77 votes
Accepted

Is Earth's moon the only one where a total eclipse of the sun covers the entire sun?

As planets get farther from the Sun, the Sun takes up a smaller part of the sky. The Sun is about 31 arc-minutes when viewed from Earth, but just 6-7 from Jupiter and 3-4 from Saturn. Less than 2 ...
userLTK's user avatar
  • 23.9k
73 votes

Why do Jupiter’s moons have so much water?

tl;dr: They have more water because they captured it as ice, and it’s easier to hold onto ice than water vapor. Planets (and by extension, moons) beyond the frost line were formed with ice as a part ...
Justin T's user avatar
  • 3,404
48 votes
Accepted

Why don't the inner moons of Jupiter have tidally-induced volcanism?

It’s because they are much smaller than Io. Tidal forces are differential forces, that is, they result from the difference in gravitational pull on one side of a body compared to the other. When an ...
Eric Jensen's user avatar
  • 4,864
44 votes

Why is there a mountain inside the Herschel crater on Mimas?

In the extreme energy of a large impact, the rock behaves like a liquid (It isn't actually completely melted, though some is. The extreme forces cause the rock to flow). As the impactor hits the moon, ...
James K's user avatar
  • 119k
38 votes

Do celestial objects need to be big to have liquid water on their surfaces?

Liquid water can't exist in a vacuum. If there is no pressure, then the boiling point will drop to the freezing point and so there will either be ice or water vapour. And if the world is "small&...
James K's user avatar
  • 119k
36 votes
Accepted

Why do planets and satellites in the Solar system look so wildly different if they came from more or less the same matter?

This questions can be split in two; for planets and satellites. The diversity of planets reflects in part the diversity in terms of chemical composition of the protoplanetary disk. We know that UV ...
Swike's user avatar
  • 3,896
34 votes

Is it possible to have satellites (natural or not) orbit the same celestial object in different directions ? (clockwise, counterclockwise)

It is absolutely possible. Moons that formed with their planet will be in prograde orbits, but moons that are captured bodies (such as the outer satellites of Jupiter, Saturn, Uranus, and Neptune (...
notovny's user avatar
  • 4,770
33 votes

Definition of a moon in an exam: "A satellite of a planet that *doesn't produce light itself but reflects it*" - is there relevance for the emphasis?

I'd ask the test author to provide an example of an object that would be improperly defined as a moon in the typical answer ("A satellite of a planet") that would properly be defined as not ...
Connor Garcia's user avatar
  • 16.3k
23 votes
Accepted

Which Saturn satellite passes closest to Saturn's rings and at what distance?

Pan, Daphnis, and various other moonlets, I would argue, are inside the rings. If you explicitly discount the Encke gap (which Pan orbits in) and the Keeler gap (which Daphnis orbits in) as being ...
Ingolifs's user avatar
  • 4,157
22 votes

If the Earth had another moon would it be better protected from asteroids?

I don't quite buy JamesK's claim that "the moon only covers less than 0.001% of the sky, and so leaves us vulnerable to 99.999% of asteroids." That argument would work if typical asteroids 1)...
Ilmari Karonen's user avatar
20 votes

Do moons have moons?

It's theoretically possible for moons of moons to exist in stable orbits, gravitationally speaking. However, as far as I'm aware, no natural moons of moons have ever been observed as of yet. (...
ahiijny's user avatar
  • 481
20 votes
Accepted

Did we discover 10 or 12 new moons of Jupiter?

Per the Carnegie Science article that Magic Octopus Urn linked from NASA in the comments, a Carnegie Science team led by Scott S. Sheppard noticed something new in spring of 2017 (though some ...
called2voyage's user avatar
  • 6,312
20 votes
Accepted

At what distance from the Sun can planetary moons exist?

There are several factors determining the inner limit to moons. Perhaps the simplest is that it needs to stay inside the Hill sphere, the region around the planet where the planet's gravity dominates ...
Anders Sandberg's user avatar
19 votes

Would an analogue of the definition for planets also work for moons?

In 2006 the IAU had a trilemma. They could decide that Eris was a planet, and potentially allow for future discoveries of tens of new planets. They could be inconsistent, declare that Pluto was a ...
James K's user avatar
  • 119k
19 votes

Is it possible to have satellites (natural or not) orbit the same celestial object in different directions ? (clockwise, counterclockwise)

Partial and supplemental answer. As an example1 from Wikipedia's List of natural satellites; Mooons by primary: (Jupiter's) 84 known irregular moons are organized into two categories: prograde and ...
uhoh's user avatar
  • 31.3k
17 votes

Why don't the inner moons of Jupiter have tidally-induced volcanism?

There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Only one of those innermost moons (Thebe) has an ...
David Hammen's user avatar
  • 33.7k
16 votes

How would ocean tides work on a tidally-locked planet?

Since the tidal bulge is always in the same place, how would that affect ocean tides? The concept of tidal bulge is a useful fiction, but fiction nonetheless. For an object in an eccentric orbit, the ...
David Hammen's user avatar
  • 33.7k
15 votes

Do celestial objects need to be big to have liquid water on their surfaces?

Do celestial objects need to be big to have liquid water on their surfaces? Yes. In a nutshell: liquid surface water needs an atmosphere. To sustain an atmosphere, a planet must be sufficiently ...
gerrit's user avatar
  • 2,033
15 votes

Definition of a moon in an exam: "A satellite of a planet that *doesn't produce light itself but reflects it*" - is there relevance for the emphasis?

Is this a test of science or of Suomi (or English)? In English, a moon is a natural satellite of a planet, dwarf planet, or asteroid. In Suomi, Yleisnimenä kuu on planeetan, kääpiöplaneetan tai ...
James K's user avatar
  • 119k
14 votes
Accepted

Are all satellites thought to be the result of collisions with the planets?

There are three main formation scenarios for planetary moons. The giant impact hypothesis: The satellite forms as a consecuence of an impact between the planet an a large planetesimal. The Moon is an ...
Swike's user avatar
  • 3,896
13 votes
Accepted

What exactly is a "moon"?

Unlike "planet" the IAU hasn't attempted to precisely define "moon". General usage requires that a "moon" is a natural satellite of a planet (or dwarf planet, asteroid, or perhaps even of another moon?...
James K's user avatar
  • 119k
13 votes
Accepted

How many satellites orbit their planet faster than the planet rotates?

Uranus rotates once every 0.718 days. 11 of its satellites have a shorter orbital period. These are inner satellites of Uranus which are roughly in the equatorial plane of Uranus. I don't quite ...
usernumber's user avatar
  • 17.5k
13 votes
Accepted

Is there a name for a planet and its moons/satellites?

This question has been asked before on the Space Exploration page. In summary, the term used is system, e.g. the 'Jupiter system'.
christopherlovell's user avatar
13 votes

Why don't we find planetoids at L4/L5?

In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of ...
M. A. Golding's user avatar
12 votes
Accepted

How can tidal heating lower Io's orbit?

How can tidal heating lower Io's orbit? It doesn't, at least not to first order. The first order effect is that tidal heating acts to circularize Io's orbit. Counter to that, orbital resonances with ...
David Hammen's user avatar
  • 33.7k
12 votes
Accepted

Moons with curlicue paths around our Sun?

A moon will describe a path like this if its orbital speed relative to its parent planet is greater than the parent planet's orbital speed about the Sun. (Assuming the moon's orbit about the planet ...
Michael Seifert's user avatar
11 votes
Accepted

What is the longest natural bound orbit chain observed?

Certainly 4 (perhaps 5): The longest certainly-known chain is "3": Sun-Earth-Moon. Rhea (a moon of Saturn) has been observed to have a ring of orbiting material: That gives Sun-Saturn-Rhea-...
James K's user avatar
  • 119k
11 votes
Accepted

Are satellites of trans-Neptunian objects classified as trans-Neptunian objects?

Yes/No as needed. The MPC, which is fairly authoritative, lists only (134340) Pluto, and doesn't include Charon and the other satellites of Pluto. Formally, binary asteroids are given a single ...
James K's user avatar
  • 119k
11 votes

If the Earth had another moon would it be better protected from asteroids?

Neither effect is significant. Asteroids hit the Earth because they happen to be on a collision course with the Earth. The Earth's gravity can deflect some that might just miss the Earth onto a ...
James K's user avatar
  • 119k

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