This week saw just another “routine” orbit of the ringed jewel of the solar system. From a high point of nearly a million miles from Saturn on Friday, Cassini bolted close by Saturn on Monday, slipping between its atmosphere and its rings again, for the fourth time.
Wednesday, May 10 (DOY 130)
Cassini covered a distance of about 300,000 kilometers today, continuing to climb "up" from Saturn, while slowing to about 13,000 km per hour with respect to the planet by the end of the day. It would continue slowing until reaching the "top" of its orbit, apoapsis, on Friday. The spacecraft's attitude was under control of the Ultraviolet Imaging Spectrograph (UVIS), while it spent nearly 10 hours remotely observing Saturn's auroral zone in the high northern latitudes. Meanwhile, the Magnetometer (MAG), the Magnetospheric Imaging Instrument (MIMI), and the Ion and Neutral Mass Spectrometer (INMS) continued with their direct-sensing surveys of conditions in Cassini's immediate environment. A general discussion of remote-sensing and direct-sensing science investigations may be found here: https://go.nasa.gov/2qsVfE5 .
Next, Cassini received and executed commands sent by the flight team, to turn and fire its small rocket thrusters for 14 seconds. This Orbit Trim Maneuver (OTM)-471 provided a change in velocity of 21 millimeters per second. The change was needed to reduce later uncertainties in observation timing.
After the OTM, the Radio and Plasma Wave Science (RPWS) instrument began an outer-magnetosphere survey, listening in to the chorus of frequencies from audio up through very high frequencies, being generated in the Saturn system. At the same time, the Cosmic Dust Analyzer (CDA) measured the background density of dust in the vicinity of Titan's orbit.
Today's featured article profiles an engineer who works as a member of the Cassini Spacecraft Operations team: /news/13042/cassini-jpls-driving-school .
Thursday, May 11 (DOY 131)
Cassini's Imaging Science Subsystem (ISS) made another observation of Saturn's irregular moon Bebhionn, which was described on May 3. All during the 18.3-hour ISS activity, CDA independently looked for any dust that moves in a retrograde direction, out in the vicinity of Titan’s orbit.
This stunning image of Saturn, with its shadow on the ring system, was featured today: /resources/17679 .
The Cassini RPWS Science team wrapped up a three-day meeting at the University of Iowa in Iowa City, Iowa today. Analysis of their new data from Cassini's Proximal orbits was the highlight of their work.
Friday, May 12 (DOY 132)
While Cassini was busy communicating with the flight team today via the Deep Space Network (DSN), the spacecraft floated through apoapsis, marking the start of its Orbit #274. The spacecraft was slowly rolling about its longitudinal axis, keeping its high-gain antenna dish (HGA) facing Earth. This allowed MAG to make observations of Saturn's magnetic field for eight hours, as well as to calibrate the instrument. CDA, INMS, MIMI, and RPWS all continued their surveys from the rolling robot; UVIS also scanned the distances as part of a survey of hydrogen present in interplanetary space. Today's featured article offers a good look at the Cassini spacecraft: https://saturn.jpl.nasa.gov/mission/spacecraft/cassini-orbiter .
Saturday, May 13 (DOY 133)
CIRS observed Saturn's optically-dense B ring for eight hours today, measuring its emissivity at thermal-infrared wavelengths to determine its composition; UVIS and VIMS rode along. As soon as this was completed, VIMS took the reins and watched the familiar bright star Alpha Canis Majoris, also known as Sirius, as it passed behind Saturn from Cassini's vantage point. Over the course of this ingress occultation, VIMS was able to probe Saturn's atmosphere near 30 degrees south latitude. When the star was completely blocked by the planet about an hour later, ISS took five hours to study Saturn's bright, narrow F ring, with participation from CIRS, UVIS, and VIMS. Just as this finished up, Sirius began to rise from behind the gas giant, and VIMS watched it for another 5.3 hours, this time with CIRS and UVIS riding along.
Sunday, May 14 (DOY 134)
ISS turned back to take images of the F ring for another 11.5 hours, while CIRS, UVIS, and VIMS rode along. Next, ISS targeted Saturn's planet-like moon Titan to watch the weather in its atmosphere for 90 minutes, with help from CIRS and VIMS.
Monday, May 15 (DOY 135)
Speeding in over Saturn's northern hemisphere today, ISS, CIRS, and UVIS examined so-called "propeller" features in the planet's broad A ring for nearly an hour.
A close-up of the propeller named Bleriot was featured this week: /resources/17677 . Also featured was this remarkable view of a whole swarm of propellers in the A ring /resources/17678 . For scale reference, the density waves visible in the broad A ring measure about 15 km from crest to crest. In this inward-looking view, Saturn, inside the ring system, would be far past the upper-left part of the image.
Diving at enormous speed towards today's periapsis passage, the fourth of 22 interior to the rings, Cassini turned to face its HGA toward Earth for a Radio Science Subsystem (RSS) observation. The Radio Science team used the DSN, and European Space Agency ground stations, to watch an "inside-out" occultation by the rings of Cassini's three radio transmitters. During the course of this 24-hour experiment, the Radio Science team also made a study of Saturn's gravitational field at extraordinarily high resolution. The spacecraft's velocity, revealed by the Doppler shift of its radio signal, registered gravitational accelerations of less than one one-hundredth of a millimeter per second.
While passing close to the planet, Cassini also measured the gravitational pull from the rings, whose mass can now be accurately determined. The ring mass determination has special relevance because it will help constrain knowledge of the ring system's age.
Also during this periapsis period, CDA made its own unique measurements alongside RSS; its compositional results can also help determine the age of Saturn’s rings. This is because a ring particle's degree of contamination is related to age due to debris coming in from outside the Saturn system over the ages.
The Project Scientist and the Project Manager gave a presentation this evening to a packed audience at the Griffith Observatory in Los Angeles, for the Friends of the Observatory and their guests. The Deputy Project Scientist also gave a public talk today, at the Keck Institute of Space Studies at Caltech in Pasadena, California.
The MIMI science team began a two-day meeting at the Applied Physics Laboratory in Laurel, Maryland today.
Tuesday, May 16 (DOY 136)
CIRS stared at the Cassini Division, between Saturn's A and B rings, for seven hours observing the thermal-infrared emissivity of particles that inhabit the Division.
The DSN communicated with and tracked Cassini on 13 occasions this week, using stations in California, Spain, and Australia; the European Space Agency supported twice using their stations in Australia and Argentina. A total of 9,110 individual commands were uplinked, and about 2,090 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 124,426 bits per second.
Cassini is executing its set of 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. Each orbit stretches out to an apoapsis altitude of about 1,272,000 km from Saturn, where the spacecraft's planet-relative speed is around 6,000 km/hr. At periapsis, the distance shrinks to about 2,500 km above Saturn's visible atmosphere (by comparison, Saturn is about 120,660 km in diameter), and the speed is around 123,000 km/hr.
The most recent spacecraft tracking and telemetry data were obtained on May 10 using one of the 34-meter diameter DSN 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://saturn.jpl.nasa.gov/anomalies .
The countdown clock in Mission Control shows 122 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.