Retro moon: Cassini spacecraft studies moon that orbits Saturn backward
The tiny moon of Hyrrokkin is one of the Norse group of Saturn's 60 or so moons, which orbits the gas giant in a retrograde direction.
Tiny Hyrrokkin hides amongst a field of stars in this image taken by NASA's Cassini spacecraft on Aug. 7, 2015.
NASA/JPL-Caltech/Space Science Institute
Sen—When we think of the moons of Saturn, many worlds spring to mind: cloud-shrouded Titan, water-spewing Enceladus, and Death Star-like Mimas merely form the head of the line. But, while impressive, these satellites and others like them form a relatively small fraction of all Saturnian moons. Wandering much farther from Saturn is a group whose population comprises nearly half of the planet’s more than 60 satellites, and this week NASA’s Cassini spacecraft took time to study one of these distant objects.
Twenty-nine moons make up the so-called Norse group of satellites around Saturn. Only large Phoebe has been studied in detail, but this week’s target, Hyrrokkin, is much smaller— only around 8 km in size—and farther away. That means that even in Cassini’s high-powered Narrow-Angle Camera, Hyrrokkin appears as nothing more than a point of light.
What can we learn from that point of light? For one, it will help scientists pin down the moon’s orbital trajectory. Hyrrokkin’s orbit has high eccentricity, meaning it moves dramatically closer and farther from Saturn as it circles the planet. But what sets the Norse group apart is the direction they orbit: retrograde, astronomy jargon for backwards. This means that these objects probably did not form with Saturn. Instead, over the last few billion years, they were probably captured when straying too close to the planet.
By monitoring how the apparent brightness of Hyrrokkin changes over time, astronomers will also be able to deduce better constraints on the shape and rotational characteristics of the moon.
The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.
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