NASA's Cassini spacecraft peers toward a sliver of Saturn's sunlit atmosphere while the icy rings stretch across the foreground as a dark band.(NASA)
NASA's Cassini spacecraft peers toward a sliver of Saturn's sunlit atmosphere while the icy rings stretch across the foreground as a dark band.(NASA)

Cassini Takes Plunge Into Saturn, Scientists Cross-Fingered

In its line up for final plunge into Saturn’s atmosphere, Cassini has once again taken a proximal plunge into the surface of Saturn on June 29, 2017. The final plunge is scheduled for mid-September.

The Cosmic Dust Analyzer’s (CDA) science team, in Germany adjusted the instrument’s settings this week based on experience in recent “proximal” passages between Saturn’s rings and atmosphere. They have created a string of 39 commands that would set the instrument to make the best possible observations during the next proximal plunge. Now the instrument’s data-collection rate has been adjusted to 4 kilobits per second, thus making sure all ring-particle impacts would be sensed.

Here is a week-long update previous to the plunge:

Wednesday, June 21 (DOY 172)

Writers, bloggers, photographers, educators, students, artists and others who use social media to engage specific audiences are encouraged to apply for special access to Cassini’s Grand Finale event in mid-September.

Thursday, June 22 (DOY 173)

The Composite Infrared Spectrometer (CIRS) turned and looked at Saturn’s large icy moon Dione for 3.5 hours today. The Imaging Science Subsystem (ISS), the Visible and Infrared Mapping Spectrometer (VIMS), and the Ultraviolet Imaging Spectrograph (UVIS) – all the other Optical Remote-Sensing (ORS) instruments – rode along to make observations as well. CIRS’s goal was to measure Dione’s surface emissivity at thermal-infrared wavelengths, which hold clues to the composition and structure of that moon’s regolith.

Friday, June 23 (DOY 174)

Beginning late today, the spacecraft trained its High-Gain Antenna dish on the distant Earth. It then accurately tracked our planet for a total of 28 hours. Accordingly, the Radio Science Subsystem (RSS) team had Cassini power on its S-band (2 GHz) and Ka-band (32 GHz) radio transmitters, which directed their beams of energy out the HGA along with the main communications beam at X-band (8 GHz).

The result was a high-precision measurement of Saturn’s gravitation, which will be analyzed to reveal deviations from spherical symmetry.

Saturday, June 24 (DOY 175)

CIRS observed the dark side of Saturn’s A ring at far-infrared wavelengths for five hours today, with the other ORS instruments riding along. In addition to studying ring-particle compositions, the observation was part of a campaign to compare the spectral properties of ices among different regions of Saturn’s rings and icy moons.

Cassini and Titan happened to come close to one another today, to a distance about the same as that from Earth to our own Moon.

Sunday, June 25 (DOY 176)

This week’s Titan observing wrapped up with its final 4.3 hours devoted to observing clouds on the planet-like moon; VIMS rode along.

Monday, June 26 (DOY 177)

ISS turned and spent 7.7 hours observing Saturn’s irregular moon Bebhionn, an object of about six kilometers diameter, which orbits Saturn in an inclined ellipse that reaches as far as 25.1 million km from the planet. It might have a binary or contact-binary nature. Bebhionn was named after the goddess of birth in early Irish mythology.

The flight team held a Command Approval Meeting fine-tuning commands with consent from representatives from each of the affected spacecraft subsystems and instruments.

Tuesday, June 27 (DOY 178)

UVIS observed. Ten minutes after the Deep Space Network (DSN) station in Australia acquired Cassini’s downlink, its 18-kilowatt transmitter was turned on, and comands were sent. After a round-trip of 2 hours 31 minutes, telemetry confirmed that the commands had been received and were ready to take effect right before Cassini’s eleventh proximal plunge on June 29.

A total of 58 individual commands were uplinked, and about 1,625 megabytes of science and engineering telemetry data were downlinked and captured at rates as high as 142,201 bits per second.

Wrap up:

Cassini is executing its set of 22 Grand Finale Proximal orbits, which have a period of 6.5 days, in a plane inclined 61.9 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 on the planet’s total 120,660 km in diameter with a speed of 123,000 km/hr.

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