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New images of an icy world

Cassini image of Rhea

NASA's Cassini spacecraft took this raw, unprocessed image of Saturn's moon Rhea on March 10, 2012. The camera was pointing toward Rhea from a distance of approximately 41,873 kilometres.

THESE RAW, UNPROCESSED IMAGES of Saturn’s second largest moon, Rhea, were taken on March 10, 2012, by NASA’s Cassini spacecraft. This was a relatively distant flyby with a close-approach distance of 42,000 kilometres, well suited for global geologic mapping.

At 1,530 kilometres diameter, Rhea is the ninth-largest moon in the Solar System.

During the flyby, Cassini captured these views of the moon’s cratered surface, creating a 30-frame mosaic of Rhea’s leading hemisphere and the side of the moon that faces away from Saturn.

The observations included the large Mamaldi (480 kilometres across) and Tirawa (360 kilometres across) impact basins and the 47-kilometre-wide “ray crater”Inktomi, one of the youngest surface features on Rhea.

Cassini image of Rhea

This second raw, unprocessed Cassini image of Rhea was taken from a distance of approximately 42,258 kilometres, and shows the moon's icy, cratered surface. The streaks on the right are an artefact of the imaging.

Cassini image of Rhea

Shadows help to give a 3D effect to Rhea's craters in this raw, unprocessed Cassini shot taken from a distance of approximately 42,096 kilometres.

Cassini image of Rhea

This raw, unprocessed shot was taken from much further away, approximately 115,060 kilometres, and shows Rhea's "terminator"—the dividing line between day and night.

Cassini has been investigating Saturn and its moons since 2004. This included dropping a probe called Huygens onto the surface of Saturn’s largest moon, Titan, in 2005. Launched in 1997, Cassini-Huygens mission is a co-operative project of NASA, the European Space Agency and the Italian Space Agency.

See all of Cassini’s raw images at NASA’s Saturn page.

Adapted from information issued by NASA / JPL-Caltech / SSI.

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Pluto has a new moon

Artist's impression of Pluto and Charon

Artist's impression of Pluto and its large moon Charon (diameter 1,043km). Hubble telescope observations have uncovered a previously unseen Plutonian moon, probably only 13 to 34km across.

ASTRONOMERS USING THE Hubble Space Telescope have discovered a fourth moon orbiting the icy dwarf planet Pluto. The tiny, new satellite, temporarily designated P4, was uncovered in a Hubble survey searching for rings around the dwarf planet.

The new moon is the smallest discovered circling Pluto. It has an estimated diameter of 13 to 34 km. By comparison, Charon, Pluto’s largest moon, is 1,043 km across, and the other moons, Nix and Hydra, are in the range of 32 to 113 km in diameter.

“I find it remarkable that Hubble’s cameras enabled us to see such a tiny object so clearly from a distance of more than 5 billion km,” said Mark Showalter of the SETI Institute, who led this observing programme with Hubble.

Hubble image showing motion of Pluto's four moons

This composite of two Hubble images—taken on June 28, 2011 and July 3, 2011—shows Pluto's four satellites in motion. P4 is the as-yet-unnamed new moon.

Mission to Pluto

The finding is a result of ongoing work to support NASA’s New Horizons mission, scheduled to fly through the Pluto system in 2015. The mission is designed to provide new insights about worlds at the edge of our Solar System.

Hubble’s mapping of Pluto’s surface and discovery of its satellites have been invaluable to planning for New Horizons’ close encounter.

“This is a fantastic discovery,” said New Horizons’ principal investigator Alan Stern of the Southwest Research Institute. “Now that we know there’s another moon in the Pluto system, we can plan close-up observations of it during our flyby.”

Moons formed in a smash-up

The new moon is located between the orbits of Nix and Hydra, which Hubble discovered in 2005. Charon was discovered in 1978 at the US Naval Observatory and first resolved using Hubble in 1990 as a separate body from Pluto.

The dwarf planet’s entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the Solar System. The smash-up flung material that coalesced into the family of satellites observed around Pluto.

Artist's concept of Pluto's satellite system

An artist's concept of Pluto's satellite system with newly discovered moon P4 highlighted.

Lunar rocks returned to Earth from the Apollo missions led to the theory that our moon was the result of a similar collision between Earth and a Mars-sized body 4.4 billion years ago.

Scientists believe material blasted off Pluto’s moons by micrometeoroid impacts may form rings around the dwarf planet, but the Hubble photographs have not detected any so far.

No sign of rings yet

“This surprising observation is a powerful reminder of Hubble’s ability as a general purpose astronomical observatory to make astounding, unintended discoveries,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington.

P4 was first seen in a photo taken with Hubble’s Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18. The moon was not seen in earlier Hubble images because the exposure times were shorter.

There is a chance it appeared as a very faint smudge in 2006 images, but was overlooked because it was obscured.

Adapted from information issued by NASA. Images and graphics courtesy NASA / A. Feild (STScI) / ESA.

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How Kleopatra gave birth to twins

  • Asteroid Kleopatra is a rubble pile shaped like a dog bone, 217km long
  • Found to have twin 8km-wide moons that orbit it
  • Moons have been named after queen Cleopatra’s twins

ASTRONOMERS STUDYING two mini-moons orbiting an asteroid called Kleopatra have confirmed that the dog bone-shaped asteroid is probably a pile of rocky rubble instead of a solid body.

The French and American team, which includes Franck Marchis (University of California, Berkeley) and Pascal Descamps (Institut de Mecanique Celeste et de Calculs des Ephemerides, Observatoire de Paris), report their findings in the journal Icarus.

Kleopatra was discovered in 1880. Observations made in 2000 showed it to have an unusual, elongated shape reminiscent of a dog bone.

Subsequent radar observations confirmed the shape, but Marchis and his colleagues wanted higher resolution images to determine whether the two lobes of the dog bone are touching or are two separate bodies, and also to calculate its density.

The video above is a rotation of a computer model produced from the radar data.

Using the Keck II telescope in Hawaii, in 2008 the astronomers obtained the best images yet and confirmed that the asteroid is a double-lobed body. They also discovered the two small moons.

Space rubble

The team charted the orbits of the moons (diameters 3 and 5 kilometres), from which they could calculate the mass of the asteroid. Given its size, shape and mass, the astronomers then calculated the asteroid’s density—3.6 grams per cubic centimetre. (As a comparison, Earth’s average density is 5.5 grams per cubic centimetre.)

If the bulk of the asteroid is made of iron—a common component with a density of about 5 grams per cubic centimetre—then it must be between 30 and 50 percent empty space, the team concludes.

“Our observations of the orbits of the two satellites of … Kleopatra imply that this large metallic asteroid is a rubble pile, which is a surprise,” said Marchis, who is also a planetary scientist at the SETI Institute. “Asteroids this big are supposed to be solid, not rubble piles.”

Asteroid Kleopatra and its two tiny moons

Asteroid Kleopatra (overexposed in the left-hand image) and its two tiny moons. The image on the right has been processed to reduce the glare and more easily show the moons, which are now called Alexhelios and Cleoselene.

Kleopatra, about 217 kilometres long, is one of several large asteroids recently found to be composed of rocky rubble held together by mutual gravitational attraction. Others are Sylvia (280 kilometres in diameter), Antiope (86km), Hermione (190km) and 22 Kalliope (166km). Each of these has one or more moons, or in the case of Hermione, is itself a double asteroid.

How to grow a planet

The proportion of large asteroids in the Solar System that are rubble piles is unknown. But the fact that, so far, all multiple asteroids are porous collections of gravitationally bound chunks could have implications for how planets form, Marchis said.

Astronomers think planets are built up by rocks and asteroids crashing into each other and merging, with the resulting bodies gradually growing bigger and bigger.

But collisions between two asteroids are just as likely to smash both bodies to pieces ars they are to coalesce into a single large one, potentially making planet formation a slow process.

Rubble pile asteroids, however, would merge more easily during a collision.

Asteroid Kleopatra

Radar image of dog bone-shaped Kleopatra. The asteroid is thought be made of rocky rubble held together under its own weak gravitational field.

“If a large proportion of asteroids in the early Solar System were rubble-pile, then the formation of the cores of planets would be much faster,” Marchis said.

The twins leave home

Kleopatra probably coalesced from the remains of a rocky, metallic asteroid smashed to smithereens after a collision with another asteroid, which could have occurred any time since the origin of the Solar System 4.5 billion years ago.

Based on a theory of “binary asteroid” formation, the rubble pile would have been set spinning faster by another, oblique impact 100 million years ago. The spinning asteroid would have slowly elongated and eventually split off the most distant of its moons.

The inner moon was likely shed more recently, perhaps 10 million years ago.

The International Astronomical Union’s Committee on Small Body Nomenclature has accepted the proposal of Marchis and his collaborators to name the moons after Cleopatra’s twin children—Cleopatra Selene II and Alexander Helios.

The outermost moon has been named Alexhelios and the innermost moon Cleoselene. In Greek mythology, Helios and Selene represented the Sun and Moon, respectively.

Adapted from information issued by the University of California, Berkeley / NSSDC / NASA / Stephen Ostro et al. (JPL) / Arecibo Radio Telescope / NSF.

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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.

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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,

Image courtesy NASA / JPL / Space Science Institute.

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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.

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Saturn’s icy moon Dione

One side of Saturn's moon, Dione

Icy terrain with wispy patterns covers one side of Saturn's moon Dione.

Wispy terrain stretches across the trailing hemisphere of Saturn’s moon Dione in this view taken by NASA’s Cassini spacecraft during its January 27, 2010, non-targeted flyby.

Cassini came within about 45,000 kilometres of the moon during this flyby, but this image was acquired at a distance of approximately 137,000 kilometres from Dione. This view looks toward the side of the moon that was facing away from Saturn, and in particular its trailing hemisphere (the half of Dione that faces “backwards” as the moon orbits the planet).

Dione (pronounced dy-OH-nee) is 1,123 kilometres wide, and this image shows detail down to 819 metres per pixel.

Dione is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.

Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.

Contrary to what scientists had expected upon studying this fascinating moon, much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This anomaly suggests that during the period of heavy meteors bombardment, Dione was ‘tidally locked’ to Saturn in the opposite orientation. (A moon is tidally locked when it keeps the same face to its parent planet at all times.)

Because Dione is relatively small, an impact big enough to cause a 35-kilometre-diameter crater could have spun the moon. Since there are many craters larger than 35-kilometres, Dione could easily have been spun more than once. The moon has probably been tidally locked in its current orientation for the past several billion years.

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