APOD: Moons of Saturn (2019 Oct 17)

[APOD Robot]

Moons of Saturn

Explanation: On July 29, 2011 the Cassini spacecraft's narrow-angle camera took this snapshot and captured 5 of Saturn's moons, from just above the ringplane. Left to right are small moons Janus and Pandora respectively 179 and 81 kilometers across, shiny 504 kilometer diameter Enceladus, and Mimas, 396 kilometers across, seen just next to Rhea. Cut off by the right edge of the frame, Rhea is Saturn's second largest moon at 1,528 kilometers across. So how many moons does Saturn have? Twenty new found outer satellites bring its total to 82 known moons, and since Jupiter's moon total stands at 79, Saturn is the Solar System's new moon king. The newly announced Saturnian satellites are all very small, 5 kilometers or so in diameter, and most are in retrograde orbits inclined to Saturn's ringplane. You can help name Saturn's new moons, but you should understand the rules. Hint: A knowledge of Norse, Inuit, and Gallic mythology will help.

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[Ann]

I'm often somewhat bored by pictures of the planets in the Solar system, but I must admire the composition of this APOD!

Little moon Pandora is sitting smack dab in what I assume is the F-ring of Saturn. Rhea is positioned so perfectly that it almost looks as if it had replaced Saturn in the center of the rings. Also I'm always happy to see smooth shiny Enceladus, and I like the heavily cratered surface of one of the Solar system's smallest spherical bodies, Mimas. And Janus at far left looks like a "little man" with a long nose and at least one eye, looking with some concern at the scene to the right of him.

This APOD is surely going to be one that generates enormous traffic, as people will no doubt be happy to suggest names for Saturn's new moons. So I'll suggest two names from the Norse mythology, Hel, goddess of the underworld, and Vale, avenger of the murder of Balder, who was everyone's favorite god in the Norse mythology. Or Vidar. I just like the name, and unlike the two others I suggested, it is a fairly popular name in Sweden.

Ann

[sym666]

Cassini, my love! I'm sure you can see us wherever you are!

[gmPhil]

You can help name Saturn's new moons, but you should understand the rules.

So... no "Moony McMoonface then?

[Boomer12k]

Awesome image... great timing...

:---[===] *

[orin stepanek]

Saturn may have even more moons when you consider what makes up the rings!

[orin stepanek]

Awesome image... great timing...

:---[===] *

+1

[BDanielMayfield]

Saturn may have even more moons when you consider what makes up the rings! PIA12797-full.jpg

In the finding of new, even tinier moons there is no end. Oh no, I see the making of a new category, the “dwarf moon”. They’ll call any non round moon a dwarf.

[Ann]

Saturn may have even more moons when you consider what makes up the rings! PIA12797-full.jpg

In the finding of new, even tinier moons there is no end. Oh no, I see the making of a new category, the “dwarf moon”. They’ll call any non round moon a dwarf.

Than Janus may not be a moon! Janus does not look spherical to me.

Janus may be demoted and called a "dwarf moon" in the future!

Ann

[Chris Peterson]

Saturn may have even more moons when you consider what makes up the rings! :mrgreen: PIA12797-full.jpg

In the finding of new, even tinier moons there is no end. Oh no, I see the making of a new category, the “dwarf moon”. They’ll call any non round moon a dwarf.

Than Janus may not be a moon! Janus does not look spherical to me.

Janus may be demoted and called a "dwarf moon" in the future! :shock:

Well, a "dwarf moon" would still be a moon, right? (I don't imagine the category will be invented, however. "Moon" just means "natural satellite", and there's never been a concern about the shape or size or process by which it came into being. At times the Earth has had a second moon when asteroids were briefly captured in orbit.)

[sillyworm 2]

Jaw dropping image....beautiful.

[bystander]

viewtopic.php?t=39189#p296021

[neufer]

• What is the history of the expression “many moons ago”?

[Ann]

In the finding of new, even tinier moons there is no end. Oh no, I see the making of a new category, the “dwarf moon”. They’ll call any non round moon a dwarf.

Than Janus may not be a moon! Janus does not look spherical to me.

Janus may be demoted and called a "dwarf moon" in the future!

Well, a "dwarf moon" would still be a moon, right? (I don't imagine the category will be invented, however. "Moon" just means "natural satellite", and there's never been a concern about the shape or size or process by which it came into being. At times the Earth has had a second moon when asteroids were briefly captured in orbit.)

Click to view full size image

Moon Daphnis, non-moon ring particles.
Credit: NASA/JPL-Caltech/Space Science Institute.

Right, but there has to be a difference between a moon and a ring particle!

Ann

[neufer]

Click to play embedded YouTube video.

Than [sic] Janus may not be a moon! Janus does not look spherical to me. Janus may be demoted and called a "dwarf moon" in the future!

Well, a "dwarf moon" would still be a moon, right? (I don't imagine the category will be invented, however. "Moon" just means "natural satellite", and there's never been a concern about the shape or size or process by which it came into being. At times the Earth has had a second moon when asteroids were briefly captured in orbit.)

Right, but there has to be a difference between a moon and a ring particle!

[khh]

So just 2 planets have a total of 161 moons? This is lunacy!

[MarkBour]

This is one marvelous photo! I would really like to have a summer home on Pandora.

The Jupiter vs. Saturn moon race would seem to be a never-ending, ever-growing catalog. I suppose that just as we've done with planets, someone will propose a "cleared its orbital band" criterion for a moon as opposed to a moonlet or ring particle. Otherwise, we can already say with high confidence that Saturn has a trillion moons.

[orin stepanek]

Saturn may have even more moons when you consider what makes up the rings! PIA12797-full.jpg

In the finding of new, even tinier moons there is no end. Oh no, I see the making of a new category, the “dwarf moon”. They’ll call any non round moon a dwarf.

Than Janus may not be a moon! Janus does not look spherical to me.

Janus may be demoted and called a "dwarf moon" in the future!

Ann

Maybe they'll be called moonatoids!

[neufer]

---

[Fred the Cat]

Click to play embedded YouTube video.

Well, a "dwarf moon" would still be a moon, right? (I don't imagine the category will be invented, however. "Moon" just means "natural satellite", and there's never been a concern about the shape or size or process by which it came into being. At times the Earth has had a second moon when asteroids were briefly captured in orbit.)

Right, but there has to be a difference between a moon and a ring particle!

Yes but would a moon by any other name smell as sweet?

[neufer]

The Jupiter vs. Saturn moon race would seem to be a never-ending, ever-growing catalog. I suppose that just as we've done with planets, someone will propose a "cleared its orbital band" criterion for a moon as opposed to a moonlet or ring particle. Otherwise, we can already say with high confidence that Saturn has a trillion moons.

<<A moonlet, minor moon, minor natural satellite, or minor satellite is a particularly small natural satellite orbiting a planet, dwarf planet, or other minor planet. Up until 1995, moonlets were only hypothetical components of Saturn's F-ring structure, when the Earth passed through Saturn's ring plane. The Hubble Space Telescope and the European Southern Observatory both captured objects orbiting close or near the F-ring. In 2004, when Cassini caught an object 4-5km in diameter on the outer ring of the F-ring and then 5 hours later on the inner F-ring, showing that the object had orbited.

Three different types of small moons have been called moonlets:

• A belt of objects embedded in a planetary ring, especially around Saturn, such as those in the A Ring, S/2009 S 1 in the B Ring ("propeller" moonlets), and those in the F Ring
  
  Occasionally asteroid moons, such as those of 87 Sylvia
  
  Subsatellites

Click to play embedded YouTube video.

Adaptive Optics observations of (87) Sylvia, showing its two satellites, Remus and Romulus.

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<<Sylvia (minor planet designation: 87 Sylvia) is the 8th-largest asteroid [(384 × 264 × 231) ±10 km] in the asteroid belt. It is the parent body of the Sylvia family and member of Cybele group located beyond the core of the belt. Sylvia is the first asteroid known to possess more than one moon.

Sylvia was discovered by N. R. Pogson on May 16, 1866, from Madras (Chennai), India. A. Paluzie-Borrell, writing in Paul Herget's The Names of the Minor Planets (1955), mistakenly states that the name honours Sylvie Petiaux-Hugo Flammarion, the first wife of astronomer Camille Flammarion. In fact, in the article announcing the discovery of the asteroid, Pogson explained that he selected the name in reference to Rhea Silvia, mother of Romulus and Remus (MNRAS, 1866).

Sylvia is very dark in color and probably has a very primitive composition. The discovery of its moons made possible an accurate measurement of the asteroid's mass and density. Its density was found to be very low (around 1.2 times the density of water), indicating that the asteroid is porous to very porous; from 25% to as much as 60% of it may be empty space, depending on the details of its composition. However, the mineralogy of the X-type asteroids is not known well enough to constrain this further. Either way, this suggests a loose rubble pile structure. Sylvia is also a fairly fast rotator, turning about its axis every 5.18 hours (giving an equatorial rotation velocity of about 230 km/h or 145 mph). The short axis is the rotation axis. Direct images indicate that Sylvia has an axial tilt of around 29.1°. Sylvia's shape is strongly elongated.

Sylvia has two orbiting satellites. They have been named (87) Sylvia I Romulus and (87) Sylvia II Remus, after Romulus and Remus, the children of the mythological Rhea Silvia.

Romulus, the first moon, was discovered on February 18, 2001, from the Keck II telescope by Michael E. Brown and Jean-Luc Margot. Remus, the second moon, was discovered over three years later on August 9, 2004, by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France. The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned; all planes are aligned within about 1 degree of each other, suggestive of satellite formation in or near the equatorial plane of the primary.>>