Size of moons

[Tom Mazanec]

How small should an "official" moon be? Will we someday have a total of 36,482,537 named moons of the gas giants, some the size of a breadbox?

[ToSeek]

Less than 1 kilometer, and it's just a rock.

What's the smallest Moon so far?

Answered my own question: Looks as if there are some of Jupiter's that are 1 kilometer.

[frogesque]

Personally I would be unhappy with anything less than the mass required to keep itself in a more or less spherical shape. Potatoes, breadboxes and double-deck buses would be companions or satelites otherwise we will be naming every individual dust particle orbiting in the rings around Saturn.

Phobos & Deimos orbiting Mars have historically been called moons and would be disqualified by my definition but since there are only two and they have been named for a long time it's logical to keep the distinction, at least untill they crash into Mars' surface

[Normandy6644]

Well I think it depends on how you define a moon. If you define it as a smaller body orbiting a larger one, then you don't have a real limit. You can't use mass ratios since you have planets of many different sizes, so I suppose it becomes a question of taste. I mean, if our Moon were situated between Mars and Jupiter, it would be a planet, and if Pluto were orbiting the earth, it would be a moon. So, I'm not really sure how to define it.

[ToSeek]

They need to draw a line someplace or they're going to be naming all the rocks in the rings of Saturn.

[Brady Yoon]

More than 10 km, with rings and objects that don't orbit planets not included.

[Doodler]

There was an offhand mention with the recent Cassini flyby of the rings that basically stated they considered a moon to be any individual object they could resolve on its cameras.


Now that threshold will change over time, but there's nothing wrong with establishing a "Cassini Limit" for a Saturnian moon. Basically meaning, if this satellite, which is going to be our key to understanding Saturn for some number of years on from now, be the yardstick to map the system.

Any thoughts on that?

[amstrad]

My definitions:

Satellite: Body that orbits another body. Mass size have nothing to do with it. This group can be divided into artificial and natural satellites.

Moon: Natural satellite of a major planet.

Therefore, by my definitions, Jupiter so far has 60 but possibly thousands of moons.

Dactyl however is not a moon of Ida.

[Dgennero]

I'd like to agree with froguesque - a moon, or "major moon" should be big enough to be compressed into roughly spherical shape by its own gravity.
That, of course, depends on the density, material of the body in question, and there is no definite limit like "ok, radius 200 miles - moon, radius 199 miles - satellite, but as a rule of thumb it should do, so Mimas is a moon, while Deimos is just a rock.

[Tobin Dax]

My definitions:

Satellite: Body that orbits another body. Mass size have nothing to do with it. This group can be divided into artificial and natural satellites.

Moon: Natural satellite of a major planet.

Therefore, by my definitions, Jupiter so far has 60 but possibly thousands of moons.

Dactyl however is not a moon of Ida.

So, by your definition, are Phobos and Deimos moons of Mars?

[amstrad]

So, by your definition, are Phobos and Deimos moons of Mars?

Yes, absolutely.

Natural satellites can be formed through capture (most likely to be the case with Mars and with Triton/Neptune), co-formation (possible the case with Titan/Saturn), or elastic collision (like our moon).

As far as I am concerned, particles of dust can be natural satellites. Saturn's rings are all bodies in orbit around Saturn and therefore each particle in the rings are moons.

Now, whether you name all of these or even bother to count and number them is a different story entirely.

[Moose]

Natural satellites can be formed through capture (most likely to be the case with Mars and with Triton/Neptune), co-formation (possible the case with Titan/Saturn), or elastic collision (like our moon).

Could you clarify the reasoning behind excluding Dactyl if you're including Deimos and Phobos and "particles of dust" around Saturn?

[amstrad]

Could you clarify the reasoning behind excluding Dactyl if you're including Deimos and Phobos and "particles of dust" around Saturn?

I already did. My definition says that a moon is a natural satellite of a major planet. Dactyl is a natural satellite of a minor planet and is therefore simply a satellite and not a moon.

[Moose]

Could you clarify the reasoning behind excluding Dactyl if you're including Deimos and Phobos and "particles of dust" around Saturn?

I already did. My definition says that a moon is a natural satellite of a major planet. Dactyl is a natural satellite of a minor planet and is therefore simply a satellite and not a moon.

Ah. I missed that. Thanks.

Okay, Pluto. Major or minor? The world (well, Charon anyway) wants to know.

[Tom Mazanec]

Sorry, Pluto, I vote for minor.

[Brady Yoon]

What if we divide satellites into major moons and satellites/small moons. Major moons, like people have said, would be objects large enough to form a sphere, and orbiting a planet. Minor moons would be anything else.

[amstrad]

What if we divide satellites into major moons and satellites/small moons. Major moons, like people have said, would be objects large enough to form a sphere, and orbiting a planet. Minor moons would be anything else.

You beat me to it. I logged on suggest precisely this.

Whether some object is a major or minor planet/moon is just a name game. It is meaningless to science. I will leave that game to those who care.

To make the discussion more complicated, maybe we should discuss that everything in the Solar System orbits the Solar System's center of mass. The Moon orbits the Solar Systems COM and the Earth simply purturbs its orbit (and vice versa) as is being discussed in another thread.