Cassini Significant Events 04/22/2015 – 04/28/2015

Cassini is currently orbiting Saturn with a 28-day period in a plane inclined 0.3 degree from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on April 29 using one of the 34-meter diameter Deep Space Network stations in Australia. The spacecraft continues to be in an excellent state of health with all of its subsystems operating normally except for the instrument issues described at http://go.nasa.gov/1bQ48gw .

Sequence Implementation Process (SIP) teams worked on upcoming 10-week command sequences; S89 goes up on April 29 and will begin controlling the spacecraft on May 1. The S90 sequence is planned to start clocking out on July 13, and S91 will take control of Cassini on Sept. 21 of this year. Skipping ahead, in November 2016, events during the course of S97 will bring Cassini's periapses near Saturn's F ring, and then S99 through S101 will cover the spacecraft's final activities through mid September 2017. Each SIP team's experts pull together a very wide range of considerations and spacecraft activities, all of which culminate in well-orchestrated and error-free performances at Saturn.

Activities for each sequence include myriad science observations and calibrations, plus dozens of spacecraft engineering activities and periodic maintenance exercises. Of course, SIP teams must accommodate constraints and rules and celestial events such as solar conjunctions, spacecraft ring-plane crossings, periapses, apoapses and flybys. The SIP choreography also includes constant efforts to negotiate and schedule time on the Deep Space Network antennas, and accommodate other ground system activities. This week was the final full week of S88 execution, after which the S88 SIP team will enjoy a short break before moving on to work future sequences.

Wednesday, April 22 (DOY 112)

The Ultraviolet Imaging Spectrograph (UVIS) completed a 31-hour observation of Saturn’s aurora, with the Composite Infrared Spectrometer (CIRS) and the Visible and Infrared Mapping Spectrometer (VIMS) also acquiring data in ride-along mode.

Thursday, April 23 (DOY 113)

The Imaging Science Subsystem (ISS) took control of spacecraft pointing to begin a 19.5-hour observation of Saturn’s irregular moon Paaliaq, the small, distant object described in these pages on April 2: http://go.nasa.gov/1ElnrYh .

Two years from today, Cassini’s Grand Finale, aka the Proximal Orbit phase, will begin. After a gravity assist from Titan, the spacecraft will pass through apoapsis, beginning orbit #271 and 22 orbits around Saturn with periapsis between Saturn’s atmosphere and the D Ring. These orbits are illustrated in light blue in this image: /resources/16185 .

Friday, April 24 (DOY 114)

With the Paaliaq observation finished, ISS began a 24-hour observation, tracking an irregular moon of Saturn named Bebhionn. This very dark-surfaced object is about six kilometers in diameter, and takes 835 days to go around Saturn, reaching as far as 17.12 million kilometers from the gas giant. Its orbit is highly inclined and highly eccentric. A member of the Gallic group of Saturn's moons, it was discovered from Earth, and named after a giantess in Celtic mythology.

Saturday, April 25 (DOY 115)

Cassini reached apoapsis at 3.22 million kilometers from Saturn, marking the start of its orbit #215. It had slowed to 5,364 kilometers per hour relative to the planet. For the next science activity, ISS took the reins to begin an 18-hour, edge-on observation of the faint outer rings while they were backlit by the Sun. The other telescopic instruments -- CIRS, VIMS, and UVIS -- rode along.

Sunday, April 26 (DOY 116)

CIRS carried out a 12-hour stare into Saturn’s atmosphere to better understand its chemical composition. VIMS rode along. Both preceding and following this activity, ISS made two-minute storm-watch observations.

Monday, April 27 (DOY 117)

The Cosmic Dust Analyzer (CDA) began a 37-hour observation as part of the campaign to collect and report on particles of dust that orbit Saturn in the retrograde direction.

Particles in Saturn's D ring, which is best seen when lit from behind, travel all the way around the 121,000-kilometer diameter planet in less than five hours. An image featured today contrasts this ring with the more highly visible C ring:
/resources/16185 .

Tuesday, April 28 (DOY 118)

Largely concurrent with all of the other activities Cassini performed this week, the direct-sensing Magnetospheric and Plasma Science instruments continued to make observations of the conditions in the spacecraft's immediate environment.

Saturn is rising before midnight these days, and it makes a fine appearance in almost any telescope. The wide main rings are clearly visible, and a sliver of the oblate planet's shadow can be seen on the rings behind.

During the past week, the Deep Space Network communicated with and tracked Cassini on five occasions, using stations in California and Australia. A total of eight individual commands were uplinked, and about 1,232 megabytes of telemetry data were downlinked and captured at rates as high as 142,201 bits per second.

This illustration shows Cassini's position on April 28:
https://space.jpl.nasa.gov/cgi-bin/wspace?tbody=-82&vbody=1001&month=4&day=28&year=2015&hour=23&minute=55&fovmul=1&rfov=40&bfov=30&brite=1&showsc=1&showac=1 .

Milestones spanning the whole orbital tour are listed here: http://saturn.jpl.nasa.gov/mission/saturntourdates .

Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at:
http://saturn.jpl.nasa.gov/mission/presentposition/ .