Now that Cassini is in its "ring-grazing orbits" phase, it has returned stunning new views of Daphnis and friends.
It's not every day humanity gets a fresh look at a distant world. NASA's Cassini mission recently released a new image of the tiny moon Daphnis enmeshed in the rings of Saturn.
Cassini took the image on January 16, 2017, while 17,000 miles (28,000 kilometers) away from the moon. Measuring 5 miles (8 kilometers) along its longest axis, irregular Daphnis resides in the 26-mile (42-kilometer) wide Keeler Gap in Saturn's outer A ring. The Keeler Gap seems narrower than it really is in this image because of foreshortening due to the spacecraft's viewing angle. You can just see grooves along the long axis of Daphnis in the image, as well as a few impact craters.
For context, the range at which this image was taken is about 6,000 miles closer than geosynchronous orbit (22,236 miles above Earth's surface). And for scale, Daphnis is slightly smaller than Mars' moon Deimos. At this distance, the image scale is 551 feet (168 meters) per pixel. This marks the closest flyby Cassini or any spacecraft has made past Daphnis to date.
Making Waves in Saturn's Rings
Despite its tiny size, the gravity of the diminutive moon raises ripples along the ring's edge in both the vertical and horizontal directions. In fact, if you look closely, you'll see that Cassini caught Daphnis in the act of drawing out a narrow tendril of ring material, which trails the moon in its orbit. "The gravity from this incredibly tiny moon is creating the Keeler Gap and sculpting Saturn's magnificent rings," says Cassini project scientist Linda Spilker (Jet Propulsion Laboratory). "This picture is Cassini's last close view of Daphnis."
Cassini also caught sight of Daphnis stirring things up back in 2009, during the Equinox phase of its mission. Soft-edged waves in Saturn's rings trail the moon's wake in this image too, in stark contrast to the otherwise sharp edges seen along the length of the Keeler Gap.
The Daphnis flyby is part of Cassini's recent series of ring-grazing orbits, which will span November 2016 to April 2017. "Cassini is currently orbiting Saturn... flying just inside Saturn's F ring," Says Spilker. "These grazing orbits provide some of our best views ever of Saturn's tiny ring moons, Saturn's F ring and outer A ring." In addition to Daphnis, Cassini will also make close passes of other ring residents including Methone, Pandora, Atlas, Prometheus, Aegaeon, and Pan.
The Cassini imaging team discovered Daphnis on May 6, 2005. The moon is named after the shepherd and friend of the satyr Pan in Greek mythology, appropriate as both Pan and Daphnis are "shepherd moonlets" tending to gaps in Saturn's rings. Daphnis orbits Saturn once every 14 hours. Not only does Daphnis appear “groovy” up close, but it also looks to be coated with ring material — features that are both typical of several of Saturn's inner moons.
“That's No Moon”
Except in this case, it is. Daphnis is only the latest in a series of moon cameos. On October 22, 2016, Cassini made one more flyby 115,000 miles (185,000 kilometers, about half the Earth-Moon distance) past Mimas, the moon that imitates Star Wars' Death Star in appearance. The flyby occurred just before the start of the ring-grazing orbits.
The close-up shows Herschel Crater in stark profile along the terminator. The feature is testament to an ancient impact that may have nearly shattered the moon. Herschel Crater spans 86 miles (139 kilometers) in diameter, about one-third the diameter of Mimas itself.
After this coming April, Cassini's final days begin as it completes it dramatic Grand Finale orbits and threads the 1,240-mile-wide (2,000-kilometer-wide) gap between innermost rings and Saturn itself for 22 final orbits. Then will come the bittersweet moment on September 15, 2017, when Cassini ends its spectacular career of planetary exploration, taking the plunge and burning up in Saturn's atmosphere. Though dramatic, the move is also practical as an effort to avoid any possibility of contamination on Titan or Enceladus in the far future.
Enjoy these final views from an amazing mission. Human emissaries won't make their way to Saturn again for some time to come.