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In this illustration, the multicolored (blue and green) bubble represents the new measurements of the emission of particles known as energetic neutral atoms.
› Full image and caption

* NASA's Cassini spacecraft is helping to rewrite our understanding of the shape of our solar system as it moves through the local Milky Way galaxy.
* Previous models pictured our solar system as having a comet-like appearance. The new results suggest a picture more like a bubble.
* Cassini scientists created an image from this exotic region of space by detecting particles known as energetic neutral atoms.
* It complements data collected by NASA's Interstellar Boundary Explorer.

When NASA's Cassini spacecraft began orbiting Saturn five years ago, a dozen highly-tuned science instruments set to work surveying, sniffing, analyzing and scrutinizing the Saturnian system.

But Cassini recently revealed new data that appeared to overturn the decades-old belief that our solar system resembled a comet in shape as it moves through the interstellar medium (the matter between stars in our corner of the Milky Way galaxy).

Instead, the new results suggest our heliosphere more closely resembles a bubble - or a rat - being eaten by a boa constrictor: as the solar system passes through the "belly" of the snake, the ribs, which mimic the local interstellar magnetic field, expand and contract as the rat passes. An animation is available here.

"At first I was incredulous," said Tom Krimigis, principal investigator of the Magnetospheric Imaging Instrument (MIMI) at Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. "The first thing I thought was, 'What's wrong with our data?'"

Krimigis and his colleagues on the instrument team published the Cassini findings in the Nov. 13 issue of the journal Science, which featured complementary results from NASA's Interstellar Boundary Explorer (IBEX). Together, the results create the first map of the heliosphere and its thick outer layer known as the heliosheath, where solar wind streaming out from the sun gets heated and slowed as it interacts with the interstellar medium.

The Cassini data also provide a much more direct indication of the thickness of the heliosheath, whereas scientists previously had to rely on calculations from models. The new results from Cassini show that the heliosheath is about 40 to 50 astronomical units (3.7 billion to 4.7 billion miles) thick and that NASA's twin Voyager spacecraft, which are traveling through the heliosheath now, will cross into true interstellar space well before the year 2020. Estimates as far out as 2030 had been suggested.

"These new data from Cassini really redefine our sense of our home in the galaxy, and we can now do better studies of whether our solar system resembles those elsewhere," Krimigis said.

The Voyagers have sent back rich data on the heliosphere and heliosheath, but just at two locations. Scientists want more context. One way to learn about the region is to track energetic neutral atoms streaming back toward the sun from the heliosheath.

Energetic neutral atoms form when cold, neutral gas collides with electrically-charged particles in a cloud of plasma, which is a gas-like state of matter so hot that the atoms split into an ion and an electron. The positively-charged ions in plasma can't reclaim their own electrons, which are moving too fast, but they can steal an electron from the cold gas atoms. Since the resulting particles are neutrally charged, they are able to escape magnetic fields and zoom off into space. The emission of these particles often occurs in the magnetic fields surrounding planets, but also happens when the solar wind mingles with the interstellar medium.

How did Cassini, with 22,000 wire connections and 14 kilometers (8.7 miles) of cabling specifically tweaked to get the most out of its investigation of the solar system's second largest gas bag, recently end up helping to redefine how we look at our entire solar system?

Krimigis and his Cassini colleagues working with MIMI weren't sure their instrument could pick up emissions from far-out, exotic locations, such as from the boundary of our heliosphere, the region of our sun's influence.

Last year, after spending four years focused on the energetic electrons and ions trapped in the magnetic field that surrounds Saturn, as well as the offspring of these particles known as energetic neutral atoms, the team started combing through the data from the instrument's Ion and Neutral Camera, looking for particles arriving from far beyond Saturn.

"We thought we could get some hits from energetic neutral atoms from the heliosheath because Cassini has really been in an excellent position to detect these particles," said Don Mitchell, MIMI instrument scientist and a researcher at the Applied Physics Laboratory.

Cassini was farther away from the sun than previous spacecraft trying to image the heliosphere and even swung very far away from Saturn on some of its orbits, Mitchell said. The data would likely be free of much of the interference that hampered other efforts.

Mitchell, Krimigis and their team were able to stitch together data from late 2003 to the summer of 2009. They created a color-coded map of the intensity of the energetic neutral atoms and discovered a belt of hot, high-pressure particles where the interstellar wind flowed by our heliosheath bubble.

The data matched up nicely with the IBEX images of lower-energy particles and connected that data set to the Voyager data on higher-energy particles.

"I was initially skeptical because the instrument was designed for Saturn's magnetosphere," Mitchell said, "But our camera had long exposures of months to years, so we could accumulate and map each particle that streamed through the tiny aperture from the far reaches of the heliosphere. It was luck, but also a lot of hard work."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif. manages the mission for NASA's Science Mission Directorate, Washington, D.C.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

NASA's Cassini spacecraft will fly by Saturn's moon Enceladus this weekend for a last peek at the intriguing "tiger stripes" before winter darkness blankets the area for several years.

Scientists are particularly interested in the tiger stripes, which are fissures in the south polar region, because they spew jets of water vapor and other particles hundreds of kilometers, or miles, from the surface.

The flyby, which is sometimes called "E8" because it is the eighth targeted flyby of Enceladus, is scheduled for Saturday, Nov. 21 UTC, which is the evening of Friday, Nov. 20 in U.S. time zones. Cassini team members expect to fly the spacecraft to within about 1,600 kilometers (1,000 miles) of the moon's surface, at around 82 degrees south latitude. This will be a more distant flyby than the one on Nov. 2, when Cassini flew about 100 kilometers (60 miles) above the surface.

During this flyby, scientists will focus on a tiger stripe called Baghdad Sulcus and create a contiguous thermal map of the feature. The spacecraft will also be snapping high-resolution images of the southern part of the Saturn-facing hemisphere.

For more information on the flyby, click here.

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NASA's Dawn spacecraft re-entered our solar system's asteroid belt today, Nov. 13, and this time it will stay there.

Dawn first entered the belt (whose lower boundary may be defined as the greatest distance Mars gets from the sun (249,230,000 kilometers, or 154,864,000 miles) in June 2008. It remained within the belt for 40 days before its carefully planned orbital path brought it below the asteroid belt's lower boundary.

This time around, Dawn's flight path will remain above this hypothetical lower boundary for the rest of the mission and for the foreseeable future - Dawn will become the first human-made object to take up permanent residence in the asteroid belt.

The mission of the 1,098-kilogram (2,421-pound) Dawn spacecraft is to reconnoiter Vesta and Ceres, the asteroid belt's two most massive inhabitants -- the asteroid Vesta and dwarf planet Ceres. The goal of this eight-year, 4.9-billion-kilometer (3-billion-mile) mission is to answer basic questions about the formation of planets in our solar system. NASA's unmanned Dawn spacecraft will be the first ever to orbit two planetary bodies on a single voyage. Dawn is 619 days away from arrival at its first celestial objective, asteroid Vesta.

For more information on Dawn please visit: http://dawn.jpl.nasa.gov/

Massive, bright clouds of tiny ice particles hover above the darkened rings of Saturn in an image captured by the Cassini spacecraft on Sept. 22, 2009, around the time of Saturn's equinox.

During this period, sunlight hits the rings edge-on and shines directly over the equator. The levitating icy particle clouds, which are known as "spokes" and are as wide as 10,000 kilometers (6,000 miles), appear particularly dramatic because of the unique lighting geometry of the equinox period.

The particles that make up spokes levitate above the ring plane when they acquire an electrostatic charge, the way static electricity on Earth can raise the hair on your arms. The spoke particles appear to acquire more charge during dim conditions and, during equinox, the bulk of the rings are in shadow. That angle of light also brightens features that stick out of the ring plane.

Saturn's exact equinox was Aug. 11, 2009, but there were extreme lighting conditions a few weeks before and after that date. Cassini's wide-angle camera captured this image six weeks later, when the spacecraft was about 1.3 million kilometers (808,000 miles) away from Saturn. The spokes appear in Saturn's B ring. Scientists are not yet sure how high these spokes hover above the ring plane. The bright dot on the left side of the image is Saturn's moon Janus.

Spokes in Saturn's rings were first discovered in the late 1980s by NASA's Voyager spacecraft. The features disappeared from view between 1998 and 2005, likely because of the angle of sunlight on the rings. To learn more about spokes, click here. A movie of spokes dancing around the rings in 2008 is available here.

On its way to a 2014 rendezvous with comet 67P/Churyumov-Gerasimenko, the European Space Agency's Rosetta spacecraft, with JPL instruments on board, flew past Earth today to pick up energy.

This is the third and final Earth flyby for Rosetta. It will provide exactly the boost Rosetta needs to continue into the outer solar system. The spacecraft is scheduled for a close encounter with asteroid 21 Lutetia in July 2010, before it goes into hibernation early in 2011, only to wake up in early 2014 for approach to 67P/Churyumov-Gerasimenko.

NASA has contributed an ultraviolet instrument (Alice); a plasma instrument (the Ion and Electron Sensor), and a microwave instrument (Microwave Instrument for the Rosetta Orbiter), among other contributions to this international mission. JPL manages NASA's participation in the Rosetta mission.

Learn more about NASA's contribution to Rosetta at: http://rosetta.jpl.nasa.gov