Cassini’s “dance” with Saturn’s rings continues – the probe has now completed its second dive into the rings (orbit 272), specifically the gap between the innermost rings and Saturn itself. That leaves 20 more similar dives to go, as part of the Grand Finale phase of Cassini’s mission before the fateful end on Sept. 15. This is the closest that any spacecraft has ever come to Saturn, showing the rings and the planet itself in detail never seen before.
This dive is a little different from the first one. For the first time, Cassini “rolled” to calibrate its magnetometer (MAG) for the high-intensity magnetic field observations to be performed when the spacecraft is nearest Saturn. Other highlights of this dive included:
- Cassini’s imaging cameras, the Imaging Science Subsystem (ISS), took advantage of the last opportunity to observe Saturn’s rings at extremely high phase angles while the Sun is hidden behind Saturn, allowing the instrument to survey faint ringlets in the main rings, many of which are difficult to observe outside of this geometry. The ISS also collected images to produce a movie to monitor various structures in Saturn’s D ring.
- The spacecraft’s Composite Infrared Spectrometer (CIRS) observed Saturn’s moon Rhea in infrared to narrow down the composition and structure of the moon’s surface material.
- During this orbit, Cassini was within 1,820 miles (2,930 kilometers) of Saturn’s 1-bar level. Cassini also passed within 2,980 miles (4,780 kilometers) of the inner edge of Saturn’s D ring.
The data from this second ring dive will be transmitted back to Earth on May 3. Each orbit in the Grand Finale phase takes about six and a half days to complete.
Cassini’s previous ring dive on April 26 was the first time ever that a spacecraft had entered the space inside of Saturn’s rings; this gap is about 1,200 miles (2,000 kilometers) wide. Although not 100% certain what to expect, it was considered a safe bet that the probe would survive, barring a collision with any larger-than-anticipated ring particles. Cassini did survive unscathed, and actually found the region to be even more particle-free than thought. Even the dust level was much lower than expected. That’s good news of course, but also a bit of a mystery. Scientists are now referring to the gap as “The Big Empty.”
“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”
During the first dive, Cassini used its large 13-foot (4-meter) wide saucer-shaped high-gain antenna as a shield against dust particles, protecting the instruments below. But now it seems that may no longer be necessary, except for four of the upcoming dives which pass through the innermost fringes of the rings. Orienting the spacecraft to maximize protection also meant that it was out of radio communication with Earth during that time. Fortunately, the radio signal was picked up again later, as expected.
Cassini’s Radio and Plasma Wave Science (RPWS) instrument had previously detected hundreds of ring particles per second when it crossed the ring plane just outside of Saturn’s main rings, but only detected a few on April 26.
The “hits” of the dust particles detected by RPWS were converted to audio format, which would sound likes pops and cracks. But instead of those, the Cassini team mostly just heard the usual whistles and squeaks of waves common in the charged particle environment. The data also suggested that Cassini didn’t encounter any particles larger than those in smoke.
“It was a bit disorienting – we weren’t hearing what we expected to hear,” said William Kurth, RPWS team lead at the University of Iowa, Iowa City. “I’ve listened to our data from the first dive several times and I can probably count on my hands the number of dust particle impacts I hear.”
On May 1, a newly processed image of the north polar region of Saturn’s moon Enceladus was also released (taken Nov. 27, 2016). In contrast to the south pole, this region is heavily cratered, indicating that it is much older and relatively unchanged. At the south pole, the moon’s surface has been repeatedly “smoothed over” by water vapor and ice particles erupting from the many geysers in the region. The plumes originate from the subsurface ocean, where the water erupts through large parallel cracks in the icy surface called Tiger Stripes.
Cassini also recently conducted its last-ever close flyby of the moon Titan, a world that Cassini revealed to be incredibly diverse and bizarre, with rain, rivers, lakes and seas of liquid methane/ethane (hydrocarbons) and vast fields of tall, dark dunes composed of organic tholins. Beneath all of that is thought to lie a liquid water ocean under the surface.
“Cassini’s up-close exploration of Titan is now behind us, but the rich volume of data the spacecraft has collected will fuel scientific study for decades to come,” said Linda Spilker, the mission’s project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.
There are only a few months left now in the Cassini mission. After completing the 22 ring orbits in the Grand Finale, the spacecraft will plunge into Saturn’s atmosphere, never to be heard from again. This is necessary because by that time the fuel will have run out, and scientists don’t want the probe to accidentally collide with any of Saturn’s moons, since it may still have some stowaway microbes on board. The deep descent into the turbulent atmosphere however will completely destroy the spacecraft and any microorganisms which may have hitched a ride.
Cassini has captured the imagination of people worldwide; even Google made a special doodle to celebrate the Grand Finale.
The next ring dive in the Grand Finale will be on May 8.
More information about the Grand Finale phase of the mission is here. All of Cassini’s raw images can be seen here on the mission website, and more information about the Cassini mission itself is available here.