[Yusup Hizirov (OP)]

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?
Why do asteroids have satellites, but planetary satellites do not have satellites, despite the fact that planetary satellites are much more massive than asteroids?
Saturn's two moons, Janus and Epimetheus, move in the same orbit and pass each other every four years.
Why don't Janus and Epimetheus form a double satellite? https://www.planetary.org/articles/janus-epimetheus-swap

[Origin]

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

Maybe it's just me, but I don't know what you mean.  Could you rephrase the question?

[Halc]

Maybe it's just me

Not just you.

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

I think a citation is needed. Conservation of angular momentum says that any rotating object will continue rotating unless an external torque is applied to it.  I have never heard of any artificial satellites ever being anywhere long enough for enough external torque to be applied to halt its rotation. Slow a tiny bit perhaps, but tidal forces are the only force I can think of that would do this, and they're just not strong enough when acting on something as small as any artificial satellite we've put up anywhere, regardless of which object it happens to orbit.

The Hubble telescope is an interesting example.  Its angular momentum is very near zero, and it needs to halt its rotation for hours/weeks at a time, but then it will rotate to a new orientation using gyros, all of which work without ever altering the total angular momentum of the satellite.

Also, please stop posting any topics in Question of the Week. The sub-forum is reserved for topics by radio listeners and is not for new posts by any forum members. I have been known to just delete invalid topics from there rather than move them.

[paul cotter]

Yep, I saw this just before hitting the sack last night and was tempted to reply as things have been very quiet lately. On seconds thoughts life can be quite difficult without additional aggravation-so I declined.

[Bored chemist]

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

Why do you believe that artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

Because, if it's not true, it needs no explanation.

[Janus]

I'm going to take a stab at what he might be getting at.
He may be talking about the stability of orbits around these two bodies.
If that is the case, then yes, low lunar orbits do tend to be less stable than low Earth orbits.
This is caused by mascons (mass concentrations)  randomly scattered under the Moon's surface. These are regions where the density is significantly higher than average, and this, in turn, tends to make the gravitational field nearer to the Moon a bit uneven which can disrupt the orbits of objects there.

[evan_au]

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

I agree that satellites orbit because they have inertia and a velocity relative to the parent body, which gives them angular momentum.

From a different viewpoint...
The Earth is surrounded by an atmosphere which extends beyond 100km (getting less dense the farther you go)
- This means that satellites in Low Earth Orbit (LEO) continually lose angular momentum, and their orbit decays
- The International Space Station at around 400km altitude would eventually burn up in the atmosphere if it did not get regular rocket boosts
- In contrast, the Moon has no atmosphere - any transient events like meteor impacts just result in a slightly less dense vacuum in the vicinity of the impact.
- So I expect that an artificial satellite at 150km altitude above the Moon would remain in orbit much longer than an artificial satellite at 150km above the Earth.

[Yusup Hizirov (OP)]

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

I agree that satellites orbit because they have inertia and a velocity relative to the parent body, which gives them angular momentum.

From a different viewpoint...
The Earth is surrounded by an atmosphere which extends beyond 100km (getting less dense the farther you go)
- This means that satellites in Low Earth Orbit (LEO) continually lose angular momentum, and their orbit decays
- The International Space Station at around 400km altitude would eventually burn up in the atmosphere if it did not get regular rocket boosts
- In contrast, the Moon has no atmosphere - any transient events like meteor impacts just result in a slightly less dense vacuum in the vicinity of the impact.
- So I expect that an artificial satellite at 150km altitude above the Moon would remain in orbit much longer than an artificial satellite at 150km above the Earth.

But for some reason everything happens the other way around.

[Origin]

But for some reason everything happens the other way around.

Do you have evidence of this?

[Yusup Hizirov (OP)]

But for some reason everything happens the other way around.

Do you have evidence of this?

Why do asteroids have satellites, but planetary satellites do not have satellites, despite the fact that planetary satellites are much more massive than asteroids.

[Eternal Student]

Hi.

planetary satellites do not have satellites

    There are many moons around Saturn  (146 according to the NASA website today - but new bodies get identified often).   Of those some were thought to be moons that were in a fairly stable orbit around other another moon - but carefull examinations seem to have shown they weren't.     (Reference:  https://en.wikipedia.org/wiki/Subsatellite    under discussion of "possible natural instances" of sub-satellites).
     None-the-less  there are so many things up there around Saturn that some moon will have some other body that will orbit it,  even if that sub-satellite isn't much bigger than a dust particle.

    This article  ("Can a moon have a moon?",   BBC sky at night magazine),   https://www.skyatnightmagazine.com/space-science/can-moon-have-moon       also discusses the issue.    Their main conclusion was:.......

    ....the moons probably don?t have a large enough gravitational field to control the motions of a smaller object and compel it into orbit.   The planet would likely still be the controlling influence and create an unstable orbit. So the answer is probably no: planetary moons cannot have natural satellites.

    In general, it is known that the mechanics of a gravitational 2-body system will usually result in the formation of a stable orbit of the two bodies around a common barycentre.   However, the 3-body problem is much more complicated.   For example, one of the bodies is often ejected or thrown out of the system over enough time.  It is presumably rare for a moon to retain a sub-satellite in a stable orbit around itself.

Why do asteroids have satellites,

    Many of them stay far enough away from large sources of gravitation (like a planet) that they are effectively only two-body problems,  the asteroid and its satellite.

Best Wishes.

[Halc]

Why don't Janus and Epimetheus form a double satellite?

This was seemingly never answered. They don't form a double satellite because each is moving at a speed larger than the escape velocity of the other. So when they approach each other, in the frame of their mutual center of gravity, they each pass the other in a sort of hyperbolic path. In the frame of Saturn, this results in a horseshoe orbit, https://en.wikipedia.org/wiki/Horseshoe_orbit because each moon travels in a horseshoe path relative to the rotating frame of the line between the primary (Saturn) and the other moon.

There are several objects in a horseshoe relationship with Earth. They're in orbit about the sun, but in Earth's orbital path. Each object travels faster than Earth as it orbits the sun until it catches up to Earth from the other side, at which point it reverses its progression and lags Earth until Earth catches up from the other direction. Those two moons around Saturn are doing the same thing.

[Zer0]

@Yusup

& if You permit a little optimistic speculation...
Maybe, just maybe our very own Moon..
Might have a teeny weeny Moonlet!
(: or moonletS )