Cassini Completes Final -- and Fateful -- Titan Flyby
This unprocessed image of Saturn's moon Titan was captured by NASA's Cassini spacecraft during its final close flyby of the hazy, planet-sized moon on April 21, 2017.
Credits: NASA/JPL-Caltech/Space Science Institute
This unprocessed image of Saturn's moon Titan was captured by NASA's Cassini spacecraft during its final close flyby of the hazy, planet-sized moon on April 21, 2017.
Credits: NASA/JPL-Caltech/Space Science Institute
This unprocessed image of Saturn's moon Titan was captured by NASA's Cassini spacecraft during its final close flyby of the hazy, planet-sized moon on April 21, 2017.
Credits: NASA/JPL-Caltech/Space Science Institute
NASA's Cassini spacecraft has had its last close brush with Saturn's hazy moon Titan and is now beginning its final set of 22 orbits around the ringed planet.
The spacecraft made its 127th and final close approach to Titan on April 21 at 11:08 p.m. PDT (2:08 a.m. EDT on April 22), passing at an altitude of about 608 miles (979 kilometers) above the moon's surface.
Cassini transmitted its images and other data to Earth following the encounter. Scientists with Cassini's radar investigation will be looking this week at their final set of new radar images of the hydrocarbon seas and lakes that spread across Titan's north polar region. The planned imaging coverage includes a region previously seen by Cassini's imaging cameras, but not by radar. The radar team also plans to use the new data to probe the depths and compositions of some of Titan's small lakes for the first (and last) time, and look for further evidence of the evolving feature researchers have dubbed the "magic island."
"Cassini's up-close exploration of Titan is now behind us, but the rich volume of data the spacecraft has collected will fuel scientific study for decades to come," said Linda Spilker, the mission's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California.
Gateway to the Grand Finale
The flyby also put Cassini on course for its dramatic last act, known as the Grand Finale. As the spacecraft passed over Titan, the moon's gravity bent its path, reshaping the robotic probe's orbit slightly so that instead of passing just outside Saturn's main rings, Cassini will begin a series of 22 dives between the rings and the planet on April 26. The mission will conclude with a science-rich plunge into Saturn's atmosphere on Sept. 15.
"With this flyby we're committed to the Grand Finale," said Earl Maize, Cassini project manager at JPL. "The spacecraft is now on a ballistic path, so that even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn's atmosphere on Sept. 15 no matter what."
Cassini received a large increase in velocity of approximately 1,925 mph (precisely 860.5 meters per second) with respect to Saturn from the close encounter with Titan.
After buzzing Titan, Cassini coasted onward, reaching the farthest point in its orbital path around Saturn at 8:46 p.m. PDT (11:46 p.m. EDT) on April 22. This point, called apoapse, is where each new Cassini lap around Saturn begins. Technically, Cassini began its Grand Finale orbits at this time, but since the excitement of the finale begins in earnest on April 26 with the first ultra-close dive past Saturn, the mission is celebrating the latter milestone as the formal beginning of the finale.
The spacecraft's first finale dive will take place on April 26 at 2 a.m. PDT (5 a.m. EDT). The spacecraft will be out of contact during the dive and for about a day afterward while it makes science observations from close to the planet. The earliest time Cassini is scheduled to make radio contact with Earth is 12:05 a.m. PDT (3:05 a.m. EDT) on April 27. Images and other data are expected to begin flowing in shortly after communication is established.
Some key numbers for Cassini's Grand Finale and final plunge into Saturn.
Credits: NASA/JPL-Caltech
A new narrated, 360-degree animated video gives viewers a sense of what it might be like to fly alongside Cassini as it makes one of its Grand Finale dives.
NASA VR: Cassini's Grand Finale (360 view)
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Dark Chasm
The low angle of the sun over Tethys' massive canyon, Ithaca Chasma (near the terminator, at right), highlights the contours of this enormous rift.
Ithaca Chasma is up to 60 miles (100 kilometers) wide, and runs nearly three-fourths of the way around icy Tethys (660 miles or 1,062 kilometers across). The canyon has a maximum depth of nearly 2.4 miles (4 kilometers) deep.
The giant crater Odysseus -- usually one of Tethys’ most recognizable features-- is barely seen in profile along the limb, at upper left.
This view looks toward the Saturn-facing hemisphere of Tethys. North on Tethys is up and rotated 5 degrees to the left. The image was taken in green light with the Cassini spacecraft narrow-angle camera on Jan. 30, 2017.
The view was obtained at a distance of approximately 221,000 miles (356,000 kilometers) from Tethys. Image scale is 1 mile (2 kilometers) per pixel.
The Cassini mission is a cooperative project of NASA, ESA (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 mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.
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Pan Anaglyph (3-D)
These stereo views, or anaglyphs, highlight the unusual, quirky shape of Saturn's moon Pan. They appear three-dimensional when viewed through red-blue glasses with the red lens on the left.
The views show the northern and southern hemispheres of Pan, at left and right, respectively. They have been rotated to maximize the stereo effect.
Standard (non-stereo) versions of these views are presented in PIA21436.
Pan has an average diameter of 17 miles (28 kilometers). The moon orbits within the Encke Gap in Saturn's A ring. See PIA09868 and PIA11529for more distant context views of Pan.
Both of these views look toward Pan's trailing side, which is the side opposite the moon's direction of motion as it orbits Saturn.
These views were acquired by the Cassini narrow-angle camera on March 7, 2017, at distances of approximately 16,000 miles or 25,000 kilometers (left view) and 21,000 miles or 34,000 kilometers (right view).
Image scale in the original images is about 500 feet (150 meters) per pixel (left view) and about 650 feet (200 meters) per pixel (right view). The images have been magnified by a factor of two from their original size.
Quelle: NASA
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Update: 27.04.2017
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Nasa waits on Cassini radio contact from Saturn
Controllers and scientists must wait until Thursday to hear from Cassini.
The probe was due early on Wednesday to make the first of 22 dives in between Saturn's cloudtops and the inner edge of its spectacular rings.
The daredevil flights are designed to gather pictures and other science data of unprecedented resolution.
But Cassini was out of radio contact for the duration of the plunge and is not scheduled to re-establish communications for another day.
Because the probe was moving so fast - at over 110,000km/h (70,000mph) - there was some risk attached to flying through the ring plane.
An impact with even a tiny ice or rock particle at that velocity could do a lot of damage, and so the decision was made to point Cassini's big antenna in the direction of travel, to act as a shield.
But, of course, that meant it could not also then talk to Earth at the same time.
Assuming all goes well, 21 similar dives will be made over the course of the next five months before the probe dumps itself in the atmosphere of Saturn. With so little fuel left in its tanks, Cassini cannot continue its mission for much longer.
The US space agency (Nasa) is calling the gap-runs the "grand finale", in part because of their ambition. They promise pictures of unparalleled resolution and science data that finally unlocks key puzzles about the make-up and history of this huge world.
"We're going to top off this mission with a lot of new measurements - some amazing new data," said Athena Coustenis from the Paris Observatory in Meudon, France.
"We're expecting to get the composition, structure and dynamics of the atmosphere, and fantastic information about the rings," she told the BBC.
A key objective is to determine the mass and therefore the age of the rings. The more massive they are, the older they are likely to be - perhaps as old as Saturn itself.
Scientists will do this by studying how the velocity of the probe is altered as it flies through the gravity field generated by the planet and the great encircling bands of ice.
"In the past, we were not able to determine the mass of the rings because Cassini was flying outside them," explained Luciano Iess of the Sapienza University of Rome, Italy.
"Essentially, the contribution of the rings to the gravity field was mixed up with the oblateness of Saturn. It was impossible. But by flying between the rings and the planet, Cassini will be able to disentangle the two effects.
"We're able to tell the velocity of Cassini to an accuracy of a few microns per second. This is indeed fantastic when you think Cassini is more than one billion kilometres away from the Earth."
Having the mass number might not straightforwardly resolve the age issue, however, cautioned Nicolas Altobelli, who is project scientist for Nasa's Cassini mission partner, the European Space Agency.
"We still need to understand the rings' composition. They are made of very nearly pure water-ice. If they're very old, formed at the same time as Saturn, how come they still look so fresh when they're constantly bombarded with meteorite material?" he pondered.
One possibility is that the rings are actually very young, perhaps the remains of a giant comet that got too close to Saturn and broke apart into innumerable fragments.
Coustenis, Iess and Altobelli discussed the end phases of the Cassini mission here in Vienna at the General Assembly of the European Geosciences Union.
The earliest that Cassini is expected to radio home is 07:00 GMT (08:00 BST) on Thursday. Contact should come through Nasa's 70m-wide Deep Space Network antenna at Goldstone, California.
If a stable communications link is established, pictures and other data ought to start coming down about half an hour later.