wing this, UVIS scanned across Saturn's northern polar region to image the aurorae there; CIRS rode along. Then, just 20 minutes before speeding through periapsis, Cassini flew within 12,000 kilometers of Saturn's diminutive moon Atlas -- it was less than one Earth-diameter away. ISS led all the ORS instruments observing Atlas for 2.6 hours. In all its years at Saturn, this approach was by far the closest to the little moon, which orbits Saturn in the space between the broad A ring and the bright, narrow F ring. The Magnetospheric and Plasma Science (MAPS) instruments were making in-situ measurements this whole time, which included the ring-plane crossing. The Radio and Plasma Wave Science (RPWS) instrument increased its sampling frequency to record the signature of dust impacts on the spacecraft, and to study plasma waves near Saturn's magnetic equator. With high-quality Atlas images stored aboard, ISS then turned back to view the receding A ring from its dark side, and led the ORS instruments for 1.3 hours of close-up observations of structures that scientists have dubbed "propellers" (http://go.nasa.gov/17oqTWF).
Studies of Saturn's aurora continued for another 3.7 hours, as VIMS stared at the planet's dark southern polar regions. The lack of sunlight there made it easier to image aurorae; CIRS rode along. Scientific scrutiny then turned back towards some more propellers, with ISS, CIRS, and UVIS observing. Finally, CIRS began a six-hour study of the unlit side of the A ring, with UVIS riding along. Ring temperatures drop abruptly when the particles pass through Saturn's shadow. In perpetual sunlight now that Saturn is near solstice, particles in the A ring are no longer passing through the shadow, so CIRS scanned along the A ring looking for more subtle variations in thermal emission.
Thursday, April 13 (DOY 103)
The next occultation experiment was jointly observed by VIMS and UVIS for 1.2 hours today, when the Sun set behind the A ring. Shortly after passing behind the Cassini Division, the Sun disappeared behind Saturn from Cassini's vantage point. With the Sun safely blocked, ISS turned to image the faint rings. Such geometries are conducive to studying ring structures that are dominated by small, micron-sized particles, such as Saturn's innermost D ring, and the outlying G and E rings.
The spectacular images of Atlas went right to the Cassini Raw Images website as soon as they came down to Earth (as do all Cassini's images). Here are some of them, with standard captions: /news/13025/cassini-sees-flying-saucer-moon-atlas-up-close .
News from Cassini about Saturn's moon Enceladus, and related news from the Hubble Space Telescope about Jupiter's moon Europa were the subjects of today's NASA press conference: /news/3024/nasa-missions-provide-new-insights-into-ocean-worlds-in-our-solar-system .
Three additional articles featured Enceladus on the Cassini website today:
Hydrothermal activity /news/13023/hydrothermal-activity .
Free samples /news/13022/free-samples .
The enormous, puzzling E ring /news/13021/put-a-ring-on-it .
Friday, April 14 (DOY 104)
Today is the birthday of Christiaan Huygens, the polymath who correctly surmised the configuration of Saturn’s rings, and discovered its largest moon Titan in 1655, when he was 26 years old.
CIRS spent three hours with the campaign to create a library of moderate wavelength-resolution spectra of various regions in Saturn's rings; VIMS and UVIS rode along. Later, today's occultation experiment involved VIMS tracking the bright red star Alpha Orionis, also known as Betelgeuse; CIRS rode along. The star made a full radial egress behind Saturn's main rings. This was the last science activity stored in the 10-week-long S98 sequence of commands. The command sequence S99, whose uplink last week was captured on video (https://go.nasa.gov/2oq3jnM), took over control of Cassini for the next seven weeks.
The MAPS instruments continued their direct-sensing surveys as the S99 sequence took effect; the Cosmic Dust Analyzer (CDA) looking at dust, the Ion and Neutral Mass Spectrometer (I