One of 62 moons discovered thus far orbiting giant Saturn, Janus is a 111-mile (179-km) -wide pockmarked potato composed of rock and ice rubble. The image above shows Janus as seen with Cassini’s narrow-angle camera on September 10, 2013, from a distance of 621,000 miles (1 million km), floating against the blackness of space.
Despite its apparent isolation in the image above, though, Janus isn’t alone. It shares its orbit around Saturn with its slightly smaller sister moon Epimetheus, and they regularly catch up to each other — and even switch places.
Janus and Epimetheus travel in nearly the same track, about 94,100 miles (151,500 km) out from Saturn. They occasionally pass each other, their gravity causing them to switch speeds and positions as they do; Janus goes faster and higher one time, slower and lower the next – but the two never come within more than about 6,200 miles of each other.
The two moons switch positions roughly every four years.
This scenario is referred to in astrophysics as a 1:1 resonance. Astronomers were initially confused when the moons were discovered in 1966 as it wasn’t known at the time that there were actually two separate moons in a single orbit. (This wasn’t confirmed until Voyager 1’s visit to Saturn in 1980.) It’s been suggested that Janus and Epimetheus will eventually come to orbit a single Lagrangian point around Saturn instead of trading places… in about another 20 million years.
The view above looks toward the Saturn-facing side of Janus. Covered in both dark and light colored material, Janus’ surface is thought to be coated with a layer of fine dust that slides down its steeper slopes, revealing the brighter ice beneath.
It’s a good thing NASA labeled the moons in this image of Saturn, because they are pretty hard to see. But they are there, keeping each other company in this Cassini spacecraft image of Saturn’s night side. And as the Cassini team says, it seems fitting that they should do so since in Greek mythology, their namesakes were brothers.
In Greek mythology these three were all sons of Iapetus (another of Saturn’s moons), and supposedly Prometheus and Epimetheus were tasked with creating humans. Prometheus was a pretty good sort, and gave gifts to humans like fire; Epimetheus gave humans evil – not so good. And famously, Atlas ended up having the weight of the world on his shoulders.
But in science, Prometheus the moon is about 86 kilometers across (53 miles) and is located just inside the F ring in this image, while Epimetheus is about 113 kilometers across (70 miles) and is farther from the rings, due right of Prometheus in this image. Atlas is the tiny guy (30 kilometers across (19 miles) and can be just barely seen between the A and F rings almost right below Epimetheus,
This view looks toward the unilluminated side of the rings from about 30 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on Sept. 19, 2012.
The view was obtained at a distance of approximately 2.2 million kilometers (1.4 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 96 degrees. Image scale is 128 kilometers (80 miles) per pixel. Epimetheus has been brightened by a factor of 1.5 and Atlas’ brightness has been enhanced by a factor of 3 relative to the rings and Prometheus to improve visibility.
Taken during the Cassini spacecraft’s October 1 flyby of Saturn’s ice-spewing moon, this image – released today – shows a crescent-lit Enceladus with southern geysers in action… and the much smaller Epimetheus peeking out from behind!
The 70-mile (113-km) -wide Epimetheus is dwarfed by its larger sibling Enceladus, which is 313 miles (504 km) in diameter… about the width of the state of Arizona.
One of the most reflective objects in the solar system, Enceladus appears to be casting some reflected light onto Epimetheus as well. (Image processors at the Cassini Imaging Lab have brightened the moons by a factor of 1.8 relative to the rings in order to bring out detail.)
Some bright clumps of material can also be seen orbiting within Saturn’s rings at upper left, possibly stirred up by the movement of the shepherd moon Pan.
There’s nothing up the sleeves of the Cassini imaging team in this image; it is real! Is the moon Titan being cut in half by Saturn’s rings? What is actually happening here is that the middle part of the rings are made dark as Saturn casts its shadow across them. Cassini was just in the right place at the right time, making it appear as though Titan is being sliced in half! The night side of the planet is to the left, out of the frame of the image. Illuminated Titan can be seen above, below and through gaps in the rings. Click the image for a larger version.
As an added benefit in this shot, Mimas (396 kilometers, 246 miles across) is near the bottom of the image, and Atlas (30 kilometers, 19 miles across) can barely be detected near the thin F ring just above the center right of the image. Lit terrain seen here is the area between the leading hemisphere and Saturn-facing side of Titan (5,150 kilometers, 3,200 miles across). This view looks toward the northern, sunlit side of the rings from just above the ringplane.
Below are a few more magical images from Cassini:
Here the moon Enceladus appears strung along a wispy ring of Saturn, likely the G ring. Look close and Enceladus’ plumes are visible, too.
Two of Saturn’s small moons appear to be sitting on Satun’s thin F ring in this image.
From the CICLOPS website:
Pandora (81 kilometers, 50 miles across) is on the left, and Epimetheus (113 kilometers, 70 miles across) is on the right. This view looks toward the northern, sunlit side of the rings from just above the ringplane. Both moons are closer to Cassini than the rings are. Pandora is slightly closer to Cassini than Epimetheus here.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Nov. 23, 2009. The view was acquired at a distance of approximately 1.3 million kilometers (808,000 miles) from Pandora and Epimetheus. Image scale is 8 kilometers (5 miles) per pixel.