Short answers, pulled from the story.
Giovanni Domenico Cassini discovered the moon Tethys on the 1st of January 1684 while observing from the Paris Observatory. He found it together with Dione during a night when he used an aerial telescope to identify four new moons orbiting Saturn.
The moon Tethys has a radius of about 531 kilometers and a density of only 0.98 grams per cubic centimeter, making it the lowest-density major moon in the entire Solar System. Spectroscopy confirms that water ice dominates its surface composition while radar observations reveal an icy regolith with porosity exceeding 95 percent.
Odysseus crater spans approximately 450 kilometers in diameter and covers nearly two-fifths of the moon Tethys itself. The valley known as Ithaca Chasma stretches more than 2,000 kilometers long and measures roughly 100 kilometers wide across the surface.
Two co-orbital moons named Telesto and Calypso share Tethys's Lagrange points by orbiting 60 degrees ahead and 60 degrees behind respectively. These satellites measure under 25 kilometers in diameter and were discovered by Voyager spacecraft in April and March 1980.
Pioneer 11 flew past Saturn on the 1st of September 1979 reaching closest approach to Tethys at a distance of 329,197 kilometers from its center. Voyager 1 passed within 415,670 kilometers on the 12th of November 1980 while Voyager 2 came closer at 93,010 kilometers on the 26th of August 1981.