Cassini Significant Events -- for 07/14/05 - 07/20/05

July 22, 2005

(Source: Cassini Project)


The most recent spacecraft telemetry was acquired Wednesday, July 20, from the Madrid tracking stations. 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 .

Activities this week:

Thursday, July 14 (DOY 195):

Cassini passed by Rhea today for a non-targeted encounter. The RADAR
instrument performed scatterometry and radiometry observations of that moon
to constrain the bulk density of the top decimeter of the surface, and to
help constrain the surface thermal properties.

The main event today was Cassini's second targeted flyby of Enceladus. A
non-targeted encounter took place on February 17, 2005 at a range of 1172
km. At that time the Dual Technique Magnetometer (MAG) saw a remarkable
signature in Saturn's magnetic field in the vicinity of Enceladus suggesting
the presence of ions surrounding the icy moon. Gases may be originating from
the surface or possibly the interior of Enceladus. The first targeted
encounter was on March 9, 2005 at a distance of 500 km.

The encounter this week had originally been designed to be flown at an
altitude of 1000 km. Because the February and March flybys suggested
intriguing magnetic and atmospheric signatures, the Cassini science
community was very interested in obtaining more detailed data. After
investigating the situation, the Program approved the release of a new
reference trajectory lowering the flyby altitude to 175 km. This is
Cassini's lowest-altitude flyby of any object to date.

For this encounter the entire suite of Magnetospheric and Plasma Science
instruments took high data-rate science. MAG monitored the interaction of Enceladus with the Saturnian magnetospheric plasma to determine whether it generates an internal magnetic field, and engaged in measurements that will yield unique observations of Saturn's internal magnetic field.

The Cosmic Dust Analyzer took measurements to help determine the density of the E Ring, and of the dust surrounding Enceladus.

The Radio and Plasma Wave Science instrument (RPWS) executed many
observations in the immediate vicinity of Enceladus that will help
characterize the plasma wave spectrum, and searched for evidence of pickup ions.

All the Optical Remote Sensing instruments participated in a hectic campaign to obtain high-resolution mosaics and spectra of Rhea and Enceladus during their respective flybys.

Of primary interest to the Ultraviolet Imaging Spectrograph is an
occultation by Enceladus of the star Bellatrix during Cassini's closest
approach which will help constrain Enceladus' atmospheric species and
density.

The Composite Infrared Spectrometer obtained its only temperature
measurement of Enceladus' winter north polar night region, which has been in
darkness for more than eight years, and also looked for active surface
sources that may contribute material to Saturn's E Ring.

Further icing on the cake today included additional non-targeted flybys of
the Saturnian moons Prometheus, Methone, and Epimetheus.

Friday, July 15 (DOY 196):

Following yesterday's very successful Enceladus flyby, the Radio Science
Subsystem (RSS) performed its fifth occultation experiment intended to
measure the properties of Saturn's rings and atmosphere. The orbit 11 RSS
Saturn/Rings Egress occultation completed successfully over the DSS stations
at Canberra for the first segment of the occultation. Then these stations
overlapped with the Madrid stations for 45 minutes before Madrid was left to
cover the second segment.

This was the first occultation implemented within a Ground Movable Block
(GMB) and did not require a Live Inertial Vector Propagator Update shortly
prior to the occultation. Out of the eight diametric occultations taking
place between May and September of this year, this occultation occurred over
the lowest ground antenna elevation angles, not exceeding 30 degrees of
elevation. This made it very challenging in terms of developing an optimum
pointing strategy for Ka-band.

Monday, July 18 (DOY 199):

Cassini enters Solar Conjunction today with a separation angle of 4 degrees.
During conjunction, communications with the spacecraft become degraded due
to interference from the sun. For the next nine days the Spacecraft
Operations and Mission Support and Services offices will participate in a
campaign where multiple non-operational commands are sent to the spacecraft
on a daily basis to obtain link characterizations.

In addition, Commands will be sent to the spacecraft to modify the System
Fault Protection Command Loss Timer (CLT) strategy during the conjunction
period. Part of the fault protection system monitors communications with the
ground. If for any reason a CLT command is not received by the spacecraft
before the expiration of the time limit, the spacecraft will assume there is
a "problem," call fault protection and put Cassini into safe mode until the
"problem" is resolved. This is a common feature on all spacecraft and CLT
commands are sent each time Cassini has a DSN pass to prevent the expiration
of the time limit.

As was mentioned in the report last week, the Program approved raising the
T7 flyby altitude from 1025 to 1075 km to avoid the risk of
atmosphere-induced torques that could possibly result in spacecraft loss of
attitude control and safing. Today the Navigation team released a new
reference trajectory 050207 to accommodate these changes.

The final sequence products for S13 were made available in the Program file
repository for team review.

Uplink Operations sent commands to the spacecraft to disable RPWS sounder
operations, turn the Ka-band ON for an RSS Solar Conjunction demo, load the
ISS version 1.4 flight software in to the cameras, and repair the SSR
partitions for CDS, ACS and the instruments.

A workshop on the surface of Titan was hosted by the USGS in Flagstaff,
Arizona on July 18 and 19. The goal of the workshop was to develop an
integrated picture of Titan from data sets returned over the last year
regarding primarily the surface, but also including the atmosphere,
especially as it relates to the surface, and use this as an aid to planning
the remainder of the Cassini prime mission. Items that were discussed
included a summary of the main findings from each team, opportunities for
integrated data interpretation, the development of cooperative observations
and the TITEWAD web-based GIS planning tool, ground-based support
measurements, aftermarket changes to the science operations plan, and
extended mission planning.

Tuesday, July 19 (DOY 200):

An Atmospheric Science Preview meeting was held today to highlight
interesting Saturn observations being conducted in Revs 9-15. In the
October time frame, another preview meeting will be held to cover the
Equatorial Orbits, i.e. Revs 15-24ish.

Wednesday, July 20 (DOY 201):

An S15 Science Operations Plan Update process project briefing and waiver
disposition meeting was held today.

In last week's Significant Events it was erroneously reported that an encounter strategy meeting for Enceladus 2 through Titan 5 was held on July 7. The meeting does not occur until July 22.