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Subaru Telescope Discovers 100th Moon of Saturn

May 22, 2023
Last updated: May 24, 2023

Saturn has now passed 100 known moons. The 100th moon announced around Saturn has been given the provisional designation S/2004 S43 and was first observed by the Subaru Telescope.

A team led by Scott Sheppard (Carnegie Institution for Science) first observed this moon in 2004 using the Subaru Prime Focus Camera (Suprime-Cam). The very faint 26th magnitude, 2 km sized moon required additional years to confirm its orbit. With 145 confirmed moons, 62 of which were announced this May by the International Astronomical Union (IAU), Saturn takes back the "moon king" title from Jupiter (Note 1).

Subaru Telescope Discovers 100th Moon of Saturn

Figure 1: Conceptual diagram showing the orbits of Saturn's satellites. The left and right figures represent Saturn viewed from the polar and equatorial directions, respectively. The new moons announced this time are members of the irregular outer satellites whose orbits are colored in either red, green, or blue. (Credit: Scott Shepard/Carnegie Institution for Science)

About half of the newly announced Chronian (from Chronos, the Greek name of the god known as Saturn in Latin) moons this May were first discovered by Suprime-Cam observations in 2004, 2005, 2006 and 2007, and confirmed with CFHT observations in 2019 and 2021 by a team led by Edward Ashton (Academia Sinica Institute of Astronomy and Astrophysics). The other half of the newly announced moons were discovered by the Ashton's group using CFHT, which is also at Maunakea. All of the new moons are very small, typically 2 km in size.

The new Chronian moons are interesting as they are mostly fragments of larger parent moons. The parent moons were fragmented by collisions with other moons or passing comets or asteroids. The fragments created by these impacts remain in orbits similar to that of the parent moon. Based on the number of similar satellite orbits, it appears there were at least 5 to 8 original parent moons.

These moons are important to understand because they are the last remnants of the population of objects that formed in the giant planet region as the rest of the material was incorporating into the planets. Imaging one of them up close with a spacecraft like NASA's Dragonfly mission, will give us a better understanding of the primordial material that went into making the planets.

"That is one of the reasons to find more Chronian moons," says Sheppard. "The hope is that if we find enough, one of them will just happen to be near enough to the spacecraft’s trajectory to get close up images of it while the spacecraft is passing through the outer Chronian system to the inner Chronian system."

"There are many more smaller moons we have detected around Saturn, but they still need more observations to be confirmed. Most of these unconfirmed moons were first seen at the Subaru Telescope in 2004," says Sheppard.

(Note 1) Jupiter, which currently has 95 known moons, 15 of which were announced earlier this year, only had the moon crown for a few months. "We believe we are complete for moons around Saturn down to about 3 km in size; while at Jupiter, closer to the Earth, the completeness is down to about 2 km in size. So Saturn having more known moons and yet not being complete to the smaller sizes means Saturn likely has more moons than Jupiter at the same size range," says Sheppard.

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