Cassini is currently orbiting Saturn with a 31.9-day period in a plane inclined 28.6 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Jan. 7 using one the 70-meter diameter Deep Space Network station 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://saturn.jpl.nasa.gov/news/significantevents/anomalies .
Holidays and vacation took the place of some of the flight team's meetings and routine activities during the past three weeks, but realtime operations proceeded at a normal pace; for example, the Imaging Science Subsystem (ISS) alone acquired and sent home more than two thousand images. The spacecraft also performed some important calibration activities.
Cassini continued climbing outbound from its Dec. 8 periapsis at Saturn, and reached apoapsis, the orbit's high point, on the 24th. Near that point, a short rocket-engine burn set up the trajectory for the next Titan encounter, T-108, which will occur on Jan. 11. As usual, the electrically driven reaction wheels managed the spacecraft's attitude, allowing the telescopic remote-sensing instruments to track their targets. Most of the time, the direct-sensing Magnetospheric and Plasma Science (MAPS) instruments also took data about conditions in the immediate environment.
Wednesday, Dec. 17 (DOY 351)
The S87 command sequence began executing today. These on-board commands will run until February 21, by which time Titan encounters will have decreased Cassini's orbital inclination to within 8.5 degrees of the ring plane.
The Composite Infrared Spectrometer (CIRS) stared at the sunlit side of Saturn's C ring for five hours, obtaining thermal infrared spectra to study ring particle composition; ISS and the Visible and Infrared Mapping Spectrometer (VIMS) took advantage of the pointing and "rode along" to take data. This observation repeated for ten hours on Dec. 21, Dec. 27, and Jan. 1. When today's observation was finished, CIRS began a 23-hour study of Saturn’s atmosphere in the mid-infrared part of the spectrum, in an effort to determine temperatures in the upper troposphere and tropopause.
Sometimes when the telescopic instruments are pointed near Saturn, ISS calls for a small change in attitude to carry out a two-minute storm-watch observation on the planet. This was the case today, and again on Dec. 18, when VIMS also participated. ISS repeated the observation on Dec. 21, Dec. 22, Dec. 27, on Jan. 1 with VIMS, and on Jan. 2; a second one that day also included VIMS. Jan. 5 had an ISS-only storm-watch, followed by an ISS and VIMS storm-watch on Jan. 6. Looking back over three weeks brings to light the repetitive nature of some of Cassini’s monitoring activities.
Thursday, Dec. 18 (DOY 352)
A news feature released today discusses Jupiter's moon Europa. Cassini’s flyby of Jupiter in 2001 allowed scientists a snapshot of Europa’s state of activity at that time. Cassini's discovery of jets of water issuing from Saturn's small moon Enceladus plus tantalizing evidence from the Hubble Space telescope has invigorated interest in the possibility or plume activity at Europa as well:
Sunday, Dec. 21 (DOY 355)
CIRS started its day by observing Saturn for 12 hours to better understand the atmosphere's composition; ISS and VIMS rode along. Next, ISS performed an observation in the satellite orbit campaign, looking near Saturn for small known objects and to possibly discover new ones. This 60-minute observation was repeated on Dec. 27, Jan. 1, Jan. 2, and finally on Jan. 6.
Late in the day CIRS, with ISS and VIMS riding along, began a ten-hour stare at the sunlit side of Saturn's A ring, the outermost of the three main rings visible to anyone with a small telescope. The observation gathered thermal-infrared spectra to study ring particle composition; this observation repeated on Saturday the 27th.
Monday, Dec. 22 (DOY 356)
ISS began a 37.5-hour observation of Saturn's irregular moon Thrymr. This is a member of the Norse group of Saturn's moons, which occupy distant, inclined, eccentric, retrograde orbits about the gas giant. Even though it was named for the giant who stole Thor's hammer, this very dark-surfaced object is only about six kilometers in diameter; however it does reach a "giant" distance of more than 20 million kilometers from the planet.
Wednesday, Dec. 24 (DOY 358)
The spacecraft coasted through apoapsis at 3.3 million kilometers from Saturn, having slowed to 6,064 kilometers per hour relative to the planet. This marked the start of Cassini's orbit #211. Late in the day, ISS began a 19-hour observation of the irregular moon Erriapus, one of the Gallic group of Saturn's satellites. These occupy distant, inclined, prograde orbits. Named for a Gaulish giant, Erriapus is only about eight kilometers in diameter. It has a very dark surface, and its orbit reaches 17.3 million kilometers from Saturn.
The Spacecraft and Navigation teams observed the annual Y-thruster calibrations. These calibrations are used to monitor the performance of the Y-facing thrusters and to determine if any changes should be made to the ground models.
Friday, Dec. 25 (DOY 359)
The MAPS instruments, in particular the Magnetometer (MAG) often request that the Cassini spacecraft perform slow rolls so that the they can observe while rotating through their environment, in MAG’s case, the magnetic field. These rolls are typically around the spacecraft’s Z-axis that is aligned along Cassini’s long dimension through the high gain antenna (HGA).
Today and again on Saturday, the spacecraft was commanded to roll not about the Z-axis, but about the electrical boresight of the HGA while it was directed at the earth. The purpose of this activity is to determine if Radio Science activities during the Grand Finale can be done while Cassini is rolling thus allowing MAG to share the activity time. This will be an important component of the design strategy for the Grand Finale science campaign.
Friday, Dec. 26 (DOY 360)
ISS, CIRS and VIMS made an observation in the Titan monitoring campaign today, when the target, Saturn's largest moon, was 3.5 million kilometers away. The observation was repeated on Dec. 27, Dec. 28, and Dec. 30. Each time, the distance had decreased by roughly 400,000 kilometers.
As part of a science campaign to study material that orbits Saturn in the retrograde direction, the Cosmic Dust Analyzer (CDA) observed dust particles for 11.25 hours today. The observation took place again, for 15 hours, on Dec. 29, and for 12.3 hours on the Dec. 30.
Sunday, Dec. 28 (DOY 362)
CDA performed a 9-hour observation as part of the exogenous dust campaign, studying material originating from outside the Saturn system. This recurred on Jan. 1.
Cassini's view of Saturn's heavily cratered satellite Mimas was featured today: http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=5130 .
Monday, Dec. 29 (DOY 363)
The spacecraft received commands from its dedicated engineers on Earth, who had it turn and fire its main engine for 5.5 seconds, carrying out Orbit Trim Maneuver (OTM)-399. This provided a change in velocity of 0.97 meters per second, adjusting Cassini's flight path for the upcoming Titan T-108 flyby.
Thursday, Jan. 1 (DOY 001)
The Ultraviolet Imaging Spectrograph (UVIS) tracked Saturn's small icy moon Mimas for five hours today, measuring the ultraviolet albedo of the body's anti-Saturnian hemisphere. This observation was repeated the next day for two hours.
Friday, Jan. 2 (DOY 002)
Late in the day, ISS started a 16-hour observation of the 325-kilometer wide Encke Gap in Saturn's a ring; the images will be assembled into a movie. CIRS and VIMS rode along.
Saturday, Jan. 3 (DOY 003)
ISS spent 16 hours today imaging Saturn's bright, narrow F ring to make a low-resolution movie. CIRS and VIMS rode along.
Sunday, Jan. 4 (DOY 004)
Earth went through perihelion today, its annual closest point to the Sun. Meanwhile 1,595,000,000 kilometers away, ISS's telescopes tracked Saturn's large, enigmatic moon Iapetus for 13.75 hours while it was four million kilometers from the spacecraft; CIRS and UVIS rode along. A similar observation was repeated starting on the next day and lasted 9.75 hours, with VIMS joining in. This unusual two-tone moon can be seen in a view similar to today's, from late 2013:
Cassini's image of Saturn's moon Rhea, hanging in front of the planet, became NASA's Astronomy Picture of the Day today: http://apod.nasa.gov/apod/ap150104.html .
Monday, Jan. 5 (DOY 005)
UVIS observed Saturn's 400-kilometer-wide moon Tethys, with ISS and VIMS riding along. The observation lasted 2.5 hours, and it was repeated the following day for 6.2 hours.
Several members of Cassini's Attitude and Articulation Control Subsystem team represented the project this week at the American Institute of Aeronautics and Astronautics (AIAA) Guidance, Navigation, and Control Conference in Kissimmee, Florida.
When VIMS led an observation in October 2013, of the red star L2 Puppis as it passed behind Saturn's rings, ISS took images in ride-along mode. One of them is featured today; clicking on the full-size version, the star can be seen just inward from the small Keeler Gap in the A ring:
Tuesday, Jan. 6 (DOY 006)
During the past three weeks, the DSN communicated with and tracked Cassini on 22 occasions, using stations in Australia and California. A total of 174 individual commands were uplinked, and about 3,346 megabytes of telemetry data were downlinked and captured at rates as high as 124,426 bits per second.