The moon of Saturn that occupies the leading Lagrange point of Tethys is Telesto. It shares Tethys’s orbit around Saturn and stays near the L4 point, about 60 degrees ahead of Tethys.
This arrangement makes Telesto a Trojan moon. The L4 point is a gravitationally stable region formed by the combined motion and gravity of Saturn and Tethys. A second moon, Calypso, occupies the corresponding trailing L5 point, about 60 degrees behind Tethys.
Telesto was discovered in 1980 by Bradford A. Smith, Harold Reitsema, Stephen Larson, and John Fountain. It was initially given a provisional designation before receiving the name Telesto, drawn from Greek mythology, in 1983. The moon is also known as Saturn XIII and Tethys B.
The main source of confusion is that Saturn has several Trojan moons. Helene leads Dione, while Polydeuces trails Dione. Only Telesto is at Tethys’s leading point; Calypso is Tethys’s trailing companion.