The small icy moons in the outer solar system fascinate me. Above is Helene, about twenty-three miles across at its longest. For a size comparison, here’s Manhattan superimposed:
Interesting to see rounded outlines of craters and those little ridges. What are they, and how were they formed? These are not the craggy, rough moons depicted in science fiction illustration. So dead and uninteresting. Helene may not launch a thousand ships of exploration, but the Cassini probe has uncovered an intriguing find. Indeed, this moon is made up of frozen water, with some impurities. And the thinking now is that bodies like this are piles of icy rubble, sort of like those snow banks outside my house.
Helene is also interesting in that it shares the orbit of its much larger sister, Dione. Before the small moon’s 1980 discovery (from a telescope on Earth, by the way, not from a space probe), scientists had noted the possibility that the workings of gravity make it possible for a body, usually presumed to be a human-made satellite, to reside at certain points respective to the earth and the sun.
These “L” designations are the Lagrangian points. They work for the Earth and the moon, too. In fact, placing a human station at L4 or L5, ahead or behind the moon in its orbit around the Earth, is a decades-old science fiction idea. Helene, by the way, resides in Dione’s L4 point, leading the way as they orbit the ringed planet. It is one of four known “Lagrangian moons” in the solar system.