Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius about nine times that of Earth. It has only one-eighth the average density of Earth, but with its larger volume Saturn is over 95 times more massive. Saturn is named after the Roman god of agriculture; its astronomical symbol (♄) represents the god’s sickle.
|Pronunciation||/ˈsætərn/ ( listen)|
|Aphelion||1,514.50 million km (10.1238 AU)|
|The Cassini mission to Saturn has returned its final close-up images of the gas giant’s Dione moon.
The probe passed within 500km of the pockmarked surface on Monday – its fifth such encounter in the spacecraft’s 11-year tour of the ringed planet.
Cassini is now engaged in a series of observational “lasts”.
And in 2017 it will put itself on a destructive dive into Saturn’s atmosphere.
“I am moved, as I know everyone else is, looking at these exquisite images of Dione’s surface and crescent, and knowing that they are the last we will see of this far-off world for a very long time to come,” said Carolyn Porco, who leads the imaging team on the mission.
“Right down to the last, Cassini has faithfully delivered another extraordinary set of riches. How lucky we have been.”
The closest ever approach to Dione was in 2011, when the US, European and Italian space agency mission swept just 100km above the moon.
Dione has a diameter of 1,122km. It has an icy exterior and a rocky interior.
Cassini has detected a wispy oxygen atmosphere at the world, and has also seen signs that it may still be active, with what appear to be regions on its surface that have been altered by internal processes.
Next year, Cassini will begin a series of manoeuvres to put itself in orbits that take it high above, and through, Saturn’s rings.
Then, in 2017, once the probe’s fuel has all but run out, ground controllers will command the spacecraft to plunge into the planet’s atmosphere, where it will be destroyed.
As Cassini hurtles towards Saturn, it will become incredibly hot, will melt and ultimately will be crushed by huge pressures.
The mission is being disposed of in this way to be sure there is no possibility that debris from Cassini can one day land on Enceladus and Titan. These moons have been talked of as candidates for extraterrestrial life, and scientists would not want them contaminated by any Earth microbes that might still be on the probe – however unlikely that might be.
The coming months will see Cassini make final, farewell passes of a number of moons.
Referring to Monday’s flyby of Dione, Dr Porco said: “Consider this the start of The Long Goodbye.”
No missions are presently in preparation to re-visit the Saturnian system.
Those outer-planet ventures that are being worked on will go to Jupiter.
America has its Juno probe arriving at the gas giant next year, to be followed by Europe’s Juice satellite in the 2030s.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
➡ Planet Saturn’s Moon – BURNT SIENNA
NASA’s Cassini spacecraft has been orbiting Saturn for the last 11 years taking what can only be described as some of the most stunning and humbling pictures of the planet and its 62 moons.
Despite being only the 15th largest moon, Cassini’s flybys have been more than impressive capturing some stunning images of the moon against the vast backdrop of Saturn.
These last five flybys have been vital for scientists studying Dione with the last (and closest) flyby hopefully confirming the existence of geological activity.
Dione itself is actually a giant floating ice cube, except imagine your average ice cube is now only slightly smaller than our moon and filled with actual ice volcanoes.
To see it in reality would be astonishing but thankfully Cassini’s stunning images help show not only the Moon’s incredible composition but also its relationship to Saturn.
Looking like artwork from ‘Interstellar’, Cassini’s photos paint a humbling picture about the planets we so conventionally think about in school textbooks.
Saturn’s interior is probably composed of a core of iron–nickel and rock (silicon and oxygen compounds). This core is surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium, and finally a gaseous outer layer. Saturn has a pale yellow hue due to ammonia crystals in its upper atmosphere. Electrical current within the metallic hydrogen layer is thought to give rise to Saturn’s planetary magnetic field, which is weaker than Earth’s, but has a magnetic moment 580 times that of Earth due to Saturn’s larger size. Saturn’s magnetic field strength is around one-twentieth of Jupiter’s. The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 km/h (1,100 mph; 500 m/s), higher than on Jupiter, but not as high as those on Neptune.
The planet’s most famous feature is its prominent ring system that is composed mostly of ice particles, with a smaller amount of rocky debris and dust. At least 62 moons are known to orbit Saturn, of which 53 are officially named. This does not include the hundreds of moonlets in the rings. Titan, Saturn’s largest moon, and the second-largest in the Solar System, is larger than the planet Mercury, although less massive, and is t
➡ Planet Saturn – BURNT SIENNA
Saturn pinpointed to within one mile
By Jonathan Webb
Science reporter, BBC News, Seattle
The Cassini probe set off in 1997 and arrived at Saturn seven years later
Thanks to a continent-wide radio telescope, astronomers say they know where Saturn is – to within one mile.
The calculation is many times more accurate than previous estimations and will be useful for the future study of our solar system and beyond.
It used signals sent by the spacecraft Cassini, orbiting Saturn since 2004.
Ten antennae scattered from Hawaii to the Virgin Islands performed the precise measurement, despite Saturn being nearly a billion miles away.
This powerful assembly is known as the Very Long Baseline Array (VLBA), a giant telescope in ten parts.
The findings were reported at a meeting of the American Astronomical Society in Seattle.
“Because of [the VLBA’s] large geographic extent, it has the ability to make very high resolution images – but for this study, the critical thing it can do is measure very precise angles,” explained Dr Dayton Jones of Nasa’s Jet Propulsion Laboratory.
Dr Jones and his colleagues tracked Cassini’s position relative to a reference grid of quasars – bright, ancient radio wave sources well beyond our galaxy.
Saturn, pictured by Cassini and now pinpointed in space thanks to the craft’s transmissions
Without using this type of radio astronomy, where the antennae compare notes in a technique known as “interferometry”, Dr Jones said the best estimates of Saturn’s location were about 20 times less precise.
From our distant vantage point, predicting Saturn’s trajectory to within about one mile is the equivalent of “the width of a dime at 2,000 miles”, Dr Jones said.
“This is very good, and far better than previous techniques have been able to provide,” he added, commending the “extraordinary precision” of the VLBA.
And that precision is especially important when it concerns a giant planet like Saturn.
“Getting better orbits… particularly for the two planets that dominate the dynamics of our solar system, Jupiter and Saturn, improves the basis of the entire ephemeris,” Dr Jones said.
An ephemeris is a table of predicted locations in space.
It has widespread uses in astronomy. Scientists who study the blinking light of pulsars have to be sure of their timing, and require an incredibly exact model of Earth’s own orbit.
“And all the other bodies in the solar system affect the Earth’s orbit, so you really want to have that all put together in a nice consistent system,” Dr Jones told the BBC.
Furthermore, when it comes to planning an actual mission there is little room for error.
“If you want to send a spacecraft to orbit one of the moons of Saturn or Jupiter, you really do want to know what that trajectory has to be, to get there at the same time as the planets do!”