The Dance of Helene

My favorite robotic spacecraft, Cassini, managed a flyby of Saturn’s moon Helene. The video above (by the awesome Emily Lakdawalla) collects a series of still images into a movie, with a few loops thrown in to show the details of the surface. It’s not a very big moon by any standards: about 36 kilometers (22 miles) at its widest, and as you can see, it’s far from spherical. (This is a general rule about gravity: big objects tend towards spherical shapes, but small objects like asteroids, comets, and so forth can be as lumpy as they please.)

Saturn's moon Tethys, which helps provide a gravitational point for Helene

There are a lot of interesting things about Helene that will keep planetary scientists busy for a while, and since I am not in that august company, I’ll send you to the Planetary Society blog for more information. However, one thing that struck my attention that I missed before in discussions of Helene: it’s a Trojan moon. Trojans are objects that lurk in special gravitational spots known as Lagrange points— places where the combination of gravity from two objects creates places of stability. That idea may merit a post in and of itself, but Trojans are fairly common occurrences in our Solar System; the most famous Trojans are asteroids that orbit in the same path as Jupiter — the combination of the Sun’s gravity and Jupiter’s gravity creates two Lagrange points in the orbit where asteroids have collected. These points move along with Jupiter, so the asteroids never get any closer or farther away. In the case of Helene, the Lagrange points are due to the combined gravity of Saturn and its moons Tethys (shown at the right) and Rhea, both of which are much larger than little Helene.

With the moons, moonlets, and rings all dancing together under the baton of Saturn’s gravity, but also partnering with each other in pairs and more complex associations, we see a system as complex in its way as the entire Solar System. Those who hold that understanding something detracts from its beauty will not convince me; as scientists strive to understand this elegant dance, its beauty and wonder are enhanced as we say “look what gravity can do”.

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3 Responses to “The Dance of Helene”


  1. 1 Matthew R. Francis June 20, 2011 at 19:54

    Phil Plait also wrote about Helene and its oddly two-faced nature (cratered on one side, grooved on the other): http://blogs.discovermagazine.com/badastronomy/2011/06/20/a-moody-moon-turns-its-face


  1. 1 Centrifugal Forces and Trojan Horses « Galileo's Pendulum Trackback on August 17, 2011 at 14:17
  2. 2 Moons and Junes and Ferris Wheels « Galileo's Pendulum Trackback on November 23, 2011 at 16:03
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