Status Report

NASA Cassini Significant Events for 06/10/04 – 06/16/04

By SpaceRef Editor
June 19, 2004
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NASA Cassini Significant Events for 06/10/04 – 06/16/04
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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
http://saturn.jpl.nasa.gov/operations/present-position.cfm .

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.

SpaceRef staff editor.