Posted on by Fraser Cain

Why Eros Has So Few Craters

Image credit: NASA/JPL
University of Arizona scientists have discovered why Eros, the largest near-Earth asteroid, has so few small craters.

When the Near Earth Asteroid Rendezvous (NEAR) mission orbited Eros from February 2000 to February 2001, it revealed an asteroid covered with regolith — a loose layer of rocks, gravel and dust — and embedded with numerous large boulders. The spacecraft also found places where the regolith apparently had slumped, or flowed downhill, exposing fresh surface underneath.

But what NEAR didn’t find were the many small craters that scientists expected would pock Eros’ landscape.

“Either the craters were being erased by something or there are fewer small asteroids than we thought,” James E. Richardson Jr. of UA’s planetary sciences department said.

Richardson concludes from modeling studies that seismic shaking has obliterated about 90 percent of the asteroid’s small impact craters, those less than 100 meters in diameter, or roughly the length of a football field. The seismic vibrations result when Eros collides with space debris.

Richardson, Regents’ Professor H. Jay Melosh and Professor Richard Greenberg, all with UA’s Lunar and Planetary Laboratory, report the analysis in the Nov. 26 issue of Science.

“Eros is only about the size of Lake Tahoe — 20 miles (33 kilometers) long by 8 miles (13 kilometers) wide,” Richardson said. “So it has a very small volume and a very low gravity. When a one-to-two-meter or larger object hits Eros, the impact will set off global seismic vibrations. Our analysis shows how these vibrations easily destabilize regolith overlaying the surface.”

A rock-and-dust layer creeps, rather than crashes, down shaking slopes because of Eros’ weak gravity. The regolith not only slides down horizontally, but also is launched ballistically from the surface and ‘hops’ downslope. Very slowly, over time, impact craters fill up and disappear, Richardson said.

If Eros were still in the main asteroid belt between Mars and Jupiter, a 200-meter crater would fill in about 30 million years. Because Eros is now outside the asteroid belt, that process takes a thousand times longer, he added.

Richardson’s research results match the NEAR spacecraft evidence. Instead of the expected 400 craters as small as 20 meters (about 70 feet) per square kilometer (three-fifths mile) on Eros’ surface, there are on average only about 40 such craters.

The modeling analysis also validates what scientists suspect of Eros’ internal structure.

“The NEAR mission showed Eros to most likely be a fractured monolith, a body that used to be one competent piece of material,” Richardson said. “But Eros has been fractured throughout by large impacts and is held together primarily by gravity. The evidence is seen in a series of grooves and ridges that run across the asteroid’s surface both globally and regionally.”

Large impacts fracture Eros to its core, but many smaller impacts fracture only the upper surface. This gradient of big fractures deep inside and numerous small fractures near the surface is analogous to fractures in the upper lunar crust, Richardson said. “And we understand the lunar crust — we’ve been there. We’ve put seismometers on the moon. We understand how seismic energy propagates through this kind of structure.”

The UA scientists’ analysis of how impact-induced seismic shaking has modified Eros’ surface has a couple of other important implications.

“If we eventually do send spacecraft to mine resources among the near-Earth asteroids or to deflect an asteroid from a potential collision with the Earth, knowing internal asteroid structure will help address some of the strategies we’ll need to use. In the nearer future, sample return missions will encounter successively less porous, more cohesive regolith as they dig farther down into asteroids like Eros, which has been compacted by seismic shaking,” Richardson noted.

“And it also tells us about the small asteroid environment that we’ll encounter when we do send a spacecraft out into the main asteroid belt, where Eros spent most of its lifetime. We know the small asteroids — those between the size of a beachball and a football stadium — are out there. It’s just that their ‘signature’ on asteroids such as Eros is being erased,” Richardson said.

This finding is important because the cratering record on large asteroids provides direct evidence for the size and population of small main-belt asteroids. Earth-based telescopic surveys have catalogued few main-belt asteroids that small. So scientists have to base population estimates for these objects primarily on visible cratering records and asteroid collisional history modeling, Richardson said.

Original Source: UA News Release

Posted on by Fraser Cain

Astronauts Move Soyuz on Station

Image credit: NASA
The Expedition 10 crewmembers are back inside the International Space Station after taking a short ride this morning. They flew their Soyuz spacecraft from one docking port to another to clear the way for two spacewalks next year.

Having configured Station systems for autonomous operation, Expedition 10 Flight Engineer and Soyuz Commander Salizhan Sharipov and Expedition 10 Commander Leroy Chiao undocked the Soyuz from the Station’s Pirs Docking Compartment at 4:32 a.m. EST, as they flew 225 miles over the southern Atlantic Ocean.

Sharipov, seated in the center seat of the Soyuz descent module compartment, and Chiao seated to his left, backed the capsule away from the Station approximately 98 feet. They flew the Soyuz laterally along the Station approximately 45 feet before rotating the craft 135 degrees to align it with the Earth-facing docking port on the adjacent Zarya module. The vehicle was held in position for eight minutes of station-keeping, ensuring correct alignment of docking mechanisms, before the crew began the final approach toward the Station.

Docking was at 4:53 a.m. EST, as the Soyuz and the Station passed over western Asia. Within minutes, hooks and latches engaged between the Soyuz and Zarya firmly linking the return vehicle and the Station. After a series of leak checks, the crew reentered the Station at 6:54 a.m. EST, and they began reconfiguring Station systems for normal operations.

Repositioning of the Soyuz cleared Pirs, which also serves as an airlock, for a pair of spacewalks by Chiao and Sharipov planned for early next year.

Information about crew activities on the Space Station, future launch dates and Station sighting opportunities from Earth, is available on the Internet at: http://spaceflight.nasa.gov/

Details about Station science operations are available on the Internet from NASA’s Marshall Space Flight Center, Huntsville, Ala., Payload Operations Center at: http://scipoc.msfc.nasa.gov/

For information about NASA and other agency missions, visit: http://www.nasa.gov

Original Source: NASA News Release

Posted on by Fraser Cain

Portrait of Mimas in Saturn’s Rings

In a splendid portrait created by light and gravity, Saturn’s lonely moon Mimas is seen against the cool, blue-streaked backdrop of Saturn’s northern hemisphere. Delicate shadows cast by the rings arc gracefully across the planet, fading into darkness on Saturn’s night side.

The part of the atmosphere seen here appears darker and more bluish than the warm brown and gold hues seen in Cassini images of the southern hemisphere, due to preferential scattering of blue wavelengths by the cloud-free upper atmosphere.

The bright blue swath near Mimas (398 kilometers, or 247 miles across) is created by sunlight passing through the Cassini division (4,800 kilometers, or 2,980 miles wide). The rightmost part of this distinctive feature is slightly overexposed and therefore bright white in this image. Shadows of several thin ringlets within the division can be seen here as well. The dark band that stretches across the center of the image is the shadow of Saturn’s B ring, the densest of the main rings. Part of the actual Cassini division appears at the bottom, along with the A ring and the narrow, outer F ring. The A ring is transparent enough that, from this viewing angle, the atmosphere and threadlike shadows cast by the inner C ring are visible through it.

Images taken with red, green and blue filters were combined to create this color view. The images were obtained with the Cassini spacecraft narrow angle camera on Nov. 7, 2004, at a distance of 3.7 million kilometers (2.3 million miles) from Saturn. The image scale is 22 kilometers (14 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org .

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Future Robots May “Hop” Across Mars

Image credit: Pioneer Astro
Part lander, part aircraft, the gashopper (no, not grasshopper) is a unique concept being considered by NASA for future robotic exploration of Mars. Unlike landers, such as the Viking spacecraft, Beagle 2, or the upcoming Phoenix lander which can only examine a few square metres of ground, the gashopper could land, perform scientific analysis and launch itself back into the air to fly hundreds of kilometres to a new location.

The gashopper would get its electricity from a large set of solar panels built on top of its wings. It would use this electricity to retrieve carbon dioxide from the Martian atmosphere, and then store it as a liquid inside the aircraft. When enough gas was stored up to make a flight, it would heat up a hot bed of pellets and then pass the CO2 through it. Now hot, the gas would act as a propellant, and allow the gashopper to lift off vertically from the surface of Mars. Once airborne, it could then fire more gas out a rear thruster and begin flying as an airplane, using its large wings for lift and maneuverability. When it was ready to land, the aircraft could slow its airspeed, and then touch down gently as a vertical lander.

The proposal comes from the mind of Robert Zubrin, author of The Case for Mars, President of the Mars Society, and the President of Pioneer Astronautics. It’s one of 219 research projects selected by NASA for Small Business Research and Development contract awards.

Zubrin sees the gashopper not only as a technology for exploring Mars, but as a proof of concept for many engineering challenges that NASA will have to overcome in future missions, both robotic and human. “If we’re going to do a sample return mission, we’ll want to know how to make propellant for the return journey,” explains Zubrin, “and the gashopper will also let us test many liftoffs and landings with hazard avoidance in all kinds of terrain.

“The gashopper will be using native carbon dioxide for fuel, so it won’t contaminate the soil with hydrocarbons,” continues Zubrin. This is important, because spacecraft from Earth using hydrocarbons for fuel could contaminate the landing site with chemicals that could confuse the search for life. “Once the gashopper gets moving, it’ll find a pristine Martian surface to explore.”

The simplest gashopper could actually be quite light, as little as 50 kg (110 pounds). Compare this to the current Mars Exploration Rovers, which both weigh in at 185 kg (380 pounds). Tack on some more weight, and the gashopper could carry a few mini-rovers, like the tiny Sojourner that visited Mars as part of the Pathfinder mission. These could be targeted at the most interesting features based on the gashopper’s aerial reconnaissance of the area.

Image credit: Pioneer Astro
Another advantage of the gashopper is that is could completely ignore terrain. When NASA selected the landing sites for its Mars landers, it purposefully chose locations that were relatively flat, so the rovers could drive at a useful speed. The gashopper could land at the edge of a deep chasm, examine the area, jump down to the bottom and get back out again. It would give scientists unprecedented range and flexibility when searching for evidence of past water or life on Mars.

Of course, there’s a catch. The limiting feature of the gashopper is the electricity required to pressurize and heat the carbon dioxide propellant. This process consumes a lot of power, and the gashopper would need more than a month using its solar cells to refuel and recharge its batteries before it could take off again.

To generate more electricity, NASA could consider using a Radioisotope Thermal Generator, similar to those carried by Cassini, the Viking landers, or the upcoming Mars Science Laboratory (due for launch in 2009). With a more powerful electrical system, the gashopper could lift off every few days, and essentially be able to roam the entire planet of Mars.

Zubrin’s company, Pioneer Astronautics, has already done a significant amount of testing and research for the concept, and they developed a prototype ballistic gashopper for NASA’s Jet Propulsion Lab in 2000. The engine worked well in the lab, and they were able to get a remote-controlled vehicle with a mass of 50 kg to fly in a simulated Martian gravity (using a helium balloon to provide stability).

Instead of sitting on one spot, or slowly crawling across the surface of Mars, future robotic explorers to visit the Red Planet may take to the skies and soar. Well… hop, anyway.

Written by Fraser Cain

Posted on by Fraser Cain

Ingredients are There to Make Rocky Planets

One of the currently hottest astrophysical topics – the hunt for Earth-like planets around other stars – has just received an important impetus from new spectral observations with the MIDI instrument at the ESO VLT Interferometer (VLTI).

An international team of astronomers [2] has obtained unique infrared spectra of the dust in the innermost regions of the proto-planetary discs around three young stars – now in a state possibly very similar to that of our solar system in the making, some 4,500 million years ago.

Reporting in this week’s issue of the science journal Nature, and thanks to the unequalled, sharp and penetrating view of interferometry, they show that in all three, the right ingredients are present in the right place to start formation of rocky planets at these stars.

“Sand” in the inner regions of stellar discs
The Sun was born about 4,500 million years ago from a cold and massive cloud of interstellar gas and dust that collapsed under its own gravitational pull. A dusty disc was present around the young star, in which the Earth and other planets, as well as comets and asteroids were later formed.

This epoch is long gone, but we may still witness that same process by observing the infrared emission from very young stars and the dusty protoplanetary discs around them. So far, however, the available instrumentation did not allow a study of the distribution of the different components of the dust in such discs; even the closest known are too far away for the best single telescopes to resolve them. But now, as Francesco Paresce, Project Scientist for the VLT Interferometer and a member of the team from ESO explains, “With the VLTI we can combine the light from two well-separated large telescopes to obtain unprecedented angular resolution. This has allowed us, for the first time, to peer directly into the innermost region of the discs around some nearby young stars, right in the place where we expect planets like our Earth are forming or will soon form”.

Specifically, new interferometric observations of three young stars by an international team [2], using the combined power of two 8.2-m VLT telescopes a hundred metres apart, has achieved sufficient image sharpness (about 0.02 arcsec) to measure the infrared emission from the inner region of the discs around three stars (corresponding approximately to the size of the Earth’s orbit around the Sun) and the emission from the outer part of those discs. The corresponding infrared spectra have provided crucial information about the chemical composition of the dust in the discs and also about the average grain size.

These trailblazing observations show that the inner part of the discs is very rich in crystalline silicate grains (“sand”) with an average diameter of about 0.001 mm. They are formed by coagulation of much smaller, amorphous dust grains that were omnipresent in the interstellar cloud that gave birth to the stars and their discs.

Model calculations show that crystalline grains should be abundantly present in the inner part of the disc at the time of formation of the Earth. In fact, the meteorites in our own solar system are mainly composed of this kind of silicate.

Dutch astronomer Rens Waters, a member of the team from the Astronomical Institute of University of Amsterdam, is enthusiastic: “With all the ingredients in place and the formation of larger grains from dust already started, the formation of bigger and bigger chunks of stone and, finally, Earth-like planets from these discs is almost unavoidable!”

Transforming the grains
It has been known for some time that most of the dust in discs around newborn stars is made up of silicates. In the natal cloud this dust is amorphous, i.e. the atoms and molecules that make up a dust grain are put together in a chaotic way, and the grains are fluffy and very small, typically about 0.0001 mm in size. However, near the young star where the temperature and density are highest, the dust particles in the circumstellar disc tend to stick together so that the grains become larger. Moreover, the dust is heated by stellar radiation and this causes the molecules in the grains to re-arrange themselves in geometric (crystalline) patterns.

Accordingly, the dust in the disc regions that are closest to the star is soon transformed from “pristine” (small and amorphous) to “processed” (larger and crystalline) grains.

Spectral observations of silicate grains in the mid-infrared wavelength region (around 10 ?m) will tell whether they are “pristine” or “processed”. Earlier observations of discs around young stars have shown a mixture of pristine and processed material to be present, but it was so far impossible to tell where the different grains resided in the disc.

Thanks to a hundred-fold increase in angular resolution with the VLTI and the highly sensitive MIDI instrument, detailed infrared spectra of the various regions of the protoplanetary discs around three newborn stars, only a few million years old, now show that the dust close to the star is much more processed than the dust in the outer disc regions. In two stars (HD 144432 and HD 163296) the dust in the inner disc is fairly processed whereas the dust in the outer disc is nearly pristine. In the third star (HD 142527) the dust is processed in the entire disc. In the central region of this disc, it is extremely processed, consistent with completely crystalline dust.

An important conclusion from the VLTI observations is therefore that the building blocks for Earth-like planets are present in circumstellar discs from the very start. This is of great importance as it indicates that planets of the terrestrial (rocky) type like the Earth are most probably quite common in planetary systems, also outside the solar system.

The pristine comets
The present observations also have implications for the study of comets. Some – perhaps all – comets in the solar system do contain both pristine (amorphous) and processed (crystalline) dust. Comets were definitely formed at large distances from the Sun, in the outer regions of the solar system where it has always been very cold. It is therefore not clear how processed dust grains may end up in comets.

In one theory, processed dust is transported outwards from the young Sun by turbulence in the rather dense circumsolar disc. Other theories claim that the processed dust in comets was produced locally in the cold regions over a much longer time, perhaps by shock waves or lightning bolts in the disc, or by frequent collisions between bigger fragments.

The present team of astronomers now conclude that the first theory is the most likely explanation for the presence of processed dust in comets. This also implies that the long-period comets that sometimes visit us from the outer reaches of our solar system are truly pristine bodies, dating back to an era when the Earth and the other planets had not yet been formed.

Studies of such comets, especially when performed in-situ, will therefore provide direct access to the original material from which the solar system was formed.

More information
The results reported in this ESO PR are presented in more detail in a research paper “The building blocks of planets within the “terrestrial” region of protoplanetary disks”, by Roy van Boekel and co-authors (Nature, November 25, 2004). The observations were made in the course of ESO’s early science demonstration programme.

Notes

[1]: This ESO press release is issued in collaboration with the Astronomical Institute of the University of Amsterdam, The Netherlands (NOVA PR) and the Max-Planck-Institut f?r Astronomie (Heidelberg, Germany (MPG PR).

[2]: The team consists of Roy van Boekel, Michiel Min, Rens Waters, Carsten Dominik and Alex de Koter (Astronomical Institute, University of Amsterdam, The Netherlands), Christoph Leinert, Olivier Chesneau, Uwe Graser, Thomas Henning, Rainer K?hler and Frank Przygodda (Max-Planck-Institut f?r Astronomie, Heidelberg, Germany), Andrea Richichi, Sebastien Morel, Francesco Paresce, Markus Sch?ller and Markus Wittkowski (ESO), Walter Jaffe and Jeroen de Jong (Leiden Observatory, The Netherlands), Anne Dutrey and Fabien Malbet (Observatoire de Bordeaux, France), Bruno Lopez (Observatoire de la Cote d’Azur, Nice, France), Guy Perrin (LESIA, Observatoire de Paris, France) and Thomas Preibisch (Max-Planck-Institut f?r Radioastronomie, Bonn, Germany).

[3]: The MIDI instrument is the result of a collaboration between German, Dutch and French institutes. See ESO PR 17/03 and ESO PR 25/02 for more information.

Original Source: ESO News Release

Posted on by Fraser Cain

Detailed View of Dione

A gorgeous Dione poses for Cassini, with shadowed craters and bright, wispy streaks first observed by the Voyager spacecraft 24 years ago. The wispy areas will be imaged at higher resolution in mid-December 2004. Subtle variations in brightness across the surface of this moon are visible here as well. Dione’s diameter is 1,118 kilometers, (695 miles).

The image shows primarily the trailing hemisphere of Dione, which is the side opposite the moon’s direction of motion in its orbit. The image has been rotated so that north is up.

The image was taken in visible light with the Cassini spacecraft narrow angle camera on Oct. 27, 2004, at a distance of about 1.2 million kilometers (746,000 miles) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 28 degrees. The image scale is 3.5 kilometers (2.2 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org .

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Next Station Crew Named

Image credit: NASA
Veteran NASA astronaut John Phillips and seasoned Russian Cosmonaut Sergei Krikalev are the next crew of the International Space Station. Their six-month mission is set for launch in April 2005.

Phillips and Krikalev are the eleventh crew for the orbiting research complex. Krikalev will serve as Station Commander, and Phillips is Flight Engineer and NASA International Space Station Science Officer. Designated Expedition 11, they will be on board the Station when the Space Shuttle makes its first Return to Flight mission. The Shuttle is scheduled to dock with the Space Station in May 2005.

Both crewmembers have previously been to the International Space Station. Phillips flew to the Station aboard the Shuttle on the STS-100 mission in 2001. During that 12-day mission, the crew installed the Canadarm2 Station robotic arm.

In 2000, Krikalev was a member of Expedition 1, the first International Space Station crew. Expedition 11 will be his sixth space flight and fourth long-duration mission. He has the most flights for any Russian cosmonaut.

Selected in 1985, Krikalev flew aboard the Mir Space Station in 1988-89, 1991-92 and the International Space Station in 2000-01. He flew aboard the Shuttle on the first joint U.S.-Russian mission, STS-60 in 1994, and on the first International Space Station assembly mission, STS-88 in 1998. Krikalev has accumulated 625 days in space. At the completion of a six-month stay aboard the Station on Expedition 11, Krikalev will have spent more time in space than any other person.

The Expedition 11 backup crewmembers are astronaut Daniel Tani and cosmonaut Mikhail Tyurin.

For more information about NASA astronauts, Russian cosmonauts, and the Space Station, on the Web, visit:

http://spaceflight.nasa.gov

For information about NASA missions and projects, visit: http://www.nasa.gov

Original Source: NASA News Release

Posted on by Fraser Cain

Crater Hale on Mars

This image, taken by the High Resolution Stereo Camera (HRSC) on board ESA?s Mars Express spacecraft, shows Crater Hale in the Argyre basin of the southern hemisphere of Mars.

The image shows an area close to the northern rim of the Argyre basin, located at latitude 36? South and longitude 324? East, and was taken with a ground resolution of about 40 metres per pixel during Mars Express orbit 533 in June 2004.

Slight periodic colour and brightness variations in parts of the image indicate atmospheric waves in clouds.

Crater Hale, with its terraced walls, central peak and a part of the inner ring is visible in the upper (eastern) part of the image. The region has been eroded heavily by deposits caused by this impact, and subsequent processes.

On the southern rim of Hale, parts of the crater wall have moved downslope towards the crater?s centre. At the bottom (western) part of the picture, the surface shows a network of fluvial channels which may have been caused by running water.

The HRSC experiment on ESA?s Mars Express mission is led by the Principal Investigator Prof. Dr Gerhard Neukum, of the Freie Universitaet Berlin, who also designed the camera. The science team for the experiment consists of 45 Co-Investigators from 32 institutions and 10 nations.

The camera was developed at the German Aerospace Center (DLR) and built in co-operation with industrial partners EADS Astrium, Lewicki Microelectronic GmbH and Jena-Optronik GmbH.

The HRSC is operated by the DLR Institute of Planetary Research, through ESA?s European Space Operations Centre in Darmstadt, Germany.

Image resolution has been decreased for use on the internet. The colour image was processed using the HRSC nadir (vertical view) and three colour channels.

Original Source: ESA News Release

Posted on by Fraser Cain

Best Views of Titan and Tethys

Image credit: NASA/JPL/SSI
New views of two of Saturn’s moons, Titan and Tethys, represent the most detailed look at these moons to date and show a sharp contrast between them — one is foggy and one is cratered.

The Cassini spacecraft captured the puzzle pieces for the full-disc view of the mysterious Titan during its first close encounter on Oct. 26, 2004. The mosaic comprises nine images taken at distances ranging from 650,000 kilometers (400,000 miles) to 300,000 kilometers (200,000 miles).

The pictures are available at http://saturn.jpl.nasa.gov, http://www.nasa.gov/cassini and http://ciclops.org.

The images that make up the mosaic were processed to reduce effects of the atmosphere and to sharpen surface features. The mosaic of images has been trimmed to show only the illuminated surface and not the atmosphere around the edge of the moon. The Sun was behind Cassini, so nearly the full disc was illuminated. South polar clouds are seen at the bottom.

Surface features are best seen near the center of the moon. The surface features become fuzzier toward the outside of the image, where the spacecraft is peering through more haze. The brighter region on the right side near the equator is named Xanadu Regio. Scientists are debating what processes may have created the bizarre surface brightness patterns seen there. Titan’s lack of obvious craters is a hint of a young surface. However, the exact nature of that activity, whether tectonic, wind-blown, river-related, marine or volcanic, is still unknown.

Two days after the close encounter with icy Titan, Cassini captured the images used in the mosaic of the battered and cratered moon Tethys. The result is the best-ever natural color view of Titan.

As seen here, the surface of Tethys has a neutral hue. Three images form this natural color composite. The mosaic reveals a world nearly saturated with craters — many small craters lie on top of older, larger ones, suggesting an ancient surface. Grooves can be seen at the top and along the boundary between day and night.

Tethys is known to have a density very close to that of water, indicating that it is likely composed mainly of water ice. Its frozen mysteries await Cassini’s planned close flyby in September 2005.

The images to create this mosaic were taken on Oct. 28, 2004, at a distance of about 256,000 kilometers (159,000 miles) from Tethys. This view shows the trailing hemisphere of Tethys, which is the side opposite the moon’s direction of motion in its orbit.

Both images were taken with the narrow angle camera onboard the Cassini spacecraft. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Tethys Hangs Under Saturn

This dazzling view looks beyond gigantic storms near Saturn’s south pole to the small but clear disc of Tethys (1,060 kilometers, or 659 miles, across). Clouds and ribbons of gas swirl about in the planet’s atmosphere in the foreground, while a tremendous chasm is visible on the icy moon.

The image was taken with the Cassini spacecraft narrow angle camera on Oct. 18, 2004, at a distance of 3.9 million kilometers (2.4 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 61 degrees. The view is in wavelengths of visible red light centered at 619 nanometers. The image scale is 23 kilometers (14 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org .

Original Source: NASA/JPL News Release