Cassini Significant Event Report

For Week Ending 06/18/04

The most recent spacecraft telemetry was acquired from the Goldstone tracking station on Wednesday, June 16th. 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 .

On Friday, June 11th, 2004, Cassini made its first encounter with a member of the Saturn system. This week's Phoebe encounter, the only flyby of an outer Saturnian satellite in the mission, and the first close flyby ever of an irregular Saturnian satellite, was spectacularly successful! Cassini came within approximately 2,068 kilometers of the dark moon. It has been 23 years since the Voyager 2 flyby of Phoebe in 1981 at 2.2 million kilometers, more than 1,000 times farther away.

Since all of the optical remote sensing instruments were pointing towards Phoebe during the flyby, it was not until several hours later that the spacecraft turned to relay the data back to Earth. The signal was received on Saturday, June 12th, through the Deep Space Network antennas in Madrid, Spain and Goldstone, California. Five instruments reported taking significant data: the Composite Infrared Spectrometer (CIRS), Imaging Science Subsystem (ISS), RADAR, Ultraviolet Imaging Spectrograph (UVIS), and the Visible and Infrared Mapping Spectrometer (VIMS).

These instruments returned a wealth of scientific data on this tortured and battered moon, thanks to the success of the Live Update process that enabled a last minute modification in the pointing of the spacecraft to more accurately target Phoebe. Further analysis of the data over the next several weeks will begin to unravel the moon's past and possible age and origin, resolve its mass and physical properties, and begin to shed light on the moon's composition, surface properties, and topography.

VIMS will provide the first ever resolved spectra of the surface of Phoebe, up to 0.5 km/pixel at closest approach, with full range 0.4 to 5-micron spectra. This data will be used to derive compositional maps of Phoebe's surface.

Phoebe is an exceptionally interesting target for CIRS, due to its unusual surface composition compared to most of the icy satellites and its relatively warm temperatures, which will provide high signal to noise data. CIRS performed compositional and thermal observations to assist in identifying Phoebe's origins and surface properties including global mapping of composition and both day and night temperature distributions.

UVIS measured Phoebe's ultraviolet surface reflectance, providing the first ultraviolet albedo map of this interesting body. The UVIS measurements will aid in understanding Phoebe's compositional makeup and distribution of volatiles.

RADAR team members were excited, as it has been a long time between observations. The last opportunity was 5 years ago at the time of the Earth flyby in June of 1999. RADAR observations of Phoebe penetrated to between 2 cm and 20 cm, and will constrain the bulk density and/or the relative ice cleanliness in the upper layer of regolith. This along with volume derived from imaging data will help determine if it is a highly porous 'rubble pile' based on its low density, or a more compact body as might be suggested from its roughly spherical shape. The density value will also be used to constrain its composition and indicate the rough proportions of rock and ice in its make-up.

ISS will contribute multi-color mapping of almost the entire surface at 0.3 to 2.1 kilometers per pixel resolution. The first high-resolution images of Phoebe show a scarred surface, covered with craters of all sizes and large variations of brightness across the surface, giving strong evidence that the tiny moon may be rich in ice and covered by a thin layer of darker material. Phoebe is a world of dramatic landforms, with landslides and linear structures such as grooves, ridges and chains of pits.

Although working groups must convene to discuss and refine conclusions, the Cassini science community and flight team are extremely pleased and excitement is running high. This attitude is reflected in the response of the general public to this event.

On June 14, a Cassini picture of Phoebe was Astronomy Picture of the Day. Also last week the flyby achieved the top science story spot on the Google website.

JPL has released multiple press releases and images, and, NASA Ames Research Center, and ESA published additional articles. Go to to access these items. Links to other sites referenced in this status report are listed below.

Astronomy Picture of the Day

If "today" for you is no longer June 14, 2004, then go to:

Google - Space probe fly-by of Saturn's moon - Jun 11, 2004


June 9, 2004

Cassini-Huygens looks at Phoebe's distant past

European Space Agency

14 June 2004

Additional spacecraft activities this week included the successful execution of Trajectory Correction Maneuver (TCM) -21. The "quick look" immediately after the maneuver showed the burn duration was 38.38 sec, giving a change in velocity of 3.68 meters/second.

The SOI critical sequence was also successfully uplinked to the Command and Data Subsystem's main memory. The critical sequence is on board and waiting for the "activate" command which will be radiated this coming Thursday, June 17, 2004. Additional on-board instrument activities included a Radio and Plasma Wave Science instrument Periodic Instrument Maintenance.

In the last week, 994 ISS images were obtained and were distributed along with 1160 VIMS cubes. The total number of ISS images acquired since the start of Approach Science is now 11976, and the number of VIMS cubes is 4085.

Sequence development activities are proceeding on schedule. The official port #2 for Science Operations Plan (SOP) Implementation of tour sequences S29 and S30 occurred this week, as well as the official port #1 for tour sequences S31 and S32. All products have been merged and handed to the ACS team for the end-to-end pointing validation.

Preliminary port #1 for SOP Update for tour sequence S04 occurred this week. The products were merged and reports delivered to the instrument team. As part of the Aftermarket process for S06, an assessment meeting was held. Since changes to the sequence were minimal, they were determined to fit within the available resources.

The S02 background sequence and Probe Checkout #14 sequence were approved at the S02 Sequence Approval meeting. Both files will be uplinked to the spacecraft next week. S02 begins execution on Saturday June 19. The Probe Checkout activity is designed to simulate to release of the probe from the orbiter.

In support of SOI activities, a Playback Assessment Meeting has been scheduled for July 2. The purpose of this meeting is for the instruments to report whether or not they found the SOI playback to be sufficient, based on pre-determined criteria. This is part of the contingency plan to ensure success of the overall orbit insertion activity.

Outreach activities include the formal release of the "Reading, Writing, and Rings" lesson set for grades 3 and 4. This release marks the culmination of development on Cassini's blended language arts and science lesson sets for elementary school classrooms. All materials are now available to download from the education section of the Cassini website.

Additional information about Cassini-Huygens is online at

Cassini will begin orbiting Saturn on July 1, 2004, and release its piggybacked Huygens probe about six months later for descent through the thick atmosphere of the moon Titan. Cassini-Huygens is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.

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