Cassini Current Events

Cassini Significant Events for:

07/08/04 - 07/14/04

The most recent spacecraft telemetry was acquired from the Goldstone tracking station on Wednesday, July 14. The Cassini spacecraft is in an excellent state of health and is operating normally. Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" web page located at http://saturn.jpl.nasa.gov/operations/present-position.cfm .

Cassini exited the Solar Conjunction period this week. Telecommunications performance was limited as expected due to the position of the Sun between the spacecraft and Earth. Full communications capability was restored once a 2-degree Sun-Earth-Probe separation angle was achieved. After conjunction all instruments were once again configured for science acquisition.

On or about July 13, Cassini exited Saturn's magnetosphere and the Magnetospheric and Plasma Science instruments began to monitor the solar wind. The unique geometry of this period of high altitude and phase angles of ~100 degrees will allow the Ultraviolet Imaging Spectrograph to simultaneously measure Saturn's aurora and magnetosphere.

On-board activities this week included a reaction wheel assembly bias, uplink of a ram flight software patch and table load for the Cassini Plasma Spectrometer, Probe Checkout (PCO) #14, and a test of the Probe Mission Timer Unit (MTU). This is the first checkout of the probe since Saturn Orbit Insertion. All probe instruments and the MTU are functioning properly. In preparation for probe release and relay, the checkout was utilized by members of the ground system to practice the generation of 30 minute IDRs and the monitoring of the data flow to the Huygens Probe Operations Center.

Members of the Cosmic Dust Analyzer (CDA) instrument team escorted an engineering model of the instrument to JPL where it was connected to a test bed in the Integrated Test Laboratory for flight software testing. Next week the ITL will resume Probe Relay testing.

Official port#2 of Science Operations Plan (SOP) Implementation of tour sequences S31/S32 occurred this week. The products were merged and delivered to ACS for end-to-end pointing analysis. Preliminary port#1 of SOP Implementation of tour sequences S33/S34 also occurred this week. The files were merged and a report was delivered identifying issues to be worked.

The S05 SOP Update process began this week, and an assessment meeting to review requested changes to S07 was held as part of the Aftermarket process. Initially it appears that all requested changes can fit within available resources. The Target Working Teams and Orbiter Science Teams will be reviewing the requests over the next two weeks and provide their recommendations at the decision meeting scheduled for July 27.

The Huygens Probe Mission Risk Review was held on Wednesday, July 14. This was an external review convened to assess the risks and risk mitigation measures associated with the probe mission. The board was comprised of independent reviewers from JPL, other NASA centers, and industry. The review was very successful with positive closing comments from the board.

All teams and offices supported this month's Cassini monthly management review.

A Software Review/Certification Requirements meeting was held for CDA flight software (FSW) version 9.2.4 Two bug fixes were previously approved for this "patch" delivery. The next full delivery of this FSW is scheduled for Dec 2004, with uplink planned for post-Probe mission.

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

Cassini Significant Events

for 06/24/04 - 06/30/04 The most recent spacecraft telemetry was acquired from the Canberra tracking station on Wednesday, June 30th. The Cassini spacecraft is in an excellent state of health and is operating normally. Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" web page located at http://saturn.jpl.nasa.gov/operations/present-position.cfm .

During the quiet period leading up to Saturn Orbit Insertion (SOI), members of the Spacecraft Operations Office (SCO) monitored spacecraft real-time telemetry on a continuous basis. All operations were nominal and per predict. Monitoring continued through the pre-SOI critical commanding period and up until the swap to low gain antenna-1 was commanded. From that time until a scheduled "call home" after the burn, spacecraft performance was monitored via the Radio Science Receivers (RSR).

After the antenna swap, the spacecraft was oriented so that the high gain antenna (HGA) could be used as a shield, protecting Cassini from potential dust impacts as the spacecraft performed its ascending ring plane crossing through the gap between the F and G rings.

Traveling at a speed of over 20 km/sec kilometers per second, the spacecraft was reoriented for a 96-minute main engine burn. This slowed the spacecraft by 626 meters per second and allowed it to be captured by the gravitational pull of Saturn. During this time, five science instruments remained on collecting data that will be unique in the lifetime of the Cassini mission. Never again will Cassini travel as close to Saturn as it did at 9:03 p.m. PDT when it reached closest approach of 19,980 kilometers from the cloud tops.

After completion of the burn, Cassini turned so that the HGA was aimed back toward Earth for a 20-second burst of telemetry. This "call home" confirmed for the flight team that the spacecraft was operating normally. Cassini then turned away and began execution of a science observation sequence. Science obtained at this time was key, in that the spacecraft was within 15000 kilometers from Saturn's main rings, ten times closer to the rings than at any other point in the mission, and in a region of space that had not been previously observed.

Unique post-SOI science activities included: measurement of the strength and direction of the magnetic field by the Cassini Magnetometer (MAG), ring observations by the Optical Remote Sensing Instruments, measurement of the very sparse neutral molecules in Saturn's atmosphere by the Ion and Neutral Mass Spectrometer (INMS), measurement of the charged particles by the Cassini Plasma Spectrometer (CAPS), and detection of radio emissions emitted by lightning strokes in Saturn's atmosphere by the Radio and Plasma Wave Science instrument (RPWS).

Science data playback began in the early hours of Thursday July 1. Images are now available for viewing at: http://saturn.jpl.nasa.gov .

A very significant event to occur immediately after SOI was the jettison of the INMS cover. The cover was left on the instrument since launch until after the SOI burn was complete. This was to prevent potential contamination of the instrument by exhaust material from the long burn. Prior to jettison, INMS was filled with argon to insulate and protect the interior walls. Team members were able to confirm a successful jettison when sensors noted the depletion of the argon gas. INMS was powered up at 3:39 SCET and is now taking data for the first time in the mission.

ACS analysis of official port #1 products from Science Operations Plan (SOP) implementation of tour sequences S31/S32 has been completed. The teams are working off issues in preparation for preliminary port #2.

Due to SOI activities, the Project Briefing and Waiver Disposition meeting for the SOP Update process of S04 was canceled. The handoff product will be generated and delivered to the sequencing team next week. The Aftermarket decision meeting for S06 was canceled since the number of changes requested for this sequence was minimal. System Engineering hosted a Phoebe lessons learned discussion this week.

The Phoebe flyby was the first time an IVP update was required. This activity will be performed numerous times throughout the mission so the capturing of information now will assist in future operations. Items on the agenda included discussion of target motion compensation and live update, planning and testing for the flyby beginning with the SOP Update process and including operations readiness tests that were performed for Phoebe, overview and recommendations for an end-to-end "once the dust settles" assessment of the process, general lessons learned, and follow ups. As expected, quite a number of articles and interviews have appeared on Cassini's Saturn Orbit Insertion in the last 24 to 48 hours. A sample of titles and links to the articles appear below:

STORY WRITTEN FOR CBS NEWS "SPACE PLACE" Cassini successfully arrives at Saturn July 1, 2004 http://www.spaceflightnow.com/cassini/040630soi.html

NASA Headquarters, Washington News Release: 2004-168 June 30, 2004 Cassini Spacecraft Arrives At Saturn http://www.jpl.nasa.gov

Cassini gets ringside view of Saturn's mysteries and Cassini enters Saturn orbit http://www.cnn.com/TECH/space/

Cassini Set for 4-Year Orbit of Saturn Associated Press June 27, 2004 http://story.news.yahoo.com/news?tmpl=story&cid=624&ncid=624&e=1&u=/ap/20040628/ap_on_sc/saturn_cassini

The Johns Hopkins University Applied Physics Laboratory Office of Communications and Public Affairs Laurel, Maryland July 1, 2004 APL Instrument Aims at Saturn's Space Environment http://saturn.jpl.nasa.gov/

For information on the MIMI instrument, visit: http://sd-www.jhuapl.edu/CASSINI/

News Release: 2004-168 June 30, 2004 Cassini Spacecraft Arrives At Saturn NASA Headquarters, Washington http://www.jpl.nasa.gov

A special section from AGU: JGR-Space Physics, Volume 109, A9, 2004 (Cassini Flyby of Jupiter) http://www.agu.org/journals/ss/CASINIJUP1/

University of Iowa, Iowa City News Release: 2004-164 June 28, 2004 Scientists Find That Saturn's Rotation Period Is A Puzzle http://www.jpl.nasa.gov

NPR's Richard Harris filed a story on the Cassini SOI (Morning Edition on 7/1/2004). You can hear the story in its entirety (~4m 7s) at the following link: http://www.npr.org/features/feature.php?wfId=3063001

Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, Calif., manages the Cassini mission for NASA's Office of Space Science, Washington, D.C. Cassini Outreach Cassini Mission to Saturn and Titan Jet Propulsion Laboratory California Institute of Technology National Aeronautics and Space Administration

July 2, 2004

Cassini Exposes Puzzles About Ingredients In Saturn's Rings

Just two days after the Cassini spacecraft entered Saturn orbit, preliminary science results are already beginning to show a complex and fascinating planetary system.

One early result intriguing scientists concerns Saturn's Cassini Division, the large gap between the A and B rings. While Saturn's rings are almost exclusively composed of water ice, new findings show the Cassini Division contains relatively more "dirt" than ice. Further, the particles between the rings seem remarkably similar to the dark material that scientists saw on Saturn's moon, Phoebe. These dark particles refuel the theory that the rings might be the remnants of a moon. The F ring was also found to contain more dirt.

Another instrument on Cassini has detected large quantities of oxygen at the edge of the rings. Scientists are still trying to understand these results, but they think the oxygen may be left over from a collision that occurred as recently as January of this year.

"In just two days, our ideas about the rings have been expanded tremendously," said Dr. Linda Spilker, of NASA's Jet Propulsion Laboratory, Pasadena, Calif., deputy project scientist for the Cassini-Huygens mission. "The Phoebe-like material is a big surprise. What puzzles us is that the A and B rings are so clean and the Cassini Division between them appears so dirty."

The visual and infrared mapping spectrometer onboard Cassini revealed the dirt mixed with the ice in the Cassini Division and in other small gaps in the rings, as well as in the F ring.

"The surprising fingerprint in the data is that the dirt appears similar to what we saw at Phoebe. In the next several months we will be looking for the origin of this material," said Dr. Roger Clark, of the U.S. Geological Survey, Denver, Colo., and a member of the Cassini science team.

Cassini's ultraviolet imaging instrument detected the sudden and surprising increase in the amount of atomic oxygen at the edge of the rings. The finding leads scientists to hypothesize that something may have collided with the main rings, producing the excess oxygen.

Dr. Donald Shemansky of the University of Southern California, Los Angeles, co-investigator for Cassini's ultraviolet imaging spectrograph instrument, said, "What is surprising is the evidence of a strong, sudden event during the observation period causing substantial variation in the oxygen distribution and abundance." Although atomic oxygen has not been previously observed, its presence is not a surprise because hydroxyl was discovered earlier from Hubble Space Telescope observations, and these chemicals are both products of water chemistry.

Cassini's examination of Saturn's atmosphere began while the spacecraft was still approaching the planet. Winds on Saturn near the equator decrease dramatically with altitude above the cloud tops. The winds fall off by as much 140 meters per second (approximately 300 miles per hour) over an altitude range of 300 kilometers (approximately 200 miles) in the upper stratosphere. This is the first time winds have been measured at altitudes so high in Saturn's atmosphere.

"We are finally defining the wind field in three dimensions, and it is very complex," said Dr. Michael Flasar of NASA Goddard Space Flight Center, Greenbelt, Md., principal investigator for Cassini's composite infrared spectrometer. "Temperature maps obtained now that Cassini is orbiting Saturn are expected to show more detail, helping us to unravel the riddles of Saturn's winds above the cloud tops."

Early Friday (Pacific Time), Cassini imaged Saturn's largest moon Titan, one of the prime targets for the mission. Titan is thought to harbor simple organic compounds that may be important in understanding the chemical building blocks that led to life on Earth. Although too cold to support life now, Titan serves as a frozen vault to see what early Earth might have been like. Scientists will receive the new data and images from Titan later Friday.

June 30, 2004

Cassini Spacecraft Arrives At Saturn

The international Cassini-Huygens mission has successfully entered orbit around Saturn. At 9:12 p.m. PDT on Wednesday, flight controllers received confirmation that Cassini had completed the engine burn needed to place the spacecraft into the correct orbit. This begins a four-year study of the giant planet, its majestic rings and 31 known moons.

"This is a tribute to the team at NASA and our partners at the European Space Agency and the Italian Space Agency, to accomplish this feat taking place 934 million miles [1.5 billion kilometers] away from Earth," said Dr. Ed Weiler, associate administrator for space science at NASA Headquarters, Washington, D.C. "What Cassini-Huygens will reveal during its tour of Saturn and its many moons, including Titan, will astonish scientists and the public. Everyone is invited to come along for the ride and see all this as it is happening. It truly is a voyage of discovery."

Members of the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory, Pasadena, Calif., broke into cheers and high-fives as NASA's Deep Space Network confirmed receipt of the signal indicating successful entry into orbit.

"We didn't expect anything less and couldn't have asked for anything more from the spacecraft and the team," said Robert T. Mitchell, program manager for the Cassini-Huygens mission at JPL. "This speaks volumes to the tremendous team that made it all happen."

Dr. Charles Elachi, JPL director and team leader on the radar instrument onboard Cassini, said, "It feels awfully good to be in orbit around the lord of the rings. This is the result of 22 years of effort, of commitment, of ingenuity, and that's what exploration is all about."

The mission will face another dramatic challenge in December, when the spacecraft will release the piggybacked Huygens probe - provided by the European Space Agency - which will plunge through the hazy atmosphere of Saturn's largest moon, Titan.

"This was America's night. This was NASA doing it right," said Dr. David Southwood, director of scientific programs for the European Space Agency. "They really gave those of us in Europe a challenge. We've got six months to go until we land on Titan. We're just praying that everything will go as well."

Julie Webster, Cassini-Huygens spacecraft team chief, said, "The spacecraft has been an incredible joy to fly. We stand on the shoulders of people who had 40 years of experience building and designing spacecraft."

Saturn is the sixth planet from the Sun. It is the second largest planet in our solar system, after Jupiter. The planet and ring system serve as a miniature model of the disc of gas and dust surrounding our early Sun that eventually formed the planets. Detailed knowledge of the dynamics of interactions among Saturn's elaborate rings and numerous moons will provide valuable data for understanding how each of the solar system's planets evolved.

Cassini traveled nearly 3.5 billion kilometers (2.2 billion miles) to reach Saturn after its launch from Cape Canaveral Air Force Station, Fla., on Oct. 15, 1997. During Cassini's four-year mission, it will execute 52 close encounters with seven of Saturn's 31 known moons.

The first images are expected to return Thursday morning. Science measurements gathered Wednesday are the closest ever obtained of Saturn. Those measurements may reveal details of the gravitational and magnetic fields that tell scientists about Saturn's interior.

June 29, 2004

Cassini Spacecraft At Saturn's Doorstep

Saturn is now a day away for the Cassini spacecraft, a seasoned traveler that began its journey nearly seven years ago.

On June 30 at 7:36 p.m. Pacific Time (10:36 p.m. EDT), Cassini will begin executing a series of commands to enter orbit around the ringed planet. The spacecraft will fire its main engine for a crucial 96 minutes to slow down and be captured in orbit about Saturn.

Besides launch, orbit insertion is the next most critical part of the mission. "Everything has to go just right. The burn must occur for all 96 minutes, the turns must occur at the right time, the computers must keep the sequence going even in the event something unexpected should happen," said Robert T. Mitchell, program manager for the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The spacecraft has been programmed to continue even in the event of an emergency. With a one-way light time of 1 hour and 24 minutes, we had to teach the spacecraft to take care of itself. We don't want Cassini to call home if a problem arises, we want it to keep going. That is precisely what we've told the spacecraft: Don't stop, keep going until you've put in all 96 minutes of burn," he said.

During the orbit insertion, Cassini will fly closer to Saturn than at any other time during the spacecraft's planned four-year tour of Saturn. This provides an unprecedented opportunity to study the planet and rings at close range. It will pass approximately 20,000 kilometers (12,427 miles) above Saturn's cloud tops, closer than any other spacecraft in history. It will also be flying about 10 times closer to the rings than at any other point in the mission.

Cassini carries 12 instruments that will study the planet, rings and moons in extensive detail. Riding aboard Cassini is a second spacecraft, the Huygens probe, built by the European Space Agency. It carries half a dozen instruments that will study Titan, Saturn's largest moon, a prime target for both Cassini and the Huygens probe. Titan is the only moon in the solar system to have a dense atmosphere and resembles the early Earth in deep freeze.

"In a sense, Cassini and the Huygens probe are like time machines that will take us back to examine a world we've never seen before, a world that may resemble what our own world was like 4.5 billion years ago," said Dr. Jean-Pierre Lebreton of the European Space Agency, who is mission manager and project scientist for the Huygens probe.

Eighty-five minutes before the engine burn, Cassini will rotate to point its main antenna dish forward. The Italian-built antenna, 4 meters (13 feet) in diameter, will offer shielding against dust particles the spacecraft may hit as it crosses a gap in the rings. The spacecraft will continue transmitting a monotone "carrier" signal with a secondary antenna for tracking from Earth. Cassini will pass twice through a known gap between the F and G rings, first while ascending shortly before the burn, then while descending shortly after the burn.

The engine burn will slow the spacecraft by 626 meters per second (1,400 miles per hour). Five science instruments will be on during the burn, and others will be used shortly after the engine cuts off. The magnetometer will measure the strength and direction of the magnetic field to understand the physics of Saturn's magnetic dynamics. Lightning may also be detected. Another instrument will provide a record of the dust hits as the spacecraft flies through the ring plane. These observations may tell scientists the size of these tiny particles and the thickness of that ring region. The remote sensing instruments will assess the rings' composition, temperature, and structure. Then the spacecraft will be oriented for the outbound ring plane crossing. After crossing the ring plane in the descending mode, Cassini will look back at the sunlit face of the rings to take more data before turning to Earth to transmit its data.

"Should something happen during the burn, the science sequence will stop," said Dr. Dennis Matson, project scientist for the Cassini-Huygens mission at JPL. "We are prepared to live with this outcome. Getting into orbit is the priority. Getting the science is extra credit."

June 28, 2004

Scientists Find That Saturn's Rotation Period Is A Puzzle

On approach to Saturn, data obtained by the Cassini spacecraft are already posing a puzzling question: How long is the day on Saturn?

Cassini took readings of the day-length indicator regarded as most reliable, the rhythm of natural radio signals from the planet. The results give 10 hours, 45 minutes, 45 seconds (plus or minus 36 seconds) as the length of time it takes Saturn to complete each rotation. Here's the puzzle: That is about 6 minutes, or one percent, longer than the radio rotational period measured by the Voyager 1 and Voyager 2 spacecraft, which flew by Saturn in 1980 and 1981.

Cassini scientists are not questioning Voyager's careful measurements. And they definitely do not think the whole planet of Saturn is actually rotating that much slower than it did two decades ago. Instead, they are looking for an explanation based on some variability in how the rotation deep inside Saturn drives the radio pulse.

The radio sounds of Saturn's rotation, which are also the first sounds from Saturn studied by Cassini, are like a heartbeat and can be heard by visiting http://www.jpl.nasa.gov/videos/cassini/0604/ and http://www-pw.physics.uiowa.edu/space-audio

"The rotational modulation of radio emissions from distant astronomical objects has long been used to provide very accurate measurements of their rotation period," said Dr. Don Gurnett, principal investigator for the Cassini Radio and Plasma Wave Science instrument, University of Iowa, Iowa City. "The technique is particularly useful for the giant gas planets, such as Jupiter and Saturn, which have no surfaces and are covered by clouds that make direct visual measurements impossible."

The first hint of something strange about that type of measurement at Saturn was in 1997, when a researcher from Observatoire de Paris reported that Saturn's radio rotation period differed substantially from Voyager.

Dr. Michael D. Desch, Cassini Radio Plasma Wave Science team member, and scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., has analyzed Saturn radio data collected by Cassini from April 29, 2003, to June 10, 2004. "We all agree that the radio rotation period of Saturn is longer today than it was in during the Voyager flyby in 1980," he said.

Gurnett said, "Although Saturn's radio rotation period has clearly shifted substantially since the Voyager measurements, I don't think any of us could conceive of any process that would cause the rotation of the entire planet to actually slow down. So it appears that there is some kind of slippage between the deep interior of the planet and the magnetic field, which controls the charged particles responsible for the radio emission." He suggests the solution may be tied to the fact that Saturn's rotational axis is nearly identical to its magnetic axis. Jupiter, with a more substantial difference between its magnetic axis and its rotational axis, shows no comparable irregularities in its radio rotation period.

"This finding is very significant. It demonstrates that the idea of a rigidly rotating magnetic field is wrong," said Dr. Alex Dessler, a senior research scientist at the University of Arizona, Tucson. In that way, the magnetic fields of gas giant planets may resemble that of the Sun. The Sun's magnetic field does not rotate uniformly. Instead, its rotation period varies with latitude. "Saturn's magnetic field has more in common with the Sun than the Earth. The measurement can be interpreted as showing that the part of Saturn's magnetic field that controls the radio emissions has moved to a higher latitude during the last two decades," said Dressler.

"I think we will be able to unravel the puzzle, but it's going to take some time," said Gurnett. "With Cassini in orbit around Saturn for four years or more, we will be in an excellent position to monitor long-term variations in the radio period, as well as investigate the rotational period using other techniques."

Cassini, carrying 12 scientific instruments, is just two days from its planetary rendezvous with Saturn. On June 30 it will become the first spacecraft to orbit Saturn, when it begins a four-year study of the planet, its rings and its 31 known moons. The spacecraft recently flew past Saturn's cratered moon Phoebe, where it captured spectacular images as well as data on its mass and composition.

June 28, 2004

Scientists Find That Saturn's Rotation Period Is A Puzzle

On approach to Saturn, data obtained by the Cassini spacecraft are already posing a puzzling question: How long is the day on Saturn?

June 25, 2004

Getting Closer To Titan

Irregular bright and dark regions of yet unidentified composition and character are becoming increasingly visible on Titan's surface as Cassini approaches its scheduled first flyby of Saturn's largest moon on July 2, 2004.

This view represents an improvement in resolution of nearly three times over the previous Cassini images of Titan. Titan's surface is difficult to study, veiled by a dense hydrocarbon haze that forms in the high stratosphere as methane is destroyed by sunlight. This image is different from previous Titan images by Cassini because it was taken through a special filter, called a polarizer, which is designed to see through the atmosphere to the surface.

Cassini will conduct a critical 96-minute burn before going into orbit around Saturn on June 30 (July 1 Universal Time), with its first scheduled flyby of Titan on July 2.

The Cassini-Huygens mission is a cooperative project of NASA, 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 Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

June 23, 2004

Cassini Opens A Cosmic Time Capsule

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."

June 17, 2004

Cassini-Huygens Mission Status Report

After completing a successful trajectory correction maneuver on Wednesday, the Cassini spacecraft is now on its final approach to Saturn. The spacecraft is operating normally and is in excellent health.

The maneuver was necessary to adjust the spacecraft's course to achieve the desired ring plane crossing conditions on June 30. Cassini will pass through a known gap between two of Saturn's rings, called the F and G rings. The region of passage through the ring plane was searched for hazards with the best Earth- and space-based telescopes and by Cassini itself. To protect the spacecraft from particles too small to be detected from Earth, Cassini will be turned to use its high-gain antenna as a shield.

"This should be our final approach maneuver. It's on to Saturn and orbit insertion," said Earl Maize, deputy program manager for the Cassini-Huygens mission at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

During Wednesday's maneuver, Cassini's main engine burned for 38 seconds to slow the spacecraft by about 3.6 meters per second (about 8 miles per hour). In the next few days, mission managers will evaluate the tracking data to ensure the spacecraft is on the correct path for the Saturn encounter. All indications show everything is on target. Subsequent maneuvers are possible should tracking data indicate they are needed to correct the course of the spacecraft.

The Cassini-Huygens mission is a cooperative project of NASA, 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 Cassini-Huygens mission for NASA's office of Space Science, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter.

May 28, 2004

Cassini-Huygens Mission Status Report

The Cassini spacecraft successfully performed a critical six-minute trajectory correction maneuver May 27 to put it on course with its first encounter, Saturn's outermost moon Phoebe, set for June 11. The spacecraft is operating normally and is in excellent health.

"The maneuver is very critical for getting us into Saturn orbit because it is the first checkout of the bipropellant pressurization system after nearly five years of dormancy," said Todd Barber, propulsion engineer for Cassini at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It sets the stage for Saturn orbit insertion on June 30."

During the course of its trip, Cassini has traveled 3.4 billion kilometers (2.1 billion miles). "We couldn't have asked for a smoother ride," said Robert T. Mitchell, program manager for the Cassini-Huygens mission at JPL. "All the instruments are performing well, and for almost seven years we have traveled without any major hitches. The excitement is building as we are getting ready to put Cassini in orbit around the ringed planet." The orbiter has relied on three radioisotope thermoelectric generators to power all the electrical components, including the 12 science instruments. The European-built Huygens probe on board Cassini carries six instruments.

"If the road to Saturn were a highway, the Cassini orbiter would have passed the sign along the road that says 'Saturnian County line,'" said Jeremy Jones, chief navigator for the Cassini-Huygens mission at JPL. "The next exits are Phoebe, 9 million kilometers (5.4 million miles) ahead, Saturn 19 million kilometers (12 million miles) ahead."

Phoebe is an oddly shaped moon with a dark surface. It orbits in the opposite direction from the motion of most other bodies in the solar system. The backwards-revolution leads scientists to believe that it is an object captured from distant Kuiper Belt, making it an interesting target. "The Phoebe flyby may offer the first glimpse of what the frigid bodies at the edge of the solar system look like," said Dr. Bonnie Buratti, scientist on the Cassini-Huygens mission at JPL. "These bodies, which include Pluto and its satellite Charon, are believed to be remnant objects left over from the formation of the planets 4.5 billion years ago."

After the Phoebe flyby, Cassini will be on course for Saturn. On arrival date June 30 (July 1 Universal Time), Cassini will become the first orbiter around Saturn. "The two Voyager and Pioneer spacecraft flew by the planet and saw it from a distance two or three days at a time. With Cassini, we will be in the city limits for four years," said Dr. Dennis Matson, project scientist for Cassini at JPL. "The difference is like driving by the Grand Canyon versus stopping, getting off and enjoying the sights for a while."

On arrival, Cassini will begin a 96-minute burn designed to put the spacecraft into Saturn's orbit. As part of getting the spacecraft into orbit, Cassini will twice cross between known gaps in the rings. As a precautionary measure, the spacecraft will use its antenna as a shield to protect it from tiny particle hits.

A prime target for Cassini and the piggyback Huygens probe built by the European Space Agency is the smoggy moon Titan. "In the 350 years since the discovery of Titan we have come to see it as a world with surprising similarities to our own, yet located almost 1.5 billion kilometers (900 million miles) from the Sun," said Dr. Jonathan Lunine, Huygens interdisciplinary scientist and professor of planetary science and physics at the University of Arizona, Tucson. "With a thick, nitrogen-rich atmosphere and possible hydrocarbon seas, Titan may harbor organic compounds important in the chain of chemistry that led to life on Earth."

Six months after reaching Saturn, Cassini will release the wok-shaped Huygens probe towards Titan on Dec. 24, 2004 (Dec. 25 Universal Time). The event will be by far the most distant descent of a robotic probe on another object in the solar system. On Jan. 14, 2005 (Jan. 15 Universal Time), Huygens will enter Titan's atmosphere, deploy its parachute, and begin its scientific observations of Titan.

May 26, 2004

Saturn Seen From Far And Near

As Saturn grows closer through the eyes of the Cassini spacecraft, which is hurtling toward a rendezvous with the ringed world on June 30 (July 1, Universal Time), both Cassini and the Earth-orbiting Hubble Space Telescope snapped spectacular pictures of the planet and its magnificent rings.

Cassini is approaching Saturn at an oblique angle to the Sun and from below the ecliptic plane. Cassini has a very different view of Saturn than Hubble's Earth-centered view. For the first time, astronomers can compare views of equal sharpness of Saturn from two very different perspectives.

The view from Hubble, taken on March 22, 2004, is so sharp that many individual Saturnian ringlets can be seen. When Cassini returned its picture of Saturn on May 16, it was so close to the planet that the imaging science subsystem narrow-angle camera could not fit the whole planet in its field-of-view. Cassini is still about 20 million kilometers (about 12.4 million miles) away and only 36 days from reaching Saturn.

Hubble's exquisite optics, coupled with the high resolution of its Advanced Camera for Surveys, allow it to take pictures of Saturn which are nearly as sharp as Cassini's, even though Hubble is nearly a billion miles farther from Saturn than Cassini. Cassini will ultimately far exceed the resolution of Hubble during its close encounter with Saturn. Cassini's sharpness began to surpass Hubble's when it came to within 23 million kilometers (14 million miles) of Saturn earlier this month.

Camera exposures in four filters (blue, blue-green, green and red) were combined to form the Hubble image, to render colors similar to what the eye would see through a telescope focused on Saturn. The subtle pastel colors of ammonia-methane clouds trace a variety of atmospheric dynamics. Saturn displays its familiar banded structure, and haze and clouds of various altitudes. Like Jupiter, all bands are parallel to Saturn's equator. Even the magnificent rings, at nearly their maximum tilt toward Earth, show subtle hues, which indicate the trace chemical differences in their icy composition.

Cassini has two cameras, a wide angle and narrow angle. This narrow angle image was made using a combination of three filters (red, green, blue) and was taken at a range of 24.3 million kilometers (15.1 million miles). The view is from 13 degrees below the equator. Enceladus, one of Saturn's 31 known moons, appears near the south pole at the bottom of the image.

The color differences between the Hubble and Cassini images are mainly due to the different sets of filters used.

More than two decades have passed since a spacecraft last visited Saturn -- NASA's Voyager-2 flew by Saturn in August 1981. Since 1990, Hubble has produced high-resolution Saturn images, tracking storms and auroral activity while providing crisp views of the rings over time and from various angles.

Cassini will begin a four-year mission in orbit around Saturn when it arrives on June 30, 2004 (July 1, 2004 Univeral Time). Six months later it will release its piggybacked Huygens probe for descent through Titan's thick atmosphere.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, Md. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. The Cassini-Huygens mission is a cooperative mission of NASA, 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 Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.