RSSAll Entries Tagged With: "rings"

Uranus fly-by – 25 years ago today

True- and false-colour images of Uranus

Two views of Uranus—one in true colour (left) and the other in false colour—were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million kilometres from the planet, and several days from closest approach.

AS NASA’S VOYAGER 2 spacecraft made the only close approach to date of our mysterious seventh planet Uranus 25 years ago, Project Scientist Ed Stone and the Voyager team gathered at NASA’s Jet Propulsion Laboratory, Pasadena, California, to pore over the data coming in.

Images of the small, icy Uranus moon Miranda were particularly surprising. Since small moons tend to cool and freeze over rapidly after their formation, scientists had expected a boring, ancient surface, pockmarked by crater-upon-weathered-crater.

Instead they saw grooved terrain with linear valleys and ridges cutting through the older terrain and sometimes coming together in chevron shapes. They also saw dramatic fault scarps, or cliffs. All of this indicated that periods of tectonic and thermal activity had rocked Miranda’s surface in the past.

Surface of Miranda

A section of the surface of Miranda, innermost of Uranus' large satellites, as seen by Voyager 2 from 36,000 kilometres away. A complex topography of high and low terrain, craters and scarps can be seen.

The scientists were also shocked by data showing that Uranus’s magnetic north and south poles were not closely aligned with the north-south axis of the planet’s rotation. Instead, the planet’s magnetic field poles were closer to the Uranian equator. This suggested that the material flows in the planet’s interior that are generating the magnetic field are closer to the surface of Uranus than the flows inside Earth, Jupiter and Saturn are to their respective surfaces.

“Voyager 2’s visit to Uranus expanded our knowledge of the unexpected diversity of bodies that share the solar system with Earth,” said Stone, who is based at the California Institute of Technology in Pasadena. “Even though similar in many ways, the worlds we encounter can still surprise us.”

Here’s NASA’s pre-encounter video from the 1980s, showing how Voyager 2 sped past the planet while collecting its data:

A host of new discoveries

Voyager 2 was launched on August 20, 1977, 16 days before its twin, Voyager 1. After completing its prime mission of flying by Jupiter and Saturn, Voyager 2 was sent on the right flight path to visit Uranus, which is about 3 billion kilometres away from the Sun. Voyager 2 made its closest approach—within 81,500 kilometres of the Uranian cloud tops—on January 24, 1986.

Before Voyager 2’s visit, scientists had to learn about Uranus by using Earth-based and airborne telescopes. By observing dips in starlight as a star passed behind Uranus, scientists knew Uranus had nine narrow rings.

But it wasn’t until the Voyager 2 flyby that scientists were able to capture for the first time images of the rings and the tiny shepherding moons that sculpted them. Unlike Saturn’s icy rings, they found Uranus’ rings to be dark grey, reflecting only a few percent of the incident sunlight.

Voyager image of Uranus' rings and two moons

Voyager 2 discovered two "shepherd" moons associated with Uranus' thin rings.

Scientists had also determined an average temperature for Uranus—minus 214 degrees Celsius—before this encounter, but the distribution of that temperature came as a surprise. Voyager showed there was heat transport from pole to pole in Uranus’ atmosphere that maintained the same temperature at both poles, even though the Sun was shining directly for decades on one pole and not the other.

By the end of the Uranus encounter and science analysis, data from Voyager 2 enabled the discovery of 11 new moons and two new rings, and generated dozens of science papers about the quirky seventh planet.

Interstellar mission

Voyager 2 moved on to explore Neptune, the last planetary target, in August 1989. It is now hurtling toward interstellar space, which is the space between stars. It is about 14 billion kilometres away from the Sun.

Voyager 1, which explored only Jupiter and Saturn before heading on a faster track toward interstellar space, is about 17 billion kilometres away from the Sun.

“The Uranus encounter was one of a kind,” said Suzanne Dodd, Voyager project manager, based at JPL. “Voyager 2 was healthy and durable enough to make it to Uranus and then to Neptune.”

“Currently both Voyager spacecraft are on the cusp of leaving the Sun’s sphere of influence and once again blazing a trail of scientific discovery.”

The Voyagers were built by NASA’s Jet Propulsion Laboratory in Pasadena, California, which continues to operate both spacecraft.

Link: More information about the Voyager spacecraft

Adapted from information issued by NASA Jet Propulsion Laboratory.

Get SpaceInfo.com.au daily updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates. Or follow us on Twitter, @spaceinfo_oz

Moody Saturn

Saturn as seen from the Cassini spacecraft

Saturn as seen from the Cassini spacecraft during the planet's equinox in July 2009. The rings are edge-on to the Sun.

The shadows of Saturn’s rings are cast onto the planet and appear as a thin band at the equator in this image taken as the planet approached its August 2009 equinox.

Approximately every 15 years, Saturn’s experiences equinox. Just like on Earth, the equinox occurs when the Sun is directly over the equator. And because Saturn’s rings orbit around its equator, the Saturnian equinox also means that the rings are exactly edge on to the Sun.

This angle makes the rings appear significantly darken than normal, and causes anything sticking up out of the plane of the rings to look anomalously bright and to cast shadows across the rings.

These sorts of scenes are possible only during the few months before and after Saturn’s equinox, at which times Cassini’s cameras have spotted not only the predictable shadows of some of Saturn’s moons, but also the shadows of newly revealed vertical structures in the rings themselves.

The planet’s southern hemisphere can be seen through the transparent D ring in the lower right of the image. The rings have been brightened by a factor of 9.5 relative to the planet to enhance visibility.

The view overall looks toward the northern, unilluminated side of the rings from about 30 degrees above the ringplane.

See a larger version of the image here.

Images taken using red, green and blue spectral filters were combined to create this natural colour view. The images were obtained with the Cassini spacecraft wide-angle camera on July 18, 2009 at a distance of approximately 2.1 million kilometres from Saturn. Image scale is 122 kilometres (76 miles) per pixel.

Saturn facts:

  • Saturn is a gas giant planet, composed mainly of hydrogen and helium.
  • There are trace amounts of ammonia, phosphine, methane and other gases.
  • We see only the tops of Saturn’s clouds and outer atmospheric layers.
  • Underneath the clouds is thought to be a thick layer of liquid hydrogen, under which is a layer of metallic hydrogen.
  • Deep inside is thought to be a core about 12,000km wide, made of rock plus water and other gases solidified under extreme pressure.
  • The core temperature is probably 10,000 to 15,000 degrees Celsius.
  • Saturn has clouds, winds, rain, snow, storms and lightning.
  • Saturn’s overall density is less than that of water…so if you could find a lake large and deep enough, Saturn would float in it!

Adapted from information issued by NASA / JPL / Space Science Institute.

Get SpaceInfo.com.au daily updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates. Or follow us on Twitter, @spaceinfo_oz

Saturn’s shepherd moon

Saturn's moon Daphnis in the Keeler Gap in Saturn's rings

One of Saturn's two shepherd moons, Daphnis (upper left), inhabits the Keeler Gap in the planet's famous ring system. The moon's gravity causes a wave-like effect on the inner and outer edge of the Gap.

Saturn has more than 60 moons of many different sizes and shapes. Most of them orbit well outside the realm of the planet’s rings, but some live within the rings.

One such is Daphnis, a tiny 7-kilometre-diameter body that circles Saturn within a gap in the rings known as the Keeler Gap, which itself is only 42 kilometres wide.

The moon is named after a figure from Greek mythology. Daphnis was a shepherd, the some of Hermes and brother of Pan. (Saturn’s other shepherd moon is named Pan.)

Daphnis orbit is not perfectly circular; it is ever so slightly elliptical. Plus, it doesn’t orbit cleanly in the same plane as the rings, but has an inclined orbit that makes it range up to about 8 kilometres above and about 8 kilometres below the ring plane.

As it zips along through the Keeler Gap, Daphnis’ gravity disturbs the material in the rings on each edge of the Gap, resulting in the edges forming a “wavy” appearance. The wave on the inside edge of the Gap moves ahead of Daphnis, while the wave on the outer edge of the Gap lags behind, due to the different speeds at which Daphnis and material in the inner and outer edges circle Saturn.

Scientists had suspected that an undiscovered moon was causing the wavy edges of the Keeler Gap, but it wasn’t until May 2005 that it was spotted. The Cassini Imaging Science Team made the discovery on May 6, 2005 from images obtained five days earlier. It was subsequently spotted in other images taken on May 2, and earlier images taken in April 2005.

The image shown here was taken on July 5, 2010 by NASA’s Cassini spacecraft, and transmitted to Earth the following day.

Story by Jonathan Nally, Editor, SpaceInfo.com.au

Image courtesy NASA / JPL / Space Science Institute.

Get daily SpaceInfo.com.au updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates. Or follow us on Twitter, @spaceinfo_oz

The twist in Saturn’s rings

A propeller-shaped disturbance in Saturn's rings

A propeller-shaped structure formed by the influence of an unseen moon on the particles in Saturn's rings, is brightly illuminated by sunlight in this image obtained by NASA’s Cassini spacecraft.

  • Dozens of unseen moons within Saturn’s rings
  • Cause surrounding ring material to twist and warp
  • Twists look like giant propellers

Scientists using NASA’s Cassini spacecraft—currently orbiting through the Saturnian system—are stalking a new class of moons that create distinctive “propeller-shaped” gaps in Saturn’s rings.

Scientists first discovered double-armed propeller features in 2006 in an area now known as the “propeller belts” in the middle of Saturn’s outermost dense ring, the A ring.

The gaps in the rings are created by a new class of moonlets—smaller than known moons, but larger than the particles in the rings—that could clear the space immediately around them.

Those moonlets, estimated to number in the millions, are not large enough to have cleared out their entire path around Saturn, as do the moons Pan and Daphnis.

The new research, led by Matthew Tiscareno, a Cassini imaging team associate based at Cornell University, reveals a new cohort of larger and rarer moons in another part of the A ring farther out from Saturn.

A propeller-shaped disturbance in Saturn's rings

A "propeller" is brightly illuminated on the sunlit side of Saturn's rings, where the ring has been disturbed and ring material boosted above the ringplane.

With “propellers” as much as hundreds of times as large as those previously described, the scientists have been able to track them for as long as four years.

The features are up to several thousand kilometres long and several kilometres wide. The gravitational effect of the moons embedded in the ring appears to kick up ring material as high as 500 metres above and below the flat ring plane, which is well beyond the typical ring thickness of about 10 metres.

Dozens of giant “propellers”

Cassini is too far away to see the moons amid the swirling ring material around them, but scientists estimate that they are about a kilometre in diameter because of the size of the propellers.

Tiscareno and colleagues estimate that there are dozens of these giant propellers; 11 of them were imaged multiple times between 2005 and 2009.

One of them, nicknamed Bleriot after the famous aviator Louis Bleriot, has been a veritable Forrest Gump, showing up in more than 100 separate Cassini images and one ultraviolet imaging spectrograph observation over this time.

“Scientists have never tracked disc-embedded objects anywhere in the universe before now,” Tiscareno said. “All the moons and planets we knew about before orbit in empty space.”

“In the propeller belts, we saw a swarm in one image and then had no idea later on if we were seeing the same individual objects,” added Tiscareno. “With this new discovery, we can now track disc-embedded moons individually over many years.”

“Propellers give us unexpected insight into the larger objects in the rings,” said Linda Spilker, Cassini project scientist based at NASA’s Jet Propulsion Laboratory. “Over the next seven years, Cassini will have the opportunity to watch the evolution of these objects and to figure out why their orbits are changing.”

A short movie of one of Saturn's propellers

A short movie of one of Saturn's propellers

A glimpse into the Solar System’s past

The observations also mark the first time scientists have been able to track the orbits of individual objects in a “debris disc”, which is what the rings are—billions upon billions of chunks of ice of all sizes, encircling the planet.

This gives scientists an opportunity to “time-travel” back into the history of our Solar System, when the planets were forming within a much bigger version of Saturn’s rings, circling the youthful Sun.

“Observing the motions of these disc-embedded objects provides a rare opportunity to gauge how the planets grew from, and interacted with, the disc of material surrounding the early Sun,” said Carolyn Porco, Cassini imaging team lead based at the Space Science Institute.

“It allows us a glimpse into how the Solar System ended up looking the way it does.” And, by extension, what by might be happening in other star systems.

Adapted from information issued by NASA / JPL / SSI.

Get daily SpaceInfo.com.au updates by RSS or email! Click the RSS Feed link at the top right-hand corner of this page, and then save the RSS Feed page to your bookmarks. Or, enter your email address (privacy assured) and we’ll send you daily updates.

Saturnian moons line up

Image showing the moons Rhea and Epimetheus with Saturn and its rings in the background.

Image showing the moons Rhea and Epimetheus with Saturn and its rings in the background.

This amazing black and white image shows two of Saturn’s moons, Rhea and Epimetheus, against a backdrop of the planet and its rings.

Saturn has more than 60 known moons, each a different size and orbiting at different distances from the planet. They orbit at different speeds, and often overtake each other, leading to views like this when the Cassini spacecraft’s camera is pointed in the right direction.

Although they look close, the two moons are actually far apart. The view was obtained at a distance of approximately 1.2 million kilometres (746,000 miles) from Rhea, while Epimetheus is 400,000 kilometres further away at 1.6 million kilometres (994,000 miles).

The image gives a good indication of the scale of things in the Saturnian system. At 1,528 kilometres (949 miles) diameter, Rhea is by no means Saturn’s largest moon, yet it is more than one-tenth the width of Earth. Compare that with the huge bulk of Saturn in the background.

Epimetheus is tiny, only 113 kilometres (70 miles) wide.

At Cassini’s huge distance when it took this image, detail as small as 7 kilometres (4 miles) per pixel can be seen on Rhea, and 10 kilometres (6 miles) per pixel on Epimetheus.

Adapted from information issued by NASA / JPL / Space Science Institute.

Rings and moons

Cassini spacecraft image of Rhea, Prometheus and Saturn's rings.

Cassini spacecraft image of Rhea, Prometheus and Saturn's rings.

  • Rhea & Prometheus
  • Saturn’s rings seen edge-on
  • Images by the Cassini spacecraft

From just below the plane of Saturn’s thin rings, the Cassini spacecraft took this image of the rings edge-on with the planet’s second largest moon, Rhea, beyond.

Although Rhea may appear to be in the foreground of this image, it isn’t. The rings are closer. The small moon Prometheus, orbiting between the A ring and the thin F ring, is also visible within the rings near the upper middle of the image.

This view looks toward the Saturn-facing side of Rhea (1,528 kilometres wide) and the leading hemisphere of Prometheus (86 kilometres wide). This view looks toward the southern, unilluminated side of the rings from just below the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on January 31, 2010. The view was obtained at a distance of approximately 2.5 million kilometres from Rhea and approximately 2 million kilometres from Prometheus. Image scale is 15 kilometres per pixel on Rhea and 12 kilometres per pixel on Prometheus.

Adapted from information issued by NASA / JPL / Space Science Institute.