News | December 3, 2015
Cassini Significant Events 11/18/15 - 12/01/15
Cassini is currently orbiting Saturn with a period of 12.7 days in a plane inclined 1.3 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on Dec. 2 using 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 .
While many of Cassini's flight team members were enjoying the US Thanksgiving holiday schedule, the spacecraft was commanded to take something of a break as well. Each year, Earth's motion in solar orbit causes Saturn and Cassini to slip around behind the Sun. This year the superior conjunction made the spacecraft appear to be only 1.7 degrees from our star's center as viewed from the home planet. The Sun's radio noise would limit the ability to communicate via command and telemetry, and would introduce more noise in the Doppler shift and range data that the Navigation team uses to track Cassini. Therefore, planners reduced the amount of telemetry data that Cassini's scientific observations would produce during the six days in which Cassini appeared closest to the Sun in our sky, and simplified the spacecraft's activities. Its attitude remained in one orientation for that period, facing its high-gain antenna constantly toward Earth.
But one person's noise is another's data, so the Radio Science team used Cassini's signals to probe the solar corona directly, rather than via telemetry data. This Radio Science Superior Conjunction Experiment, the 12th in Cassini's long mission, started on Nov. 18 and will continue through Dec. 14. It uses the Deep Space Network (DSN) to record Cassini's three continuous radio-frequency transmissions, at S-band (5 GHz), X-band (8 GHz), and Ka-band (32 GHz), at every opportunity.
Wednesday, Nov. 18 (DOY 322)
Cassini's Imaging Science Instrument (ISS) took control of spacecraft pointing for 17.3 hours today. First, it looked at Saturn for two minutes, making a storm-watch observation. Next, ISS examined the space near Saturn for an hour, looking for small objects orbiting there, as part of the satellite orbit campaign. ISS's final target today was the tiny irregular moon Bestla. Named after a frost giantess from Norse mythology, Bestla moves in a retrograde, highly inclined and highly eccentric orbit that takes it as far as 30 million kilometers from the planet. This observation was the last opportunity for Cassini to ever observe Bestla, because of its increasing distance, and the increasing phase angle of its solar illumination.
Thursday, Nov. 19 (DOY 323)
The Cosmic Dust Analyzer (CDA) began a 31-hour observation of dust that orbits Saturn in the retrograde direction.
Friday, Nov. 20 (DOY 324)
The flight team used the 70-meter diameter DSN station at Goldstone, California today to uplink the first part of the S92 command sequence. After a round-trip light time of 3 hours 3 minutes, telemetry data showed that each of the sequence's 7,793 individual commands had been properly received and stored aboard.
Saturday, Nov. 21 (DOY 325)
Cassini’s Magnetospheric Imaging Instrument (MIMI) rotated the spacecraft for nine hours today, to study Saturn’s inner magnetosphere. When this was completed, CDA began a 24-hour period of scans while approaching and then crossing the orbit of Titan.
Sunday, Nov. 22 (DOY 326)
Once again, MIMI rotated the spacecraft for nine hours, studying the inner magnetosphere.
Monday, Nov. 23 (DOY 327)
The Visible and Infrared Mapping Spectrometer (VIMS) called the shots for 11 hours of spacecraft pointing, to map Saturn’s northern hemisphere. ISS and the Composite Infrared Spectrometer (CIRS) made their own observations while riding along. Following this, the Ultraviolet Imaging Spectrograph (UVIS) mapped the planet in the extreme- and far-ultraviolet parts of the spectrum, to study the distribution of hazes and organic compounds high in Saturn’s atmosphere. ISS and CIRS rode along with this as well.
For 3.5 hours as Saturn's large icy satellite Tethys was passing in front of the planet from Cassini’s vantage, the full suite of optical remote-sensing instruments was trained on the 1,060-km wide body. They began by observing the sunlit western boundary of Tethys's thermally anomalous region at low latitudes, then continued to observe across its night-side trailing hemisphere to finish at the region's eastern boundary. These observations will better constrain the color and surface properties both inside and outside of the anomalous region, as well as its boundaries. The viewing geometry is illustrated here: https://space.jpl.nasa.gov/cgi-bin/wspace?tbody=699&vbody=-82&month=11&day=23&year=2015&hour=20&minute=00&fovmul=1&rfov=45&bfov=30&showsc=1&showac=1 .
Tuesday, Nov. 24 (DOY 328)
Early in the day, Cassini coasted through periapsis in its Saturn Orbit #226. Coming as close as 92,425 km above Saturn's visible edge, it had reached a speed of 77,319 km per hour relative to the planet.
CIRS examined Saturn’s limb for three hours at multiple latitudes in the mid-infrared, to obtain information about the atmosphere’s thermal structure. Afterwards, Cassini's flight path took it past the tenuous Janus-Epimetheus ring. The main engine protective cover was closed, and the high-gain antenna was oriented to shield the spacecraft from errant ring particles. The Cosmic Dust Analyzer (CDA) and the Radio and Plasma Wave Science (RPWS) instrument took advantage of this ring passage to measure dust flux from the ring, and to obtain high-resolution measurements of plasma waves at Saturn's magnetic equator.
With the spacecraft safely past the dusty ring, CIRS resumed its Saturn-limb observation for six more hours, expanding on the latitude coverage just obtained. ISS then turned to image Saturn’s limb at high phase angles, basically backlit, for 3.25 hours. Finally, UVIS observed for 1.5 hours as two bright blue stars were occulted by Saturn's upper atmosphere. The stars were Epsilon and Zeta Orionis, the left and center stars of Orion's belt. The stunning viewing geometry, with Saturn's night side and a familiar star pattern, is illustrated here: https://space.jpl.nasa.gov/cgi-bin/wspace?tbody=699&vbody=-82&month=11&day=24&year=2015&hour=17&minute=20&fovmul=1&rfov=30&bfov=30&showsc=1&showac=1 . These activities were the final science observations commanded from the 10-week-long S91 sequence.
Wednesday, Nov. 25 (DOY 329)
Today, the S92 Sequence began its 74 days of controlling Cassini's activities. First off, ISS, CIRS and VIMS made a 90-minute observation in the Titan monitoring campaign, from a distance of 1.4 million km to the hazy target, Saturn’s largest moon. Next, during a routine DSN communications session, there was an opportunity to fire Cassini's rocket thrusters to refine its flight path for the upcoming Enceladus flyby, E-22 on Dec. 19, which will be Cassini's final targeted encounter with the enigmatic moon with a plume. The Navigation team's solutions showed Cassini to be close enough to its reference trajectory, though, so the maneuver, Orbit Trim Maneuver (OTM)-432, was not needed, and it was cancelled in time for US NASA members of the flight team to enjoy a Thanksgiving holiday.
Thursday, Nov. 26 (DOY 330)
ISS spent nearly 15 hours observing Saturn’s shadow on the distant, faint Phoebe ring. CIRS rode along. A NASA Science News item from 2009 illustrates the Phoebe ring here: http://science.nasa.gov/science-news/science-at-nasa/2009/07oct_giantring .
Friday, Nov. 27 (DOY 331)
From today through Dec. 2, the spacecraft remained earth-pointed as Saturn entered superior conjunction. The secondary axis of orientation was chosen for the benefit of the Magnetospheric and Plasma Science (MAPS) instruments, allowing for collection of magnetotail data during the period.
Sunday, Nov. 29 (DOY 333)
In one year from now, the gravity-assist nudge that Cassini will take when flying by Titan, during the T-125 encounter at just over 3,000 km altitude, will reduce the periapsis altitude of the spacecraft's orbits to about 150,000 from the center of Saturn. Saturn's narrow, ever-changing F ring circles the planet at about 140,000 km out. Cassini will then have 20 periapses at that altitude, with extraordinary opportunities to study the F ring. Thereafter, the final proximal orbits will carry Cassini between the innermost ring and the upper atmosphere of Saturn.
Monday, Nov. 30 (DOY 334)
Cassini coasted through apoapsis today, marking the start of its Saturn Orbit #227. Reaching an altitude of 990,808 km from the planet's center, the spacecraft had slowed to 6,056 km per hour relative to Saturn.
Saturn's small moon Enceladus appears in front of the rings in an image featured today: /resources/16275 .
Tuesday, Dec. 1 (DOY 335)
During the past two weeks, the Deep Space Network communicated with and tracked Cassini on nine occasions, using stations in Australia and California. A total of 7,836 individual commands were uplinked, and about 1,728 megabytes of telemetry data were downlinked and captured at rates as high as 110,601 bits per second.
This illustration shows Cassini’s position on Dec. 1: http://go.nasa.gov/1QSGkKs . The format shows Cassini's path over most of its current orbit up to today; looking down from the north, all depicted objects (except the background stars of course) revolve counter-clockwise, including Saturn along its orange-colored orbit of the Sun.