Science Institute

We’ll soon miss the steady stream of artistic images of Saturn, its rings and moons by the Cassini team. The probe will make its final close flyby of the planet’s largest moon, fog-enshrouded Titan, at 1:08 a.m. April 22, at a distance of just 608 miles (979 km). That night at 10:46 p.m. CDT, Cassini will enter the first of its Grand Finale orbits, a series of 22 weekly dives between the planet and the rings. The first ring plane crossing is slated for midnight CDT April 25-26.

Cassini at Saturn and the Grand Finale

The coming week will be a busy one for Cassini. On each orbit, the probe will draw closer and closer to the butterscotch ball of Saturn until it finally tears across the cloud tops and burns up as a spectacular fireball on September 15. Scientists would rather see the craft burn up in Saturn’s atmosphere instead crash into a moon and possibly contaminate it.

Cassini will become a brilliant fireball streaking over Saturn’s cloud tops on the last day of its operation on September 15. Credit: NASA/JPL-Caltech

After nearly 20 years in space, seven of them spent traveling to the ringed planet, Cassini feels like family. It won’t be easy to say goodbye, but thanks to the probe, Saturn’s family album is bursting with remarkable images that will forever remind us the tenacity of this amazing machine and the vision and work of those who kept it operating for so many years.

April 17, 2017May 6, 2020 by Matt Williams

The Orbit of Saturn. How Long is a Year on Saturn?

Every planet in the Solar System takes a certain amount of time to complete a single orbit around the Sun. Here on Earth, this period lasts 365.25 days – a period that we refer to as a year. When it comes to the other planets, we use this measurement to characterize their orbital periods. And what we have found is that on many of these planets, depending on their distance from the Sun, a year can last a very long time!

Consider Saturn, which orbits the Sun at a distance of about 9.5 AU – i.e. nine and a half times the distance between the Earth and the Sun. Because of this, the speed with which it orbits the Sun is also considerably slower. As a result, a single year on Saturn lasts an average of about twenty-nine and a half years. And during that time, some interesting changes happen for the planet’s weather systems.

Orbital Period:

Saturn orbits the Sun at an average distance (semi-major axis) of 1.429 billion km (887.9 million mi; 9.5549 AU). Because its orbit is elliptical – with an eccentricity of 0.05555 – its distance from the Sun ranges from 1.35 billion km (838.8 million mi; 9.024 AU) at its closest (perihelion) to 1.509 billion km (937.6 million mi; 10.086 AU) at its farthest (aphelion).

*A diagram showing the orbits of the outer Solar planets. Saturn’s orbit is represented in yellow Credit: NASA*

With an average orbital speed of 9.69 km/s, it takes Saturn 29.457 Earth years (or 10,759 Earth days) to complete a single revolution around the Sun. In other words, a year on Saturn lasts about as long as 29.5 years here on Earth. However, Saturn also takes just over 10 and a half hours (10 hours 33 minutes) to rotate once on its axis. This means that a single year on Saturn lasts about 24,491 Saturnian solar days.

It is because of this that what we can see of Saturn’s rings from Earth changes over time. For part of its orbit, Saturn’s rings are seen at their widest point. But as it continues on its orbit around the Sun, the angle of Saturn’s rings decreases until they disappear entirely from our point of view. This is because we are seeing them edge-on. After a few more years, our angle improves and we can see the beautiful ring system again.

Orbital Inclination and Axial Tilt:

Another interesting thing about Saturn is the fact that its axis is tilted off the plane of the ecliptic. Essentially, its orbit is inclined 2.48° relative to the orbital plane of the Earth. Its axis is also tilted by 26.73° relative to the ecliptic of the Sun, which is similar to Earth’s 23.5° tilt. The result of this is that, like Earth, Saturn goes through seasonal changes during the course of its orbital period.

*R. G. French (Wellesley College) et al., NASA, ESA, and The Hubble Heritage Team (STScI/AURA)*

Seasonal Changes:

For half of its orbit, Saturn’s northern hemisphere receives more of the Sun’s radiation than the southern hemisphere. For other half of its orbit, the situation is reversed, with the southern hemisphere receiving more sunlight than the northern hemisphere. This creates storm systems that dramatically change depending on which part of its orbit Saturn is in.

For staters, winds in the upper atmosphere can reach speeds of up to 5oo meters per second (1,600 feet per second) around the equatorial region. On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval).

This unique but short-lived phenomenon occurs once every Saturnian year, around the time of the northern hemisphere’s summer solstice. These spots can be several thousands of kilometers wide, and have been observed on many occasions throughout the past – in 1876, 1903, 1933, 1960, and 1990.

Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed, which was spotted by the Cassini space probe. Given the periodic nature of these storms, another one is expected to happen in 2020, coinciding with Saturn’s next summer in the northern hemisphere.

The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI

Similarly, seasonal changes affect the very large weather patterns that exist around Saturn’s northern and southern polar regions. At the north pole, Saturn experiences a hexagonal wave pattern which measures some 30,000 km (20,000 mi) in diameter, while each of it six sides measure about 13,800 km (8,600 mi). This persistent storm can reach speeds of about 322 km per hour (200 mph).

Thanks to images taken by the Cassini probe between 2012 and 2016, the storm appears to undergo changes in color (from a bluish haze to a golden-brown hue) that coincide with the approach of the summer solstice. This was attributed to an increase in the production of photochemical hazes in the atmosphere, which is due to increased exposure to sunlight.

Similarly, in the southern hemisphere, images acquired by the Hubble Space Telescope have indicated the existence of large jet stream. This storm resembles a hurricane from orbit, has a clearly defined eyewall, and can reach speeds of up to 550 km/h (~342 mph). And much like the northern hexagonal storm, the southern jet stream undergoes changes as a result of increased exposure to sunlight.

*Saturn makes a beautifully striped ornament in this natural-color image, showing its north polar hexagon and central vortex (Credit: NASA/JPL-Caltech/Space Science Institute)*

Cassini was able to captured images of the south polar region in 2007, which coincided with late fall in the southern hemisphere. At the time, the polar region was becoming increasingly “smoggy”, while the northern polar region was becoming increasingly clear. The reason for this, it was argued, was that decreases in sunlight led to the formation of methane aerosols and the creation of cloud cover.

From this, it has been surmised that the polar regions become increasingly obscured by methane clouds as their respective hemisphere approaches their winter solstice, and clearer as they approach their summer solstice. And the mid-latitudes certainly show their share of changes thanks to increases/decreases in exposure to solar radiation.

Much like the length of a single year, what we know about Saturn has a lot to do with its considerable distance from the Sun. In short, few missions have been able to study it in depth, and the length of a single year means it is difficult for a probe to witness all the seasonal changes the planet goes through. Still, what we have learned has been considerable, and also quite impressive!

If you’d like more information on Saturn, check out Hubblesite’s News Releases about Saturn. And here’s a link to the homepage of NASA’s Cassini spacecraft, which is orbiting Saturn.

We have also recorded an entire episode of Astronomy Cast that’s just about Saturn. Listen here, Episode 59: Saturn.

Sources:

April 4, 2017April 5, 2017 by Nancy Atkinson

Cassini’s Final Mission to Annihilation Starts April 22

Grab the tissues. This video nearly had the Cassini team all choked up during today’s press briefing, and virtual sobs and sniffs were abundant on social media posts sharing the video.

“We get goosebumps and get emotional every time we see it,” said Earl Maize, Cassini project manager at JPL.

On April 22 the Cassini spacecraft will begin its ‘Grand Finale’ — the beginning of the end of this tremendous mission that has provided breathtaking images and so many new discoveries of Saturn, its rings and moons. The mission will end on September 15, 2017, when it makes a dramatic plunge into the gas giant.

Here’s the video that had everyone teary-eyed. Be prepared for some stunning visuals:

Today, Maize talked about how nineteen countries and three space agencies contributed to the success of the Cassini/Huygens mission, saying the mission has been truly an international triumph and a phenomenal achievement.

“Cassini’s legacy is assured. We are in the books!” Maize said. “But the best is yet to come. We are going to dive into the gap between the rings of Saturn and Saturn’s atmosphere, a place where no spacecraft has ever gone. We’ll be going 70,000 mph (112,634 km/hr) into a 1,500-mile-wide (2,400-kilometer) gap, operating the spacecraft from a billion miles away.”

Cassini has been a relatively trouble free mission, and has made many discoveries about the Saturn system. So why crash the spacecraft?

Cassini is running out of fuel, basically running on fumes at this point.* And NASA needs to follow the protocol of planetary protection, and not allow a spacecraft with possible microbes from Earth to crash into a potentially habitable moon such as Enceladus or Titan.

“Cassini’s own discoveries were its demise,” Maize said. “Enceladus has a warm, salt water ocean. We can’t risk an inadvertent contact with this pristine body. The only choice was to destroy it (Cassini) in a designed fashion.”

Maize said that back in 2010, the team decided they would make the mission last as long as possible and use every last kilogram of propellant to explore the Saturn system as thoroughly as they could.

Cassini vs. Saturn. As depicted in this illustration, Cassini will plunge into Saturn’s atmosphere on Sept. 15, 2017. Using its attitude control thrusters, the spacecraft will work to keep its antenna pointed at Earth while it sends its final data, including the composition of Saturn’s upper atmosphere. Credit: NASA/JPL-Caltech

The final flyby of Titan on April 22 will ultimately alter Cassini’s trajectory and push it toward the spacecraft’s final demise. Maize described the gravity slingshot from Titan as a “last kiss goodbye that will push Cassini into Saturn. This is a roller coaster ride that we’re not coming out of.”

You can plot Cassini’s trajectory in JPL’s “Eyes on Cassini” special section of their Eyes on the Solar System website.

Cassini will make 22 passes through the gap, and in doing so, further our understanding of how giant planets, and planetary systems everywhere, form and evolve.

There’s the risk of dust or debris hitting the spacecraft, potentially crippling Cassini. But the risk is worth it, because if the spacecraft survives through even just a few of the close passes, the scientific payback will be incredible. However, even if the spacecraft is crippled and can’t send back its final science observations, the end is inevitable, as the path toward destruction will be written by the final ‘kiss’ from Titan.

“This is something we couldn’t try at any other time,” Maize said. “But now is time.”

A computer-generated representation of all Cassini’s Saturn orbits -affectionately called the “ball of yarn” by mission planners. The time frame spans Saturn Orbit Insertion on July 1, 2004 to the end of mission on Sept. 15, 2017. Credit: NASA/JPL-Caltech.

The Cassini team said the end of the mission will likely be a combination of excitement, pride and a sense of loss.

“I think that once the signal is lost, it would mean the heartbeat of Cassini is gone,” said Spilker. “I think there will be tremendous cheers and applause for the completion of an absolutely incredible mission. Hugs, tears — the Kleenex box will be passed around — but we will rejoice at being part of such a wonderful mission.”

See more images and information about the Grand Finale here.

For more of an inside look at Cassini, I devote a chapter of my book to the mission, with more insight from Earl Maize, Linda Spilker and others about the history and discoveries of the Cassini/Huygens mission, and additional details about the Grand Finale. “Incredible Stories From Space: A Behind-the-Scenes Look at the Missions Channging Our View of the Cosmos.”

Artist’s concept of Cassini orbiter crossing Saturn’s ring plane.
Credit: NASA/Jet Propulsion Laboratory.

*One of the Cassini team members said that as of today (April 4, 2017) Cassini has 36kg of hydrazine left for the thrusters, which are used everyday to orient the spacecraft, point the antenna towards Earth, point the instruments to their desired targer, etc. For the Titan flyby on April 22, about 10-15 kg. As for the bipropellant that runs the main engines, that’s a little more unknown and the one the team is worried most about running out of fuel. The team member said there is about 10 kg of that fuel left, “plus or minus 20 kilos [meaning there is true uncertainty about how much of this fuel remains]. We could run out today, or we could have 30 kilos left.”