Cassini History and Tidbits

August 20, 2004
The two newly discovered moons each measure less than 4 kilometers (2.5 miles) in diameter and might be the smallest bodies detected so far around Saturn. One circles the planet at a distance of 211,000 kilometers (131,000 miles) and the other at 194,000 kilometers (120,000 miles). They’ve been given the temporary designations S/2004 S1 and S/2004 S2. Up to this point the smallest known moon measured about 20 kilometers (12 miles) across.

In the April 26 update, you can see a graph showing all the moons of Saturn that were known at the time. Click to see a new graph that includes the Charnoz-Brahic discoveries. The vertical axis of the graph has been recalculated as a logarithmic scale to better distinguish the smaller satellites. Also the data has been replotted to accurately reflect the time period over which the discoveries of smaller and smaller moons have taken place.

(click image for larger version)

July 9, 2004
The voyager spacecraft took 30,000 photographs of the Saturnian system during their flybys in 1980 and 1981. Cassini will take 300,000 photographs if all goes well in the next four years. This represents a ten-fold increase in imagery alone. The instrument that took this week’s release, the Ultraviolet Imaging Spectrograph, offers 100 times the resolution of ultraviolet data provided by the Voyager 2 spacecraft. When combined with the improvements in the sensitivity of the Cassini instruments, the contrast between the two missions helps to demonstrate the relative quality of the data sets with which the Cassini scientific team will work. If all goes as planned, the lucky scientists on the Cassini-Huygens team will be swimming in data for years and years to come.

June 29, 2004
The Voyager 1 and 2 spacecraft rewrote the textbooks on planetary science after each of their outer solar system encounters. During the Saturn encounters of 1980 and 1981, these historic spacecraft raised entirely new questions about the nature of Saturn itself as well as its many moons—some of which were first discovered during the Voyager missions. The voyagers took over 30,000 photographs of the Saturnian system, providing the first close-up data for continuous study of this fascinating planet. The current Cassini-Huygens mission owes much to this early explorer. As a direct consequence of the Voyager’s mission, scientists have greatly increased the number and complexity of their questions. The principal investigators for each of the instruments on the Cassini-Huygens spacecraft stand to increase our understanding of Saturn by a factor of a thousand or more.

June 16, 2004
Our last update featured William Henry Pickering who first discovered Phoebe in 1898. This week we feature Dr. Bonnie Buratti, a senior scientist at the Jet Propulsion Laboratory and a member of the Visual and Infrared Mapping Spectrometer instrument team on Cassini. Dr. Buratti’s interest in science dates from the third grade. She worked at the Maria Mitchell Observatory on Nantucket Island and spent some time working in a planetarium as well. She entered graduate school at Cornell University, where she received her Ph.D. in Astronomy and Space Sciences in 1983.

Dr. Buratti first joined NASA’s Jet Propulsion Laboratory on a fellowship funded by the National Research Council just after finishing her graduate work at Cornell. Two years later JPL asked her to become a permanent member of the science team. Since that time Dr. Buratti has been a Science Team Member on many missions, and a Principal Investigator for various research programs at NASA. She is the author of dozens of research papers. In addition to Dr. Buratti’s research on Phoebe, as a member of the VIMS team she will also contribute to the research on Saturn’s largest moon, Titan.

June 10, 2004
The history of Phoebe’s discovery involves an interesting personality who was born in Boston in 1858 and lived until 1938. William Henry Pickering earned his B.S. in 1879 from the Massachusetts Institute of Technology and taught both there as well as at the Harvard Observatory. His discovery of Saturn’s moon Phoebe was the first such discovery accomplished with the use of photographic plates as opposed to direct observation via telescope. Phoebe was the 9th of Saturn’s moons to be discovered, and is one of five moons with diameters measuring in the hundreds of kilometers. Pickering was a highly regarded astronomer who led five solar eclipse expeditions between 1878 and 1901 and also established several observatories and astronomical stations. In 1905 he announced the finding of a tenth moon of Saturn that was not confirmed until 1967, when discovered by the French physicist and astronomer, Audouin Dollfus. That tenth moon was named Janus.

June 1, 2004
Our knowledge of Saturn improved significantly when Voyager-1 and Voyager-2 encountered the giant planet in 1980 and 1981. You might even conclude that the nature of some of the Voyager discoveries carried the same eye-opening impact as did the earliest discoveries and theories put forth by Huygens and his scientific contemporaries of the 17th century. For example, NASA reported in its 1991 publication “Voyager the Grandest Tour,” a quote from one scientist suggesting his or her surprise at discovering how quickly the rings rotate when compared with the planet. This scientist reports, “We never thought we would be able to see the rings spinning line this. We thought we would see the planet turning with the rings as a still life. It now appears to be more nearly the reverse.”

The important thing about this finding is its large-scale nature. Discovering that the rings rotate much faster than previously thought is nearly on par with discovering that the rings are in fact “rings” and not handles as previously suspected. It will be interesting to see how many large-scale discoveries will unfold during the four-year Cassini-Huygens tour of the Saturnian system. Such discoveries are of particular interest because no detailed understanding of atmospheric chemistry, imaging filters, etc. is required to appreciate the nature of the discovery.

May 25, 2004
In our last update we introduced Johannes Kepler, who made use of the careful observations of Danish astronomer, Tycho Brahe, to develop three laws of planetary motion. Although Kepler’s laws provide a close approximation of planetary motion, they were derived from observations about the way in which planetary bodies move but not on a fundamental understanding of the forces governing their motion. Here’s where we introduce the extraordinary insight of Isaac Newton, who in 1666 used his theory of the law of universal gravitational attraction to mathematically derive the work of Kepler.

Whether the story of the falling apple is true or not, Newton made the incredible intuitive leap of assuming that the motion of planetary bodies was governed by the same law that causes objects to fall toward the Earth. Equally astonishing was Newton’s assertion that any two bodies (including a sun and a planet or a planet and its moon) would attract each other with a force that is proportional to their masses and inversely proportional to their distance of separation. Using this theory of nature’s rules, Newton derived Kepler’s laws and even made a slight correction to Kepler’s third law of planetary motion by introducing the concept of mass – which Kepler had not considered. Ironically, Newton’s discovery was not published until 1687 (twenty years after his brilliant discovery) in a book that some scholars consider the greatest scientific book ever written—called the Philosophiae Naturalis Principia Mathematica or Principia as it is always known.

May 14, 2004
Last week we introduced the work of the brilliant Danish nobleman, Tycho Brahe, who created large, precision instruments to collect data on the positions of objects in the night sky. After Brahe’s death in 1601 a brilliant German-born mathematician (Johannes Kepler) used the mountain of astronomical data collected by Brahe to formulate three laws of planetary motion. Using Brahe’s observations of the planet Mars, Kepler determined that planetary motion is elliptical -- with the sun at one focus of the ellipse. This first law is an amazing conclusion since the eccentricity of Mars’ orbit is only 0.1, meaning that it is nearly circular when drawn to scale. The fact that Kepler determined the orbit to be elliptical is both a tribute to Brahe’s observations and to Kepler’s determination to accurately interpret the data.

Kepler’s second law is a little more difficult to deduce. It recognizes that planets speed up and slow down at various parts of their orbits as they move around the sun and provides a relationship for determining the speed from one part of the orbit to the next. Kepler’s third law, published in 1619, is a simple algebraic formula that relates the size of a planet’s orbit to the time it takes to complete one revolution around the sun. Kepler’s laws provide a good approximation of planetary motion. However, they were derived from observations about the way in which planetary bodies move but not on a fundamental understanding of the forces governing their motion. A complete theory of planetary motion would not be made public for another 67 years.

May 7, 2004
A thorough understanding of planetary motion is essential for accurately predicting the location of celestial bodies (like Saturn) or spacecraft traveling throughout the solar system (like Cassini). This capability developed slowly and came about only as a result of an enormous body of work developed over a period of nearly 100 years. The work involved several key players not to mention dozens of other assistants and colleagues. The data to support this effort was collected over a period of nearly 40 years by Tycho Brahe, who was born of Danish nobility and one of the most important scientists of the 16th Century. His entire professional life centered on the development and continuous refinement of precision scientific instruments, some of them very large, used for astronomical observations. Brahe demonstrated extraordinary integrity in the collection of data and the measurement of the positions of objects in the sky. This integrity, along with his intelligence and organizational ability, laid the groundwork for important later discoveries.

May 3, 2004
Our present knowledge of Saturn as a giant gas planet with a complex atmosphere, numerous moons (31 at present count) and multiple rings made of countless individual particles would be inconceivable without the development of scientific instruments and the courage to allow evidence to shape opinion. The earliest known records of Saturn date back to about 700 B.C.E. during which time the Assyrians (who occupied the territory which is modern day Iraq) identified it as a wandering star. From the time of this first recorded observation it would take about 2,300 years before empirical evidence, gathered through the use of the telescope, challenged human thinking about this mysterious wanderer. It’s quite startling to compare the results of 2300 years of conjecture with 400 years of data collection. Just imagine how much more we stand to learn with Cassini-Huygens touring the Saturnian system during the next four-year period.

April 26, 2004
The discovery of Saturn’s moons tells a great deal about the importance of getting “up close and personal” when it comes to the exploration of our solar system. By plotting the diameter of each moon by the year of its discovery we can see the importance of resolution—or getting that “closer” look. Click to view chart.

The largest moon, Titan, has a diameter of 5150 kilometers and was discovered by Huygens in 1655. From 1671 through 1684 Cassini discovered four more moons including Iapetus, Rhea, Tethys and Dione with diameters of 1435, 1530, 1048 and 1120 kilometers respectively. More than 100 years would pass before telescope resolutions would permit the discovery of smaller moons measuring only hundreds of kilometers in diameter. From 1789 through 1898 Mimas, Enceladus, Hyperion and Phoebe were discovered with diameters of 394, 502, 270 and 220 kilometers respectively. It wasn’t until 1980 (nearly another 100 years) that the Voyager spacecraft flew by Saturn and discovered moons less than 100 kilometers in diameter. These include Telesto, Calypso, Atlas, Prometheus and Pandora. With advancements in remote sensing since 1980 many more moons have been discovered bringing the total to 31. It's a near certainty that additional moons will be discovered by the Cassini-Huygens spacecraft.

April 21, 2004
Some discoveries require a combination of careful measurement, good fortune and lots of patience. Such is the case with the two moons Prometheus and Pandora—the subjects of this week’s NASA image. These two F-ring shepherds were first discovered by the Voyager-1 spacecraft nearly 24 years ago. Predictions of their positions over time were only discovered to be in error when the Hubble Space Telescope took pictures in 1995, revealing their positions to be different than expected based on the earlier Voyager data. It was the Hubble’s good fortune in 1995 to find Saturn’s rings nearly edge-on as seen from Earth. This configuration greatly reduced the normal glare of the rings, making Prometheus and Pandora more visible than normal.

April 9, 2004
A good scientific question sometimes reveals as much about what is known as what is unknown. With the Cassini-Huygens spacecraft, scientists will ask detailed questions about the composition of Saturn's rings. How did they form? When did they form? What are they made of:? Is their composition uniform? Over what time period are they stable? What governs their structure? With questions like these at the heart of the mission it's hard to imagine a time when we had no ideas regarding Saturn's beautiful rings. However, such was indeed the case.

The image below shows a total of 13 drawings made by distinguished scientists ranging from as early as 1610 to 1650. Called by various names including "arms" and "handles" these images, taken from Christiaan Huygens' Systema Saturnium, reflect data collected using the best telescopes available to these early scientists. Most believed that the "handles" were appendages and actually touched the planet. Huygens, in 1659, was the first to propose that Saturn's "handles" were actually a thin solid ring around the planet that didn't touch the planet at any point. Once the scientific community generally accepted the ring theory, an argument went on for about two hundred years about whether the ring was solid or made of individual particles. The debate wasn't settled until James Clerk Maxwell (of Maxwell's equations fame) published a mathematical analysis of Saturn's ring structure in 1858. Although it's daunting to think of how much effort went into these earlier investigations, it's equally inspiring to realize the power of combining empirical observations with powerful theories regarding the laws of nature.

Images from Christiaan Huygens' Systema Saturnium, drawn from 1610-1650.

April 2, 2004
Ever think about the effort leading up to the final stages of a planetary mission? We're poised waiting for the Cassini-Huygens probe to open a new window of understanding into one of the most intriguing objects in our solar system. But the process of getting to this point takes up the better part of a career in science and engineering—along with a few nail-biting ups and downs.

Back in 1982 the Space Science Committee of the European Space Foundation and the Space Science Board of the National Academy of Sciences planted the initial seeds that grew into the Cassini-Huygens mission. An earlier design called for a standardized spacecraft bus (called the Mariner Mark 2 spacecraft) on which different missions could be flown. A mission called the comet rendezvous asteroid flyby (CRAF) and Cassini were both approved for funding in 1989. Funding restrictions led to the cancellation of CRAF in 1992, forcing the restructuring of the Cassini mission. The House Appropriations Subcommittee sought to eliminate Cassini funding in 1995 but the project was saved. The Cassini-Huygens spacecraft was finally launched in 1997. The seven-year flight time to Saturn brings us to 2004. If all goes well the nominal mission life of four years extends until 2008. That's a total of 26 years from concept to completion. Cassini-Huygens reflects a remarkable commitment to exploration on the part of everyone involved.

March 26, 2004
Although the name of Gian Domenico Cassini gets top billing for the Saturn mission, one of the most important experiments bears the name of Dutch mathematician and scientist Christiaan Huygens (pron. Kristy-un How-kens) (1629-1695). In fact, when properly stated the spacecraft's name is Cassini-Huygens.

Born into a prominent family, Huygens and his brother built a telescope by grinding their own much improved homemade lenses and in 1655 detected Saturn's largest moon—Titan. Hence, the detachable probe on the side of the Cassini-Huygens spacecraft, called the Huygens probe, will separate from the main spacecraft on November 6th and on November 27th carry six scientific instruments down to the surface of Titan. It's one of the most exciting aspects of the entire mission.

March 19, 2004
Did you know that the rings of Saturn extend into space for a distance of 480,000 kilometers (300,000 miles) beyond the planet? That's farther than the distance from the Earth to the moon -- a mere 384,400 kilometers (240,000 miles). Also, we can't see all of Saturn's rings from an Earth-based telescope. Current estimates from high resolution images suggest that what look like continuous rings to us might turn out to be 500 - 1000 distinct rings around the planet.

March 12, 2004
Except for the part about being dead, having a spacecraft named in your honor is probably about as cool as it gets. Gian Domenico Cassini (1625-1712) (for whom the Cassini spacecraft is named) directed the prestigious Paris Observatory from 1669 - 1700. He discovered a division in the rings of Saturn that bears his name as well (called the Cassini division), and discovered the moons Iapetus, Rhea, Dione and Tethys. Three generations of Cassinis (his son, Jacques Cassini, grandson, Cesar-Francois Cassini de Thury and great-grandson Jean-Dominique Cassini) succeded him as director of the observatory.

March 5, 2004
Did you know that the rings of Saturn don't always look the same from Earth? The picture below is a composite from the Hubble Space Telescope that shows how the view of Saturn's rings have changed from 1996 to 2000.