Calypso (moon)

• 1. Introduction

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  Calypso is a moon of Saturn so small and so bright that it almost defies easy description. Its surface reflects more light than almost any other object in the entire Solar System, with a visual geometric albedo of 1.34. That number means Calypso reflects more light than a perfect mirror would be expected to, a fact that needs explaining. How does a tiny, battered, irregularly shaped rock orbiting Saturn end up being one of the most reflective bodies we know of? And how does a moon first spotted from the ground in 1980 turn out to be locked in an orbital dance with two other moons, one of which shares a name from the same legend? Those are the questions that make Calypso worth a closer look.

• 2. Discovery and Naming

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  In 1980, Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie spotted Calypso from the ground, making it one of a long string of Saturn satellites identified that year. It received the provisional designation S/1980 S 25, meaning it was the 25th satellite of Saturn found in 1980 alone. That was not the end of its story in the discovery logs. Several additional sightings of the same body were recorded in the months that followed, each logged under its own provisional label: S/1980 S 29, S/1980 S 30, S/1980 S 32, and then S/1981 S 2 the following year. Astronomers were catching repeated glimpses of the same small moon without yet realizing they were all looking at the same object. It was not until 1983 that the moon received its permanent name, Calypso, drawn from Greek mythology. It is also catalogued as Saturn XIV and as Tethys C, a label that hints at the orbital relationship at the center of its story.

• 3. The Trojan Arrangement

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  Seidelmann and colleagues identified in 1981 that Calypso shares its orbit with the much larger moon Tethys in a very specific way. Calypso sits at Tethys's trailing Lagrangian point, exactly 60 degrees behind Tethys in its orbit. Lagrangian points are gravitational sweet spots where a smaller body can follow a larger one indefinitely without falling in or drifting away. A separate moon called Telesto occupies the opposite end of the same arrangement, sitting 60 degrees ahead of Tethys in the leading Lagrangian point. Because of this configuration, Calypso and Telesto are called Tethys trojans. The term is borrowed from the trojan asteroids, groups of small bodies that trail and lead Jupiter in the same kind of gravitational lockstep. Calypso and Telesto together account for half of the four trojan moons presently known to exist anywhere in the Solar System.

• 4. A Battered, Luminous Surface

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  Calypso is irregularly shaped, like many small moons of Saturn and many small asteroids. Its surface shows overlapping large craters, signs of a long history of impacts. Yet those craters look softer than expected, because loose material across the surface appears to fill and smooth them. That same surface carries the extraordinary albedo of 1.34. The source of that brightness is the E-ring of Saturn, a faint ring made up of small water-ice particles. Those particles originate from the geysers erupting at the south pole of Enceladus. The E-ring's icy particles sandblast Calypso's surface continually, coating it in fresh, clean water ice that bounces light back with unusual efficiency. Calypso's position as a trailing trojan of Tethys keeps it immersed in the same region of space where that icy sandblasting is most active, tying its orbital life directly to the brightness that makes it stand out.