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Fresh Findings From Cassini and First Light from a Gravitational-Wave Event.

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October 16, 2017
Fresh Findings From Cassini

NASA's Cassini spacecraft ended its journey on Sept. 15 with an intentional plunge into the atmosphere of Saturn, but analysis continues on the mountain of data the spacecraft sent during its long life. Some of the Cassini team's freshest insights were presented during a news conference today at the American Astronomical Society Division for Planetary Science meeting in Provo, Utah.

Among the findings being shared:

-- Views from Cassini's Grand Finale show the beauty of the rings and demonstrate processes similar to those that form planets.

During Cassini's final months, the spacecraft's cameras captured views from within the gap between the planet and the rings, and the mission is releasing two new image mosaics showing the rings from that unique perspective. One view, from May 28, 2017, shows the rings emerging from behind the planet's hazy limb, while the planet itself is adorned with ring shadows. The other mosaic shows a panoramic view outward across the ringscape.

Researchers also shared a new movie of Saturn's auroras in ultraviolet light that represents the final such view from the spacecraft's Ultraviolet Imaging Spectrometer.

In addition, Cassini participating scientist and imaging team associate Matt Tiscareno of SETI Institute, Mountain View, California, provided new details about the whimsically named ring features called propellers, which are wakes in the rings created by small, unseen moonlets. The propellers are analogous to baby planets forming in disks around young stars, as they obey similar physical processes.

Tiscareno said that, in its last images of the rings (taken the day before the spacecraft's plunge into Saturn), Cassini successfully imaged all six of the propellers whose orbits were being tracked over the last several years of the mission. These objects are named for famous aviators: Blériot, Earhart, Santos-Dumont, Sikorsky, Post and Quimby. During its Ring-Grazing Orbits -- the four months of close orbits that preceeded the mission's Grand Finale -- Cassini obtained images showing swarms of smaller propellers, astounding Tiscareno and colleagues.

-- Cassini's electronic "nose" hit the jackpot, finding many surprises as it sniffed the gases in the previously unexplored space between the planet and the rings.

The spacecraft's Ion and Neutral Mass Spectrometer (INMS) returned a host of first-ever direct measurements of the components in Saturn's upper atmosphere, which stretches almost to the rings. From these observations, the team sees evidence that molecules from the rings are raining down onto the atmosphere. This influx of material from the rings was expected, but INMS data show hints of ingredients more complex than just water, which makes up the bulk of the rings' composition. In particular, the instrument detected methane, a volatile molecule that scientists would not expect to be abundant in the rings or found so high in Saturn's atmosphere. Cassini participating scientist and INMS team associate Mark Perry from the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, says the team is busy analyzing data from the final, lowest-altitude passes, which show even more complexity and variability. The INMS observations complement those by Cassini's Cosmic Dust Analyzer instrument, which sampled solid particles in the gap during the Grand Finale.

-- Researchers continue trying to wrangle insights about the length of the planet's day from measurements of Saturn's magnetic field.

Michele Dougherty, leader of Cassini's Magnetometer team from Imperial College London, provided an update on the team's progress in trying to determine whether Saturn's magnetic field has a detectable tilt. One aim of their work is to determine the precise length of time for the planet's internal rotation, which would help researchers nail down the true length of the planet's day. Dougherty says the sensitivity of Cassini's magnetic field measurements nearly quadrupled over the course of the spacecraft's 22 Grand Finale orbits -- meaning that, if the tilt of Saturn's field is greater than 0.016 degrees, researchers should be able to detect it. An extremely small tilt is challenging to explain with scientists' current understanding of how planetary magnetic fields are generated, thus suggesting more sophisticated dynamics inside Saturn.

-- New theoretical research explains the forces that keep Saturn's rings from spreading out and dispersing. It turns out to be a group effort.

Key among the questions scientists hope to answer using data from Cassini are the age and origins of the rings. Theoretical modeling has shown that, without forces to confine them, the rings would spread out over hundreds of millions of years -- much younger than Saturn itself. This spreading happens because faster-moving particles that orbit closer to Saturn occasionally collide with slower particles on slightly farther-out orbits. When this happens, some momentum from the faster particles is transferred to the slower particles, speeding the latter up in their orbit and causing them to move farther outward. The inverse happens to the faster, inner particles.

Previous research had shown that gravitational tugs from the moon Mimas are solely responsible for halting the outward spread of Saturn's B ring -- that ring's outer edge is defined by the dark region known as the Cassini Division. Ring scientists had thought the small moon Janus was responsible for confining the outer edge of the A ring. But a new modeling study led by Radwan Tajeddine of Cornell University, Ithaca, New York, shows that the A ring's outward creep is kept in check by a confederation of moons, including Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and Mimas.

The insight was made possible by Cassini, which provided scientists with high-resolution views of intricate waves in the rings, along with precise determinations of the masses of Saturn's moons. Analysis of these data led Tajeddine and colleagues to an understanding that a cumulative effect of waves from all these moons damps the outward transfer of momentum in the A ring and confines its edge.

Tajeddine will present these results in a poster at the DPS meeting, and they will be published Wednesday in the Astrophysical Journal.

"There are whole careers to be forged in the analysis of data from Cassini," said Linda Spilker, the mission's project scientist at NASA's Jet Propulsion Laboratory, Pasadena, California. "In a sense, the work has only just begun."

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.

More information about Cassini:



News Media Contact
Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.


NASA Missions Catch First Light from a Gravitational-Wave Event

For the first time, NASA scientists have detected light tied to a gravitational-wave event,
thanks to two merging neutron stars in the galaxy NGC 4993, located about 130 million
light-years from Earth in the constellation Hydra.

Shortly after 5:41 a.m. PDT (8:41 a.m. EDT) on Aug. 17, NASA's Fermi Gamma-ray
Space Telescope picked up a pulse of high-energy light from a powerful explosion,
which was immediately reported to astronomers around the globe as a short gamma-ray burst.
The scientists at the National Science Foundation's Laser Interferometer Gravitational-wave
Observatory (LIGO) detected gravitational waves dubbed GW170817 from a pair of smashing stars
tied to the gamma-ray burst, encouraging astronomers to look for the aftermath
of the explosion. Shortly thereafter, the burst was detected as part of a follow-up analysis
by ESA's (European Space Agency's) INTEGRAL satellite.

NASA's Swift, Hubble, Chandra and Spitzer missions, along with dozens
of ground-based observatories, including the NASA-funded Pan-STARRS survey,
later captured the fading glow of the blast's expanding debris.

"This is extremely exciting science," said Paul Hertz, director of NASA's Astrophysics
Division at the agency's headquarters in Washington. "Now, for the first time, we've seen
light and gravitational waves produced by the same event. The detection
of a gravitational-wave source's light has revealed details of the event that cannot
be determined from gravitational waves alone. The multiplier effect
of study with many observatories is incredible."

Neutron stars are the crushed, leftover cores of massive stars that previously
exploded as supernovas long ago. The merging stars likely had masses between
10 and 60 percent greater than that of our Sun, but they were no wider
than Washington, D.C. The pair whirled around each other hundreds of times
a second, producing gravitational waves at the same frequency.
As they drew closer and orbited faster, the stars eventually broke apart
and merged, producing both a gamma-ray burst and a rarely seen
flare-up called a "kilonova."

"This is the one we've all been waiting for," said David Reitze, executive director
of the LIGO Laboratory at Caltech in Pasadena, California. "Neutron star mergers produce
a wide variety of light because the objects form a maelstrom
of hot debris when they collide. Merging black holes -- the types of events LIGO
and its European counterpart, Virgo, have previously seen --
very likely consume any matter around them long before they crash,
so we don't expect the same kind of light show."

"The favored explanation for short gamma-ray bursts is that they're
caused by a jet of debris moving near the speed of light produced
in the merger of neutron stars or a neutron star and a black hole,"
said Eric Burns, a member of Fermi's Gamma-ray Burst Monitor team
at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"LIGO tells us there was a merger of compact objects, and Fermi tells us
there was a short gamma-ray burst. Together, we know that what we observed
was the merging of two neutron stars, dramatically confirming the relationship."

Within hours of the initial Fermi detection, LIGO and the Virgo detector
at the European Gravitational Observatory near Pisa, Italy, greatly refined
the event's position in the sky with additional analysis of gravitational wave data.
Ground-based observatories then quickly located a new optical and infrared source --
the kilonova -- in NGC 4993.

To Fermi, this appeared to be a typical short gamma-ray burst, but it occurred
less than one-tenth as far away as any other short burst with a known distance,
making it among the faintest known. Astronomers are still trying
to figure out why this burst is so odd, and how this event relates to the more
luminous gamma-ray bursts seen at much greater distances.

NASA's Swift, Hubble and Spitzer missions followed the evolution of the kilonova
to better understand the composition of this slower-moving material,
while Chandra searched for X-rays associated with the remains of the ultra-fast jet.

When Swift turned to the galaxy shortly after Fermi's gamma-ray burst detection,
it found a bright and quickly fading ultraviolet (UV) source.

"We did not expect a kilonova to produce bright UV emission,"
said Goddard's S. Bradley Cenko, principal investigator for Swift.
"We think this was produced by the short-lived disk of debris
that powered the gamma-ray burst."

Over time, material hurled out by the jet slows and widens as it sweeps up
and heats interstellar material, producing so-called afterglow emission that includes X-rays.

But the spacecraft saw no X-rays -- a surprise for an event that produced higher-energy gamma rays.

NASA's Chandra X-ray Observatory clearly detected X-rays nine days after the source was discovered.
Scientists think the delay was a result of our viewing angle, and it took time for the jet directed toward Earth
to expand into our line of sight.

"The detection of X-rays demonstrates that neutron star mergers can form powerful
jets streaming out at near light speed," said Goddard's Eleonora Troja, who led one
of the Chandra teams and found the X-ray emission.We had to wait for nine days
to detect it because we viewed it from the side, unlike anything we had seen before."

On Aug. 22, NASA's Hubble Space Telescope began imaging the kilonova and capturing
its near-infrared spectrum, which revealed the motion and chemical composition
of the expanding debris.

"The spectrum looked exactly like how theoretical physicists had predicted the outcome
of the merger of two neutron stars would appear," said Andrew Levan at the University
of Warwick in Coventry, England, who led one of the proposals for Hubble spectral observations.
"It tied this object to the gravitational wave source beyond all reasonable doubt."

Astronomers think a kilonova's visible and infrared light primarily arises through heating
from the decay of radioactive elements formed in the neutron-rich debris.
Crashing neutron stars may be the universe's dominant source for many
of the heaviest elements, including platinum and gold.

Because of its Earth-trailing orbit, Spitzer was uniquely situated to observe
the kilonova long after the Sun moved too close to the galaxy
for other telescopes to see it. Spitzer's Sept. 30 observation captured
the longest-wavelength infrared light from the kilonova, which unveils
the quantity of heavy elements forged.

"Spitzer was the last to join the party, but it will have the final word
on how much gold was forged," says Mansi Kasliwal, Caltech assistant professor
and principal investigator of the Spitzer observing program.

Numerous scientific papers describing and interpreting these observations
have been published in Science, Nature, Physical Review Letters
and The Astrophysical Journal.

Gravitational waves were directly detected for the first time in 2015 by LIGO,
whose architects were awarded the 2017 Nobel Prize in physics for the discovery.

News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, CA

Felicia Chou
NASA Headquarters, Washington

Dewayne Washington
Goddard Space Flight Center, Greenbelt, Md.

Molly Porter
Marshall Space Flight Center, Huntsville, Ala.



George Stephen Morrison (Rome (Géorgie), 7 janvier 1919 - Coronado (Californie),
17 novembre 2008) est un amiral et aviateur naval de la marine des États-Unis.
Morrison a été le commandant des forces navales américaines dans le golfe du Tonkin
au cours de l'incident du Golfe du Tonkin d'août 1964, qui a servi de prétexte à l'engagement
des États-Unis dans la guerre du Viêt Nam.

Il est le père de Jim Morrison, le chanteur du groupe de rock The Doors1,2,3.

La chouette effraie

Voir le profil de l'utilisateur
At #DPS17? Visit the #K2Mission and @NASA_
TESS booths. We provide high-precision lightcurves
for hundreds of faint Solar System objects!

One of the most mysterious stellar objects may be revealing some of its secrets at last.

Called KIC 8462852, also known as Boyajian’s Star, or Tabby's Star, the object has experienced unusual dips in brightness -- NASA's Kepler space telescope even observed dimming of up to 20 percent over a matter of days. In addition, the star has had much subtler but longer-term enigmatic dimming trends, with one continuing today. None of this behavior is expected for normal stars slightly more massive than the Sun. Speculations have included the idea that the star swallowed a planet that it is unstable, and a more imaginative theory involves a giant contraption or "megastructure" built by an advanced civilization, which could be harvesting energy from the star and causing its brightness to decrease.

A new study using NASA's Spitzer and Swift missions, as well as the Belgian AstroLAB IRIS observatory, suggests that the cause of the dimming over long periods is likely an uneven dust cloud moving around the star. This flies in the face of the "alien megastructure" idea and the other more exotic speculations.

The smoking gun: Researchers found less dimming in the infrared light from the star than in its ultraviolet light. Any object larger than dust particles would dim all wavelengths of light equally when passing in front of Tabby's Star.

"This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming," said Huan Meng, at the University of Arizona, Tucson, who is lead author of the new study published in The Astrophysical Journal. "We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period."

Why Dust is Likely

We experience the uniform dimming of light often in everyday life: If you go to the beach on a bright, sunny day and sit under an umbrella, the umbrella reduces the amount of sunlight hitting your eyes in all wavelengths. But if you wait for the sunset, the sun looks red because the blue and ultraviolet light is scattered away by tiny particles. The new study suggests the objects causing the long-period dimming of Tabby's Star can be no more than a few micrometers in diameter (about one ten-thousandth of an inch).

From January to December 2016, the researchers observed Tabby's Star in ultraviolet using Swift, and in infrared using Spitzer. Supplementing the space telescopes, researchers also observed the star in visible light during the same period using AstroLAB IRIS, a public observatory with a 27-inch-wide (68 centimeter) reflecting telescope located near the Belgian village of Zillebeke.

Based on the strong ultraviolet dip, the researchers determined the blocking particles must be bigger than interstellar dust, small grains that could be located anywhere between Earth and the star. Such small particles could not remain in orbit around the star because pressure from its starlight would drive them farther into space. Dust that orbits a star, called circumstellar dust, is not so small it would fly away, but also not big enough to uniformly block light in all wavelengths. This is currently considered the best explanation, although others are possible.

Collaboration with Amateur Astronomers

Citizen scientists have had an integral part in exploring Tabby's Star since its discovery. Light from this object was first identified as "bizarre" and "interesting" by participants in the Planet Hunters project, which allows anyone to search for planets in the Kepler data. That led to a 2016 study formally introducing the object, which is nicknamed for Tabetha Boyajian, now at Louisiana State University, Baton Rouge, who was the lead author of the original paper and is a co-author of the new study. The recent work on long-period dimming involves amateur astronomers who provide technical and software support to AstroLAB.

Several AstroLAB team members who volunteer at the observatory have no formal astronomy education. Franky Dubois, who operated the telescope during the Tabby's Star observations, was the foreman at a seat belt factory until his retirement. Ludwig Logie, who helps with technical issues on the telescope, is a security coordinator in the construction industry. Steve Rau, who processes observations of star brightness, is a trainer at a Belgian railway company.

Siegfried Vanaverbeke, an AstroLAB volunteer who holds a Ph.D. in physics, became interested in Tabby's Star after reading the 2016 study, and persuaded Dubois, Logie and Rau to use Astrolab to observe it.

"I said to my colleagues: 'This would be an interesting object to follow,'" Vanaverbeke recalled. "We decided to join in."

University of Arizona astronomer George Rieke, a co-author on the new study, contacted the AstroLAB group when he saw their data on Tabby's Star posted in a public astronomy archive. The U.S. and Belgium groups teamed up to combine and analyze their results.

Future Exploration

While study authors have a good idea why Tabby's Star dims on a long-term basis, they did not address the shorter-term dimming events that happened in three-day spurts in 2017. They also did not confront the mystery of the major 20-percent dips in brightness that Kepler observed while studying the Cygnus field of its primary mission. Previous research with Spitzer and NASA's Wide-field Infrared Survey Explorer suggested a swarm of comets may be to blame for the short-period dimming. Comets are also one of the most common sources of dust that orbits stars, and so could also be related to the long-period dimming studied by Meng and colleagues.

Now that Kepler is exploring other patches of sky in its current mission, called K2, it can no longer follow up on Tabby's Star, but future telescopes may help unveil more secrets of this mysterious object.

"Tabby's Star could have something like a solar activity cycle. This is something that needs further investigation and will continue to interest scientists for many years to come," Vanaverbeke said.

NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Swift mission in collaboration with Pennsylvania State University in University Park, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corp. in Dulles, Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.

NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena, California. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit:



Related story

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.

Last Updated: Oct. 5, 2017
Editor: Tony Greicius
Tags: Jet Propulsion Laboratory, Spitzer Space Telescope, Stars, Swift, Universe


America - A Horse With No Name

George Stephen Morrison (Rome (Géorgie), 7 janvier 1919 - Coronado (Californie),
17 novembre 2008) est un amiral et aviateur naval de la marine des États-Unis.
Morrison a été le commandant des forces navales américaines dans le golfe du Tonkin
au cours de l'incident du Golfe du Tonkin d'août 1964, qui a servi de prétexte à l'engagement
des États-Unis dans la guerre du Viêt Nam.

Il est le père de Jim Morrison, le chanteur du groupe de rock The Doors1,2,3.

La chouette effraie

Voir le profil de l'utilisateur

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