This image of the tiny moon Daphnis, only about 8km in diameter, is an enhanced-color mosaic. Daphnis is creating three waves in the outer edge of the Keeler gap. Image Credit: NASA/JPL-Caltech/Space Science Institute
The Cassini mission to Saturn ended a year and a half ago, but scientific results are still coming from all of the data it collected. When Cassini moved in closer to Saturn in its final months, it took a very detailed look at the gas giant’s rings, travelling between them and the planet itself. That detailed inspection raised quite a few questions about all the interactions shaping those rings.
A new paper published in Science presents some of the results from Cassini’s close-up look at the rings.
Saturn's rings and moons have been the subject of scientific debate. A 2019 study showed that the migration of Saturn's moons has widened the Cassini Division in Saturn's rings. Image Credit: Cassini, Dante, Baillié and Noyelles
Saturn’s moon Mimas is the smallest of the gas giant’s major moons. (Saturn has 62 moons, but some of them are tiny moonlets less than 1 km in diameter.) Two new studies show how Mimas acted as a kind of snow-plow, widening the Cassini division between Saturn’s rings.
Saturn's rings in all their glory. Image from the Cassini orbiter as Saturn eclipsed the Sun. Image Credit: By NASA / JPL-Caltech / Space Science Institute
Can you imagine the Solar System without Saturn’s rings? Can you envision Earth at the time the dinosaurs roamed the planet? According to a new paper, the two may have coincided.
Data from the Cassini mission shows that Saturn’s rings may be only 10 to 100 million years old. They may not have been there during the reign of the dinosaurs, and may in fact be a fairly modern development in our Solar System.
This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
It has been almost forty years since the Voyager 1 and 2 missions visited the Saturn system. As the probes flew by the gas giant, they were able to capture some stunning, high-resolution images of the planet’s atmosphere, its many moons, and its iconic ring system. In addition, the probes also revealed that Saturn was slowly losing its rings, at a rate that would see them gone in about 100 million years.
More recently, the Cassini orbiter visited the Saturn system and spent over 12 years studying the planet, its moons and its ring system. And according to new research based on Cassini’s data, it appears that Saturn is losing its rings at the maximum rate predicted by the Voyager missions. According to the study, Saturn’s rings are being gobbled up by the gas giant at a rate that means they could be gone in less 100 million years.
Europa and Io move across the face of Jupiter, with the Great Red Spot behind them. Image: NASA/JPL/Cassini, Kevin M. Gill
What would it be like to be onboard the Cassini orbiter as it made its way around Jupiter and Saturn and their moons? Pretty cool. Now a new video made from Cassini images pieces together parts of that stately journey.
This image of Saturn shows the planet and some of its moons in opposition. It's a composite image taken by the Hubble on June 6th, 2018. Image: NASA, ESA, A. Simon (GSFC) and the OPAL Team, and J. DePasquale (STScI); CC BY 4.0
We can’t seem to get enough of Saturn. It’s the most visually distinct object in our Solar System (other than the Sun, of course, but it’s kind of hard to gaze at). The Cassini mission to Saturn wrapped up about a year ago, and since then we’re relying on the venerable Hubble telescope to satisfy our appetite for images of the ringed planet.
A new study based on data from the Cassini mission is revealing something surprising in the atmosphere of Saturn. We’ve known about the storm at the gas giant’s north pole for decades, but now it appears that this massive hexagonal storm could be a towering behemoth hundreds of kilometers in height that has its base deep in Saturn’s atmosphere.
This grey-scale image of Saturn’s northern polar vortex was captured by the Cassini spacecraft. This image was captured from a distance of about 1.2 million km. A portion of Saturn’s rings are barely visible in the top right. Image: NASA/JPL-Caltech/Space Science Institute.
Saturn, Mars and Jupiter all beckon this summer. Image credit and copyright: Sharin Ahmad (@shahgazer)
Saturn, Mars and Jupiter all beckon this summer. Image credit and copyright: Sharin Ahmad (@shahgazer).
We’re in the midst of a parade of planets crossing the evening sky. Jupiter reached opposition on May 9th, and sits high to the east at dusk. Mars heads towards a fine opposition on July 27th, nearly as favorable as the historic opposition of 2003. And Venus rules the dusk sky in the west after the setting Sun for most of 2018.
June is Saturn’s turn, as the planet reaches opposition this year on June 27th, rising opposite to the setting Sun at dusk.
In classical times, right up until just over two short centuries ago, Saturn represented the very outer limit of the solar system, the border lands where the realm of the planets came to an end. Sir William Herschel extended this view, when he spied Uranus—the first planet discovered in the telescopic era—slowly moving through the constellation Gemini just across the border of Taurus the Bull using a 7-foot reflector (in the olden days, telescopes specs were often quoted referring to their focal length versus aperture) while observing from his backyard garden in Bath, England on the night of March 13th, 1781.
Looking east tonight at sunset… note Vesta to the upper left. Credit: Stellarium.
Orbiting the Sun once every 29.5 years, Saturn is the slowest moving of the naked eye planets, fitting for a planet named after Father Time. Saturn slowly loops from one astronomical constellation along the zodiac to the next eastward, moving through one about every two years.
The path of Saturn through 2018. Image credit: Starry Night Education software.
2018 sees Saturn in the constellation Sagittarius the Archer, just above the ‘lid’ of the Teapot asterism, favoring the southern hemisphere for this apparition. Saturn won’t cross the celestial equator northward again until 2026. Not that that should discourage northern hemisphere viewers from going after this most glorious of planets. A low southerly declination also means that Saturn is also up in the evening in the summertime up north, a conducive time for observing. Taking 29-30 years to complete one lap around the ecliptic as seen from our Earthly vantage point, Saturn also makes a great timekeeper with respect to personal life milestones… where were you back in 1989, when Saturn occupied the same spot along the ecliptic?
Saturn also shows the least variation of all the planets in terms of brightness and size, owing to its immense distance 9.5 AU from the Sun, and consequently 8.5 to 10.5 AU from the Earth. Saturn actually just passed its most distant aphelion since 1959 on April 17th, 2018 at 10.066 AU from the Sun.
Saturn’s in 2018 Dates with Destiny
Saturn sits just 1.6 degrees south of the waning gibbous Moon tonight. The Moon will lap it again one lunation later on June 28th. Note that the brightest of the asteroids, +5.7 magnitude 4 Vesta is nearby in northern Sagittarius, also reaching opposition on June 19th. Can you spy Vesta with the naked eye from a dark sky site? 4 Vesta passes just 4 degrees from Saturn on September 23rd, and both flirt with the galactic plane and some famous deep sky targets, including the Trifid and Lagoon Nebulae.
Saturn reaches quadrature 90 degrees east of the Sun on September 25th, then ends its evening apparition when it reaches solar conjunction on New Year’s Day, 2019.
Saturn is well clear of the Moon’s path for most of this year, but stick around: starting on December 9th, 2018, the slow-moving planet will make a great target for the Moon, which will begin occulting it for every lunation through the end of 2019.
It’s ironic: Saturn mostly hides its beauty to unaided eye. Presenting a slight saffron color in appearance, it never strays much from magnitude -0.2 to +1.4 in brightness. One naked eye observation to watch for is a sudden spurt in brightness known as the opposition surge or Seeliger Effect. This is a retro reflector type effect, caused by all those tiny iceball moonlets in the rings reaching 100% illumination at once. Think of how the Full Moon is actually 3 to 4 times brighter than the 50% illuminated Quarter Moon… all those little peaks, ridges and crater rims no longer casting shadows do indeed add up.
Saturn in all its glory (note the moons Enceladus and Tethys, too!). Image credit and copyright: Efrain Morales.
And this effect is more prominent in recent years for another reason: Saturn’s rings passed maximum tilt (26.7 degrees) with respect to our line of sight just last year, and are still relatively wide open in 2018. They’ll start slimming down again over the next few oppositions, reaching edge-on again in 2028.
Even using a pair of 7×50 hunting binoculars on Saturn, you can tell that something is amiss. You’re getting the same view that Galileo had through his spyglass, the pinnacle of early 17th century technology. He could tell that something about the planet was awry, and drew sketches showing an oblong world with coffee cup handles on the side. Crank up the magnification using even a small 60 mm refractor, and the rings easily jump into view. This is what makes Saturn a star party staple, an eye candy feast capable of drawing the aim of all the telescopes down the row.
If seeing and atmospheric conditions allow, crank up the magnification up to 150x or higher, and the dark groove of the Cassini division snaps into view. Can you see the shadow of the disk of Saturn, cast back onto the plane of the rings? The shadow of the planet hides behind it near opposition, then becomes most prominent towards quadrature, when we get to peek around its edge. Can you spy the limb of the planet itself, through the Cassini Gap?
Though the disk of Saturn is often featureless, tiny swirls of white storms do occasionally pop up. Astrophotographer Damian Peach noted just one such short-lived storm on the ringed planet this past April 2018.
Saturn’s retinue of moons are also interesting to follow in there own right. The first one you’ll note is +8.5 magnitude smog-shrouded Titan. Larger in diameter than Mercury, Titan would easily be a planet in its own right, were it liberated from its primary’s domain.
Though Saturn has 62 known moons, only six in addition to Titan are in range of a modest backyard telescope: Enceladus, Rhea, Dione, Mimas, Tethys and Iapetus. Two-faced Iapetus is especially interesting to follow, as it varies two full magnitudes in brightness during its 79 day orbit. Arthur C. Clarke originally placed the final monolith in 2001: A Space Odyssey on this moon, its artificial coating a beacon to astronomers. Today, we know from flybys carried out by NASA’s Cassini spacecraft that the leading hemisphere of Iapetus is coated with dark in-falling material, originating from the dark Phoebe ring around Saturn.
Owners of large light bucket telescopes may also want to try from two fainter +15th magnitude moons: Hyperion and Phoebe.
Fun fact: Saturn’s moons can also cast shadows back on the planet itself, much like the Galilean moons do on Jupiter… the catch, however, is that these events only occur around equinox season in the years around when Saturn’s rings are edge-on. This next occurs starting in 2026.
Cassini finished up its thrilling 20 year mission just last year, with a dramatic plunge into Saturn itself. It will be a while before we return again, perhaps in the next decade if NASA selects a nuclear-powered helicopter to explore Titan. Until then, be sure to explore Saturn this summer, from your Earthbound backyard.
The tell-tale white notch of a new storm system emerging on Saturn on April 1st. Image credit and copyright: Damian Peach.
The tell-tale white notch of a new storm system emerging on Saturn on April 1st. Image credit and copyright: Damian Peach.
Are you following the planets this season? The planetary action is about to heat up, as Jupiter, Saturn and Mars all head towards fine oppositions over the next few months.
Spying the Storms of Saturn
Astrophotographer Damian Peach raised the alarm on Twitter this past week of a possible bright storm emerging of the planet Saturn. The spot was noticeable even with the naked eye and in the raw video Peach captured, a sure sign that the storm was a biggie.
Though outbursts of clusters of white spots on the surface of Saturn aren’t uncommon, it’s rare to see one emerge at such a high latitude. The storm had faded considerably the next observing session Peach performed on April 5th, though observers should remain vigilant.
A storm subsiding? The followup view a few days later on April 5th. Image credit and copyright: Damian Peach.
It’s sad to think: Cassini and our eyes in the outer solar system are no more… and the situation will probably remain this way for some years to come. Juno also wraps up its mission at Jupiter (pending extension) this year, and New Horizons visits its final destination Ultima Thule (neé2014 MU69) on New Year’s Day 2019, though it’ll likely continue to chronicle its journey through the outer realms of the solar system, much like the Voyager 1, 2 and Pioneer 10, 11 missions, also bound to orbit the galaxy, mute testaments to human civilization. But even though proposals for Europa Clipper, a nuclear-powered quad-copter for Saturn’s moon Titan, and a Uranus and/or Neptune Orbiter are all on the drawing board, the “gap decade” of outer solar system exploration will indeed come to pass and soon.
Catching a storm on Saturn, Cassini style. Credit: NASA/JPL-Caltech/SSI
But dedicated amateur astronomers continue to monitor the outer solar system for changes. This month sees Saturn rising around 1:30 AM local and transiting highest to the south for northern hemisphere observers at 6:00 AM local, just before sunrise. Saturn crosses the constellation Sagittarius in 2018, bottoming out at its most southerly point this year for its 29 year path around the Sun. Saturn currently shines at +0.4 magnitude, extending 40” across (including rings) as it heads towards a fine opposition on June 27th. After opposition, Saturn formally crosses into the dusk sky. The amazing rings are an automatic draw, but last week’s storm admonishes us not to forget to check out the saffron-colored disk of Saturn itself as well. For example, I’ve always wondered: why didn’t we see the hexagon before? It’s right there festooning the northern hemisphere cap, plain as day in modern amateur images… to be sure, we’re in a modern renaissance of planetary astrophotography today, what with image stacking and processing, but surely eagle-eyed observers of yore could’ve easily picked this feature out.
And the view is changing as well, as Saturn’s rings reached a maximum tilt in respect to our line of sight of 27 degrees in 2017, and now head back towards edge-on again in 2025. And be sure to check out Saturn’s retinue of moons, half a dozen of which are easily visible in a telescope at even low power.
Finally, here’s another elemental mystery poised by Saturn related to the current storm, one that Cassini sought to solve in its final days: how fast does Saturn rotate, exactly? The usual rough guesstimate quoted is usually around 10.5 hours, but we’ve yet to pin down this fundamental value with any degree of precession.
One thing’s definitely for sure: we need to go back. In the meantime, we can enjoy the early morning views of the most glorious of the planets in our Solar System.
A couple watches the Moon rise from the icy shore of Lake Superior in Duluth earlier this month on March 1. Credit: Bob King
I apologize for the end-of-the-world title, but everything in it is true. And the world will still be here after it’s all done. On Friday (March 31) at 7:36 a.m. Central Time, the Moon will be full for the second time this month, which makes it a Blue Moon according to popular usage. Enjoy it. What with January’s Blue Moon and now this, we’ve chewed through all our Blue Moons till Halloween 2020.
I look forward to every full moon. Watching a moonrise, we get to see all manner of amazing atmospheric distortions play across the squat, orange disk. Once the sky’s dark, its outpouring of light makes walking at night a pleasure.
When a full moon occurs in spring, it hurries south down the ecliptic, the imaginary circle in the sky defining Earth’s orbit around the Sun. For northern hemisphere skywatchers, this southward sprint delays its rising by more an hour each night, forcing a quick departure from the evening sky. And that means blessed darkness for hunting down favorite galaxies and star clusters.
Tiangong 1 and a reentry simulation
As the Moon rolls along, the hapless Chinese space station Tiangong 1 hurtles toward Earth. Drag caused by friction with the upper atmosphere continues to shrink the spacecraft’s orbit, bringing it closer and closer to inevitable breakup and incineration. Since the Chinese National Space Administration (CNSA) lost touch with Tiangong 1 in March 2016, mission control can no longer power thrusters to de-orbit it at chosen time over a safe location like the ocean. The 9.3-ton (8,500 kg) station will burn up somewhere anywhere over a vast swath of the planet between latitudes 43°N and 43°S. Included within this zone are the southern half of Europe, the southern two-thirds of the U.S., India, Australia and much of Africa and South America.
Not until the day of or even hours before will have a clear idea of when and where the station will meet its fate. According to the latest update from the Aerospace Corp., which monitors falling spacecraft, reentry is expected on Easter Sunday (April 1) at 10:30 UT / 5:30 a.m. Central Time plus or minus 16 hours. This morning (March 29), the space station is circling Earth at about 118 miles (190 km) altitude. The lowest a satelllite can still make a complete orbit of the planet is about 62 miles (100 km). Below that, break-up begins.
A high definition TV camera on an aircraft took this photo of the cargo ship ATV-1 reentering the atmosphere in September 2008. Tiangong 1 is about the same size and will likely shatter and burn in similar fashion. Credit: ESA/NASA
For up-to-the-minute updates on when to expect Tiangong 1’s orbit to decay and the machine to plunge to Earth, check out Joseph Remis’ Twitter page. Most of the space station is expected to burn up on reentry, but larger chunks might survive all the way to the ground. Since much more of the Earth’s surface is water these remnants will likely end up in the drink … but you never know. If Tiangong-1 does come down over a populated area, observers on the ground will witness a spectacular, manmade fireball day or night.
Mars (right) and Saturn pair up in Sagittarius this morning, March 29 at dawn seen from Duluth, Minn. The two planets were 2.2° apart. Details: 35mm lens, f/2.8, 13 second exposure at ISO 800. Credit: Bob King
On the quieter side but nearly as eye-catching, Mars will overtake Saturn in the coming week, passing just 1° south of the ringed planet in a thrilling dawn conjunction on April 2. If the weather forecast doesn’t look promising that morning, the two planets will remain within 2° of each other now through April 6th, providing plenty of opportunities for a look.
You can easily tell them apart by color: Mars is distinctly red-orange and Saturn looks creamy white. Both are bright at around magnitude 0 though Mars is now a hair brighter by two-tenths of a magnitude. Will you be able to see the difference?
Mars passes close to Saturn on Monday, April 2. Look low in the southeastern sky shortly before and at dawn. Try getting a picture of the lovely couple by setting up your camera on a tripod and doing a series of time exposures from 5-30 seconds at f/3.5 and ISO 800. No fancy telephoto equipment is needed: a 35-55mm lens is perfect. Created with Stellarium
In most telescopes at low magnification both planets will comfortably fit in the same field of view. Saturn’s rings are tilted nearly wide open and quite beautiful. Mars appears gibbous and though still rather small, it’s brightening rapidly and drawing closer in time for its closest approach to Earth since 2003. Wishing you clear skies!
