Cassini slipped in between Saturn and its innermost rings on Wednesday, setting a new record for its Saturn relative velocity as it streaked through the gap. It had been falling in orbit, gathering speed, for a million miles following Cassini's final targeted encounter with Saturn's moon Titan the week before. That encounter's gravity assist worked as precisely as JPL's world-class team of astrodynamicists had planned. It threaded the eye of the needle accurately, delivering the spacecraft inside Saturn's ring system and safely above its atmosphere. Cassini's direct measurements of the dust population in that previously unexplored region showed a surprisingly benign environment, which was promptly dubbed "the big empty;" the spacecraft's antenna dish will not have to be used as a shield on most subsequent orbits.
The new trajectory, though, is far from being empty of scientific bounty. It is proving to be a rich field for close-up remote-sensing views of Saturn, for direct-sensing investigations of the magnetic field, plasma, radio and plasma waves, for in-situ ring particle measurement, for new gravitational studies of Saturn's mass distribution, even measuring the mass of the rings for the first time. In short, Cassini's Grand Finale Proximal Orbits have started a whole new mission's worth of discovery.
Wednesday, April 26 (DOY 116)
Cassini passed over Saturn’s north polar region as close as 6,000 kilometers above the visible atmosphere, providing for enormously improved observations by the on-board Optical Remote-Sensing (ORS) instruments. For 16.5 hours, these instruments made telescopic observations of Saturn's atmosphere: the Visible and Infrared Mapping Spectrometer (VIMS), the Composite Infrared Spectrometer (CIRS), the Imaging Science Subsystem (ISS), and the Ultraviolet Imaging Spectrograph (UVIS). Meanwhile the Magnetospheric and Plasma Wave Science (MAPS) instruments recorded direct-sensing data continuously.
At 2.5 hours before crossing the ring plane, ISS controlled the spacecraft's pointing to make a high-priority 2.3-hour observation using its Wide-Angle Camera (WAC) on Saturn; CIRS and VIMS rode along. Next, Cassini turned to point its high-gain antenna dish squarely into the expected direction of the oncoming dust, in order to act as a shield in the event there were particles large enough to cause damage. Relative to any dust, Cassini's velocity was 113,400 km per hour. This maneuver only took 20 minutes away from ISS's important activity.
Right on track, periapsis passage on Cassini's orbit #271 came hair-raisingly close to the gas giant, at an altitude of only 1.047 Saturn radii from the planet's center. And, relative to the planet's center, the spacecraft's velocity peaked at 122,245 km per hour. Of course, being in orbit, the spacecraft felt no acceleration at all; only constant free-fall.
After the shielded dust crossing, Cassini gave the reins back to ISS, which continued its high-priority, close-up WAC observations for another 1.9 hours; CIRS and VIMS again rode along. For the next five hours, the ORS instruments continued to observe Saturn's southern hemisphere and its south polar region. During the day, these instruments obtained resolutions that were 10 to 100 times better than they had previously been able to achieve.
The Radio and Plasma Wave Science (RPWS) instrument measured the equatorial dust flux while crossing the ring plane, and made high-resolution measurements of plasma waves at Saturn's magnetic equator. These measurements would help in understanding whether there is a dust population migrating from the rings down to the atmosphere.
Starting 4.5 hours after periapsis, VIMS led the other ORS instruments in observing Saturn's rings from a new perspective. Largely backlit by the Sun, the observations included the outermost faint G and E rings, and all the way inward to the D ring. Next, VIMS and UVIS watched the Sun for 1.6 hours, using their solar ports, while it became occulted by the rings.
Finishing up this monumental day, UVIS took the helm for a 2.7-hour observation, staring at Saturn’s south polar auroral zone while that region was in darkness. All the other ORS instruments rode along.
Thursday, April 27 (DOY 117)
VIMS and CIRS watched the bright red star Alpha Orionis, also known as Betelgeuse, for 1.1 hours while it went into occultation behind Saturn's upper atmosphere. Next, UVIS took the lead for two hours to complete its observation of Saturn’s south polar auroral zone.
Cassini turned and pointed its four-meter-wide high-gain antenna to Earth, and the largest Deep Space Network (DSN) station in California captured Cassini's signal when it arrived 78 minutes later. Gathered at JPL near midnight local time, flight team members, along with many friends and family, cheered when the signal appeared, showing that the spacecraft had indeed survived its first encounter with the unknown. For the next 12 hours, telemetry data from Cassini flowed in, playing back the close-up science observations, and reporting on the spacecraft's excellent health. The largest DSN station in Australia picked up the signal hours later, after Saturn and Cassini had set in California's western sky.
As reported here, the news from Cassini could not have been better: /news/13032/nasa-spacecraft-dives-between-saturn-and-its-rings .
Following Cassini's sessions with the DSN, ISS turned and captured an observation of Saturn's small moon Enceladus, to monitor the activity of its plume of ejecta for 12.5 hours; all the other ORS instruments rode along.
Friday, April 28 (DOY 118)
Today, VIMS spent 3.8 hours making global pole-to-pole mosaics of Saturn, with CIRS riding along. Next, CIRS embarked on a 10.8-hour observation studying the composition of the Saturn's atmosphere; this covered one full Saturn rotation; UVIS and VIMS rode along.
Saturday, April 29 (DOY 119)
Today, ISS observed Saturn's brightly illuminated crescent limb for 1.5 hours. Riding along, VIMS provided mid-infrared-wavelength sounding to determine stratospheric thermal structure; UVIS also rode along. The beautiful view from Cassini is illustrated here.
Cassini coasted through apoapsis later today, having completed its first full Proximal Orbit of Saturn. It reached an altitude of 1.27 million km, and had slowed to 6,032 km per hour relative to the planet, less than one-20th its speed during Wednesday's close periapsis passage. This marked the start of Orbit #272.
Sunday, April 30 (DOY 120)
VIMS, ISS, and UVIS observed an eight-hour occultation by Saturn's rings of the bright star Alpha Canis Majoris, widely known as Sirius. This helped to study the radial structure of the A and B rings. The observation also helped constrain knowledge of ring-particle size distribution, by comparing the optical depths in the ultraviolet and near-infrared parts of the spectrum at the same locations.
Appropriately today, NASA selected one of Cassini's close-up views of Saturn as the Astronomy Picture of the Day: https://apod.nasa.gov/apod/ap170430.html .
Monday, May 1 (DOY 121)
ISS took images of the lit face of Saturn's bright, narrow F ring for 7.8 hours; VIMS rode along. The next science observation was of Saturn's planet-like moon Titan, from a distance of two million km. ISS, CIRS, and VIMS monitored the weather in that moon's atmosphere for 90 minutes. When this was done, CIRS observed Saturn's second-largest satellite, Rhea, to constrain knowledge of its surface emissivity at thermal infrared wavelengths. This will help determine the composition and structure of Rhea's regolith.
A narrative about the dust-free region close to Saturn was featured here: /news/13034/cassini-finds-the-big-empty-close-to-saturn .
Enceladus was the subject of the Cassini image spotlighted today: /resources/17663 .
Tuesday, May 2 (DOY 122)
ISS acquired high-resolution images of Saturn's narrow F ring and the outer edge of the broad A ring today. First, the feature dubbed "Peggy" on the A-ring’s edge was observed, and then the F-ring-crossing object known as F16QB. More information on Peggy may be found here: /resources/16008 . The goal was to recover both these objects, improve knowledge of their orbits, and investigate their evolution.
Just as the excitement from first ring-plane crossing and periapsis passage began to wane, it was time for another. Today's was the second pair in the 22 Grand Finale Proximal Orbits. The highest priority observation of this period was for the Magnetometer (MAG). The spacecraft rotated for about 18 hours to permit MAG to make observations and calibrations near the planet. This was the first time such an observation has been undertaken in the mission. It was also the first time in the mission since the Earth flyby in 1999 that the magnetometer has been able to obtain calibration data in its highest field range (>10,000 nT).
Saturn is putting on a good show this month. Finally, it's rising before midnight now, and will be up by 9:30 p.m. towards the end of the month. This 60-second video shows when and where to look: https://www.instagram.com/p/BTkm2gfl_-Q/?taken-by=nasajpl .
The DSN communicated with and tracked Cassini on seven occasions this week, using stations in California and Australia. A total of 14 individual commands were uplinked, and about 1,927 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini has finished the first of its 22 Grand Finale Proximal orbits, which have a period of 6.4 days, in a plane inclined 62.4 degrees from the planet's equatorial plane. The second proximal plunge through the ring plane occurred on Tuesday May 2.
The most recent spacecraft tracking and telemetry data were obtained on May 2 using the 70-meter diameter DSN 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/anomalies .
The countdown clock in Mission Control shows 136 days until the end of the Mission.
This page offers all the details of the Mission's ending: https://saturn.jpl.nasa.gov/mission/grand-finale/overview .
Milestones spanning the whole orbital tour are listed here:
Information on the present position and speed of the Cassini spacecraft may be found on the "Present Position" page at: