Cassini's landmark tour of Saturn begins July 1, 2004 UTC with the Saturn Orbit Insertion (SOI) engine burn. This burn will slow the spacecraft down, allowing it to be captured by Saturn's gravity. After the spacecraft is captured, it will begin a 4-year tour of scientific exploration of the ringed planet, its moons, and magnetosphere.
The spacecraft will make 74 unique orbits around the planet, using 45 close flybys of Saturn's largest moon Titan for gravity assist and science data acquisition. Because of the sheer size of Titan, the flybys will allow for major changes in orbital paths, allowing engineers to minimize fuel use while maximizing science data collection.
Cassini's tour of the Saturn system is divided into 6 different segments. Each segment contains many Saturn and Titan flybys as well as opportunities to study the different smaller satellites.
Highlights of the Saturn Tour
74 Orbits of Saturn
45 Close flybys of Titan
8 close "targeted" flybys of other satellites:
3 close flybys of Enceladus
30 additional satellite flybys at distances less than 100,000 kilometers
(about 62,100 miles)
Many Saturn and Ring occultation opportunities
One "Titan 180 degree transfer
One high inclination sequence
Cassini's Current Position
Orbit Insertion and Probe Release: July
1, 2004 through February 15, 2005 Occultation Sequence:
February 15, 2005
through September 7, 2005 Petal Rotation
& Magnetotail Petal: September 7, 2005
through July 22, 2006 Titan 180 Transfer:
July 22, 2006 through
June 30, 2007 Rotation/Icy Satellites:
June 30, 2007 through
August 31, 2007 August 31, 2007
through July 1, 2008
This is the first part of Cassini's journey at Saturn. The arrival of the spacecraft at Saturn (through SOI, Saturn Orbital Insertion) and the deployment of the Probe to Titan (January 2005) are major scientific milestones. These events determine the mission's success, and their results will shape the future of exploration around the ringed planet. and Huygens Mission websites.
Saturn's rings oscillate on a 15-year cycle. During this cycle observers on Earth can see the rings clearly because they are tilted or "open." The rings will then rotate and become nearly invisible to observers on Earth because the view becomes "edge-on" or "closed." When Cassini first arrives at Saturn, the rings are tilted, or open. This configuration of the rings allows Cassini to see the Earth and Sun become obscured by the rings. This phenomenon is called an "occultation" and provides unique scientific opportunities allowing scientists to derive information about the structure and evolution of the ring system.
Moving the spacecraft around in its orbit as viewed by Earth provides important science opportunities. It allows scientists to capture data in a wide swath of the planet's magnetosphere. One unique region is the magnetotail. This is the region facing away from the Sun where the planet's magnetosphere is squeezed by the solar wind
Early in the Saturn tour, Cassini passes Titan on its way in to Saturn. This is what engineers call an "inbound" pass. By shifting the spacecraft's encounters with Titan to the outbound leg, the spacecraft's orientation with respect to the Sun is flipped 180 degrees. This process takes several orbits of Saturn allowing engineers to move Cassini's orbit with respect to capture different views of Saturn.
Immediately following the Titan 180 transfer sequence, the spacecraft makes several close flybys of the icy satellites to study these enigmatic moons in detail
High Inclination Sequence:
Studying Saturn at steep angles (called high inclinations) is of extreme interest to scientists. Viewing Saturn's polar regions provide opportunities for studying Saturn's rings and magnetosphere. The high inclination sequence also allows for radio (from Earth), solar, and stellar occultations of Saturn, Titan, and the ring system. Such studies allow scientists to understand the nature of the material in Saturn, Titan, and the rings.
Saturn Orbit Insertion and Probe Release:
July 1, 2004 through February 15, 2005
February 15, 2005 through September 7, 2005
Petal Rotation & Magnetotail
September 7, 2005 through July 22, 2006
Titan 180 Transfer:
July 22, 2006 through June 30, 2007
June 30, 2007 through August 31, 2007
August 31, 2007 through July 1, 2008
orbit: 1,221,830 km from Saturn
Discovered by Huygens in 1655.
It was long thought that Titan
was the largest satellite in the solar system but recent observations
have shown that Titan's atmosphere is so thick that its solid surface
is slightly smaller than Ganymede's. Titan is nevertheless larger in diameter
than Mercury and larger and more massive than Pluto.
|Titan is similar in bulk properties
to Ganymede, Callisto, Triton and (probably) Pluto. It is not known whether
it has any internal structure like Ganymede or is uniform like Callisto.
Titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.
Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started.
Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.
At the surface, Titan's temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. Nevertheless, there appears to be a lot of chemistry going on; the end result seems to be a lot like a very thick smog.
There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.
It seems likely that
the ethane clouds would produce a rain of liquid ethane onto the surface
perhaps producing an "ocean" of ethane (or an ethane/methane
mixture) up to 1000 meters deep. Recent ground-based radar observations
have cast this into doubt, however. Cassini's observations are likely
to resolve this issue
serene and majestic in the first color composite made of images taken
by NASA's Cassini spacecraft on its approach to the ringed planet, with
arrival still 20 months away.
Titan, Saturn's largest moon, appears in the upper left. It is the only moon resolved from this distance. This composite uses a threefold enhancement in the brightness of Titan relative to the brightness of Saturn. Titan is a major attraction for scientists of the Cassini-Huygens mission. They will study its haze-enshrouded atmosphere and peer down, with special instrumentation, to its surface to look for evidence of organic processes similar to those that might have occurred on the early Earth, prior to the emergence of life.
Cassini will enter orbit around
Saturn on July 1, 2004. It will release a piggybacked probe, Huygens,
which will descend through Titan's atmosphere on Jan. 14, 2005.
|This image of Titan, taken by a Voyager (1981) spacecraft, shows Titan to be completely shrouded by a thick atmosphere. The atmosphere is about 95% nitrogen, the remainder methane as well as other hydrocarbons and hydrogen cyanide.|
Recent observations with the Hubble Space Telescope show remarkable near infrared views of Titan's surface.
Voyager's camera couldn't see through Titan's atmosphere but in the near infrared the haze becomes more transparent, and HST's pictures suggest that a huge bright "continent" (preliminarily called "Xanadu") exists on the hemisphere of Titan that faces forward in its orbit. These Hubble results don't prove that liquid "seas" exist, however, only that Titan has large bright and dark regions on its surface.
The landing site for the Huygens probe has been chosen in part by examining these images. It will be just "offshore" of the largest "continent" at 18.1 degrees North, 208.7 degrees longitude. Cassini's infrared images (left, click for animation; and below right) are even clearer (and much higher resolution images from Cassini are soon to come! :-)
The observations by HST also indicate that Titan's rotation is in fact synchronous like most of Saturn's other moons.
|Piercing the ubiquitous layer
of smog enshrouding Titan, these images from the Cassini visual and infrared
mapping spectrometer reveals an exotic surface covered with a variety of
materials in the southern hemisphere.
Using near-infrared colors -- some three times deeper in the red visible to the human eye -- these images reveal the surface with unusual clarity. The color image shows a false-color combination of the three previous images. The yellow areas correspond to the hydrocarbon-rich regions, while the green areas are the icier regions. Here, the methane cloud appears white, as it is bright in all three colors.
Closest Encounter June 11 2004
Phoebe is almost 4 times more distant from Saturn than its nearest neighbor (Iapetus).
orbit: 12,952,000 km from
Most of Saturn's moons are bright but Phoebe's albedo is very low (.05), as dark as lampblack
Phoebe orbits Saturn in a retrograde orbit (backwards) and is suspected of being either from the Kuiper Belt or of the Centaur Group.If so then Phoebe may be a primordial object created at the very beginning of the Solar System and kept pristine by the cold of space and its own dark surface.
The event hopes to answer questions-and perhaps generate new ones, about Phoebe and its possible role in determining how the solar system was formed.
In 1981 Voyager passed by Phoebe and took pictures from 2,200,000 kilometers.
This was our best image until now.
Image Credit: NASA/JPL/Space Science Institute
In the first image (at left) in which Phoebe looks somewhat like a sideways skull, the large crater near the bottom displays a complex and rugged interior. The lower right hand part of Phoebe appears to be covered by bright wispy material.
The second, higher resolution image further reveals the moon's battered surface, including a crater near the right hand edge with bright rays that extend outward from its center. This suggests that dark material coats the outside. Features reminiscent of those seen on the Martian moon Phobos -- such as linear grooves--are faintly visible in the upper part of this image. There are suggestions of linear ridges or grooves and of chains of craters, perhaps radial to a large crater just hidden on the un-illuminated region in the upper left.
Image Credit: NASA/JPL/Space Science Institute
Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during Cassini's flyby on June 11, 2004. The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material. Small bright craters in the image are probably fairly young features. This phenomenon has been observed on other icy satellites, such as Ganymede at Jupiter. When impactors slammed into the surface of Phoebe, the collisions excavated fresh, bright material -- probably ice -- underlying the surface layer. Further evidence for this can be seen on some crater walls where the darker material appears to have slid downwards, exposing more light-colored material. Some areas of the image that are particularly bright - especially near lower right - are over-exposed.
An accurate determination of Phoebe's density - a forthcoming result from the flyby - will help Cassini mission scientists understand how much of the little moon is comprised of ices.
This spectacular view was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 84 degrees, and from a distance of approximately 32,500 kilometers (20,200 miles). The image scale is approximately 190 meters (624 feet) per pixel. No enhancement was performed on this image.
NASA/JPL/Space Science Institute
|The view 2000 killometers above Phoebe.
Images like this one, showing bright wispy streaks thought to be ice revealed by slumping crater walls, are leading to the view that Phoebe is an ice-rich body covered with a thin layer of dark material. Obvious down slope motion of material along the walls of the major craters in this image is the cause for the bright streaks, which are over-exposed here. Significant slumping has occurred along the crater wall at top left.
The slumping of material might have been caused by a small projectile punching into the steep slope of the wall of a pre-existing larger crater. Another possibility is that the material collapsed when triggered by another impact elsewhere on Phoebe. Note that the bright, exposed areas of ice are not very uniform along the wall. Small craters are exposing bright material on the floor of the larger crater.
Image Credit: NASA/JPL/Space Science Institute
Crater Close-up on Phoebe
This image was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 78 degrees, and from a distance of 11,918 kilometers (7,407 miles). The image scale is approximately 18.5 meters (60.5 feet) per pixel. The illumination is from the right. No enhancement was performed on this image
The True Shape of Phoebe
On June 11, 2004, during its closest approach
to Phoebe, Cassini obtained this extremely high resolution view of a dark,
desolate landscape. Regions of different reflectivity are clearly visible
on what appears to be a gently rolling surface. Notable are several bright-rayed
impact craters, lots of small craters with bright-colored floors and light-colored
streaks across the landscape. Note also the several sharply defined craters
- probably fairly young features - near the upper left corner.
These set of images were created during the Phoebe flyby on June 11, 2004. The images show the location and distribution of water-ice, ferric iron, carbon dioxide and an unidentified material on the tiny moon of Saturn. The first image was taken with Cassini's narrow angle camera and is shown for comparison purposes only. The other images were taken by the visual and infrared mapping spectrometer onboard Cassini.
The infrared image of Phoebe obtained at a distance of about 16,000 km (10,000 miles) shows a large range of bright and dark features. The resolution of the image is about 4 km (2.5 miles). carbon dioxide on the surface of Phoebe is distributed globally, although it appears to be more prevalent in the darker regions of the satellite.
The existence of carbon dioxide strongly suggests that Phoebe did not originate in the asteroid belt, but rather in much colder regions of the Solar System such as the Kuiper Belt. The Kuiper Belt is a vast reservoir of small, primitive bodies beyond the orbit of Neptune. An unidentified substance also appears to be more abundant in the darker regions.
A map showing the distribution of water ice (blue), ferric iron (red), which is common in minerals on Earth and other planets, and the unidentified material (green). Water ice is associated with the brighter regions, while the other two materials are more abundant in the darker regions.
|Cassini Opens a Cosmic
June 23, 2004
(Source: Jet Propulsion Laboratory)
MEDIA RELATIONS OFFICE
Like a woolly mammoth trapped in Arctic ice, Saturn's small moon Phoebe may be a frozen artifact of a bygone era, some four billion years ago. The finding is suggested by new data from the Cassini spacecraft.
Cassini scientists reviewed data from the spacecraft's June 11, 2004, flyby of the diminutive moon. They concluded Phoebe is likely a primordial mixture of ice, rock and carbon-containing compounds similar in many ways to material seen in Pluto and Neptune's moon Triton. Scientists believe bodies like Phoebe were plentiful in the outer reaches of the solar system about four and a half billion years ago.
These icy planetesimals (small bodies) formed the building blocks of the outer solar system and some were incorporated into the giant planets Jupiter, Saturn, Uranus and Neptune. During this process, gravitational interactions ejected much of this material to distant orbits, joining a native population of similar bodies to form the Kuiper Belt.
"Phoebe apparently stayed behind, trapped in orbit about the young Saturn, waiting eons for its secrets to be revealed during its rendezvous with the Cassini spacecraft," said Dr. Torrence Johnson, Cassini imaging team member at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
"All our evidence leads us to conclude, Phoebe's surface is made of water ice, water-bearing minerals, carbon dioxide, possible clays and primitive organic chemicals in patches at different locations on the surface," said Dr. Roger N. Clark, team member for the visual and infrared mapping spectrometer, U.S. Geological Survey in Denver. "We also see spectral signatures of materials we have not yet identified." Cassini's observations gave scientists the first detailed look at one of these primitive icy planetesimals.
Phoebe's mass was determined from precise tracking of the spacecraft and optical navigation, combined with an accurate volume estimate from images. The measurements yield a density of about 1.6 grams per cubic centimeter (100 pounds per cubic foot), much lighter than most rocks, but heavier than pure ice at approximately 0.93 grams per cubic centimeter (58 pounds per cubic foot). This suggests a composition of ice and rock similar to Pluto and Triton.
Spectral measurements, light intensity as a function of color or wavelength, confirmed the presence of water ice previously detected by Earth-based telescopes. The measurements provided evidence for hydrated minerals on Phoebe's surface, and detected carbon dioxide and solid hydrocarbons similar to those found in primitive meteorites.
"One intriguing result is the discovery of possible chemical similarities between the materials on Phoebe and those seen on comets," said Dr. Robert H. Brown, team leader for the visible and infrared mapping spectrometer, University of Arizona, Tucson. Evidence that Phoebe might be chemically kin to comets strengthens the case that it is similar to Kuiper Belt Objects.
Measurements taken by the composite infrared spectrometer were used to generate temperature maps. The maps show the surface of Phoebe is very cold, only about 110 degrees above absolute zero (minus 163 degrees Celsius, or minus 261 degrees Fahrenheit). Even colder nighttime temperatures suggest a fluffy, porous surface layer.
"One of the first results from this map is the surface of Phoebe has been badly chewed up, probably by meteorite impacts," said Dr. John Pearl, a Cassini co-investigator for the composite infrared spectrometer, at NASA's Goddard Space Flight Center, Greenbelt, Md. "We are discovering Phoebe is a very complex object, with large variations in topography."
Cassini also made radar observations of Phoebe's enigmatic surface, making it the first spacecraft radar observations of an outer-planet moon. The results are consistent with the dirty, rocky, icy surface suggested by other observations.
"We have conducted our first analysis of an outer solar system resident akin to Kuiper Belt Objects," said Dr. Dennis Matson, project scientist of the Cassini-Huygens mission at JPL. "In two short weeks, we have added more to what we know about Phoebe than we had learned about it since it was discovered 100 years ago. We did this by having multiple instruments conducting investigations all at one time during our flyby."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for NASA's Office of Space Science, Washington. For the latest images and more information about the mission on the Internet, visit http://www.nasa.gov and http://saturn.jpl.nasa.gov .
Donald Savage (202) 358-1727
|Giovanni Domenico Cassini was Born: 8 June 1625
in Perinaldo, Republic of Genoa. Died: 14 Sept 1712 in Paris, France.We
know little of his parents but certainly his father was a Tuscan. In fact
Giovanni was brought up, not by his parents but by an uncle, a brother of
his mother Julia Crovesi. After spending two years being educated at Vallebone,
Cassini entered the Jesuit College at Genoa where he studied under Casselli.
After this he studied at the abbey of San Fructuoso. Taton writes "He
showed great intellectual curiosity and was especially interested in poetry,
mathematics and astronomy. "
Cassini observed a comet in 1652-3 and he published an account of his observations which he dedicated to the Duke of Modena. From the work we can see that at this time Cassini believed in an Earth centred solar system, with comets beyond Saturn but originating from the Earth. Observations would lead him to accept the model of the solar system proposed by Tycho Brahe and, in 1659, he presented an Earth centred system with the moon and sun orbiting the Earth and the other planets orbiting the sun. Later he came to accept a version of the Copernican model.
The Pope asked Cassini to take Holy Orders for he wished to see him permanently working for him. However, Cassini preferred to keep his post as professor of mathematics and astronomy at Bologna where he taught when not undertaking Papal duties. He continued with his research in astronomy, proposing a model for atmospheric refraction which turned out to be incorrect, making an intensive study of the sun, publishing tables in 1662, and continuing to search for comets. In 1664 he observed a comet which led him to propose a new theory that comets travelled in circular orbits around the sun with the centre of the orbit in the direction of the star Sirius.
Beginning in 1664 Cassini was able to observe with new powerful telescopes made by the excellent lens maker Giuseppe Campani of Rome. With these instruments Cassini made a series of new discoveries. In July 1664 he measured the period of rotation of Jupiter on its axis, discovered the bands and spots on the planet, and saw that the planet was flattened at its poles. In 1666 he measured the period of rotation of Mars on its axis, getting a value within three minutes of the correct one, and observed surface features. He published detailed series of observations of the moons of Jupiter in 1668. Now Cassini discovered discrepancies in his data which at first he attributed to light having a finite speed:-
... light takes some time to come from the satellite to us; and it takes approximately ten or eleven minutes to traverse a distance equal to the semi-diameter of the Earth's orbit.
However, Cassini was too traditional in his views to accept his own idea, and he soon rejected it and looked for other explanation for the discrepancy. It is rather ironical that it was Cassini's data that was used by Römer in calculating the speed of light seven years later.
|Christiaan Huygens was born in 1629, Huygens came
from a wealthy and well-connected Dutch family, who were traditionally in
the diplomatic service to the House of Orange. As a young boy he already
showed promise in mathematics and drawing. Descartes used to correspond
with Huygens's grandfather and, impressed with the boy's early efforts at
geometry, he was a great influence on Huygens. In 1645 he went to the University
of Leiden to study mathematics and law. Two years later he went to the College
Shortly after Galileo had first used a telescope for astronomical purposes, many other scientists decided to use this new instrument to perform their own studies. Many realised immediately that the improvement of the quality of the telescope could mean the chance to make history in astronomy.
Huygens applied himself to the manufacture of telescopes, together with his brother Constantijn, and soon after developed a theory of the telescope. Huygens discovered the law of refraction to derive the focal distances of lenses. He also realised how to optimise his telescopes by using a new way of grinding and polishing the lenses.
In 1655, he pointed one of his new telescopes, of far better quality
than that used by Galileo, towards Saturn with the intention of studying
its rings. But he was very surprised to see that, besides the rings, the
planet had also a large moon. This is now known as Titan. In 1659 he discovered
the true shape of the rings of Saturn.
Personally, the key to understanding wave interference and propagation for me was Chritian's "Huygen's wavelet" concept. Like finding the light switch in a dark room, this 17th century model gives a satisfing intuitive basis for understanding all optics.
WILLIAM PICKERING: (KIWI STUD of SPACE) 1910-2004
William Hayward Pickering was born in Roxburgh Street, Mount Victoria, Wellington in 1910. His mother died when he was six and he was sent to live with his grandparents in Havelock, in the Marlborough Sounds at the northern tip of the South Island. Here Pickering attended Havelock Primary School, the first school of the greatest New Zealand scientist, Ernest Rutherford.
In 1923 he started boarding at Wellington College. His father, a pharmacist, had left New Zealand to work in the tropics, an environment he didn’t believe was a healthy one for his sons. Pickering was inspired by his math teacher, A. C. ‘Pop’ Gifford. Mr. Gifford founded the school’s observatory, the place where young Pickering first looked through a telescope towards the heavens. Pickering’s ability to marry practical and theoretical science was coached at Wellington College. With schoolmate Fred White (later Dr. F. White CBE, CEO of the Commonwealth Scientific and Industrial Research Organization) Pickering built an early radio station. The two communicated using Morse code with others around the world.
In a 1993 lecture Pickering gave at the University of Michigan, he said the launch of Sputnik was no secret. In 1955 both the Soviet and American governments had announced their intentions to experiment with satellites. If the public was not listening when these announcements were made, two years later they certainly heard the sound of a sinister Sputnik coming over the airwaves above middle America. Or as Pickering said: "It was only the beeping reality of Sputnik that suddenly made the threat of intercontinental atomic warfare with ballistic rockets more than a science fiction story."
The Americans were working to match Sputnik. In two months the Naval
Research Laboratory launched the Vanguard. A test launch, on December
7th, 1957, was to be viewed under the glare of the international media.
Fortunately, Pickering and the JPL had been working since Sputnik on their own satellite. If their launch went successfully it would repair some of the American government’s bruised ego.
Explorer 1 was launched from Cape Canaveral, Florida on January 31, 1958, less than four months after Sputnik. It stayed orbiting the earth for the next 10 years
In 1993 Pickering was awarded the inaugural Francois-Xavier Bagnoud Aerospace Prize for his contribution to space science. In presenting him with the Prize the then president of Caltech Thomas E. Everhart said:
Bill Pickering was a Unitarian.