Can Earth "Keep" Another Moon?

[ReptilianGameplays]

Earth has a nice moon, but, could it "keep" another moon orbiting it?

Just like Kerbin and it's moons (Mun and Minmus).

Share your thoughts.

[Cal'Mihe]

I don't see why not, after all Mars keeps two despite having a lower gravity!

Gravity isn't shared, its just there after all Of course there'd be some effects on earth too, like different tides and such.

[Tiberion]

Gravity isn't something you use up, there isn't less gravity present since there is already one major satellite around Earth. So if a body came in at the right speed and angle it could be captured by the planet. It would also be affected by the moon, and having a new satellite would change some things on Earth too (most significantly the tides of the oceans)

It is fairly unlikely though, as Earth is a minor body with respect to gravity in our system. Any wanderer that enters the system (we're talking Asteroid size and not even dwarf planet sized) is likely to be on a trajectory that the Sun and the Gas Giants scoop it up or speed it up to such a trajectory that Earth couldn't capture it. Most moons are capturing during the formation of the system while asteroid belts coalesce into planetoids and settle around the bigger bodies, or 2 bigger bodies collide, and the one that "loses" the collision becomes a moon. Our system is mostly mature at this point, so new satellites are not very likely.

[lajoswinkler]

I think it can't. Moon is an anomalously large satellite (we might call this a double planet) and perturbations it induces are enormous. Given the geological timespans, other objects' orbits should decay or the Moon might launch them into outer space.

[ReptilianGameplays]

Thank you for your reply.

But how long Earth's orbit can reach?

Is it long enough to keep another moon? To avoid it from escaping and such.

Edit: Ninja'd. :I

[K^2]

I think it can't. Moon is an anomalously large satellite (we might call this a double planet) and perturbations it induces are enormous. Given the geological timespans, other objects' orbits should decay or the Moon might launch them into outer space.

Moon is anomalously large, but it's also anomalously close to Earth. Most moons orbit much further out from their parent planet. I'm pretty sure Earth could hold a small satellite at a much greater distance in a stable orbit as a second moon.

There are other possibilities, too. Perhaps, something locked in resonance with the Moon. That would allow the significant perturbations caused by the Moon to actually keep the satellite's orbit more stable. But here I'm mostly guessing, since I don't know if there are any mass ratio limits on the resonances.

But how long Earth's orbit can reach?

Gravity has an infinite range. The only limitation is interaction with other bodies. All of them, except for the Moon, are very, very far away. So you could actually place a satellite quite far from Earth and not have to worry about it.

[ReptilianGameplays]

Gravity has an infinite range. The only limitation is interaction with other bodies. All of them, except for the Moon, are very, very far away. So you could actually place a satellite quite far from Earth and not have to worry about it.

Oh, so that's why Pluto has many moons even with the size?

[BaldWombat]

I've always wondered if the Moon could capture something as it's natural satellite.

[NGTOne]

I don't see why not - whether it would be 100% stable remains to be seen, though it would likely have to be a captured object rather than an accreted planetoid like our existing Moon. It's even conceivable that the Moon itself could have its OWN moon, of the same nature - some object it captured (though this is unlikely due to its highly uneven gravity field - even if the Moon were the only body affecting an orbiting object, that object's orbit would decay due to the highly uneven distribution of mass beneath the surface).

The reason that our Moon is so anomalous is its unusual origin story - it isn't captured, nor did it accrete from early Solar System material. Rather, as the current prevailing theory goes, another body, roughly the size of Mars, smashed into Earth a few hundred million years after its formation, blasting massive amounts of planetary material into orbit (and beyond). Over time, this accreted into a large, low-orbiting satellite as the larger fragments drew pieces towards themselves, and were themselves drawn towards even larger fragments. Due to this taking place relatively quickly (because of the abundance of material), the Moon actually underwent some geological differentiation - current models predict that the Moon has a proper core and mantle like the Earth (though much smaller in relation to the crust, which dominates). The unusual story of its formation is also likely responsible for its uneven gravity field - some of the Earth-lumps (side note: anyone got a better name for them? ) were denser than others, and those concentrations of mass persisted during the formation of the Moon as we know it today.

[K^2]

Oh, so that's why Pluto has many moons even with the size?

Part of it. The other part is that being an Oort cloud object, it has had a lot of opportunities to capture moons.

[NovaSilisko]

Just plop it at L4 or L5 and never have to worry about it again.

[K^2]

Earth-Moon's L_4/5 are barely stable. They would not be able to hold even a small moon for a significant enough amount of time.

[NovaSilisko]

Earth-Moon's L_4/5 are barely stable. They would not be able to hold even a small moon for a significant enough amount of time.

I thought it was L1 and L2 that were inherently unstable?

[K^2]

L₁ and L₂ are always unstable. (Though, they have somewhat stable orbits around them.) The stability of L₄ and L₅ depends on mass ratio of the two bodies. It needs to be at least 1:25 for these to be stable. Earth-Moon is 1:80, so it is technically stable, but this is a smaller margin than you'd think. Sun-Jupiter is closer to 1:1000, and that's the sort of ratio that lets you have a nice stable L_4/5.

[Temstar]

some of the Earth-lumps (side note: anyone got a better name for them?)

The technical name for them is mascon - mass concentration. We still don't have any good idea why mascon on the moon are so massively lumpy compared to unevenness on Earth.

Mascon was actually discovered during the Lunar Orbiter program. The main reason why Apollo 11 ended up landing downrange to the original landing site was because mascon where still not well understood at the time and it perturbed Eagle's flight path. A fix was added to Apollo 12 and later navigation computers to account for mascon, hence why they were able to do pin point landing afterwards.

[metaphor]

The Moon is actually pretty far away from the Earth as far as moons go. It's orbiting at a distance of 40% of Earth's sphere of influence radius. So there's not much room for anything orbiting farther away. And since its mass is relatively close to the Earth's (more than any other moon of a planet) there would be huge tidal forces for any other orbiting body. I don't think any orbit in the Earth-Moon system would be stable over millions of years.

If you look at the Pluto-Charon system, the other discovered moons orbit only at 3 or more times Charon's distance. That would be impossible in the Earth-Moon system since 3 times the Moon's distance is outside the Earth's sphere of influence.

[Rjhere]

I heard that Pluto-Charon is actually a double dwarf planet system, were they are orbiting a barycenter. Also they have about 3 satellites, and by satellites, we mean small celestial objects like astroids just to carify for some peeps. I may be wrong tho about the moons!

[dharak1]

I heard about some speculation about if Rhea has rings/moons I'm not sure what happened but it would be cool if it did.

[Rjhere]

Did some more research, and it seems that it's called the Plutonian system. Consists of two dwarf planets (Pluto and Charon), and four smaller satellites, all orbiting the barycenter of the system.

The satellites are called (from inner to outer ones) Styx, Nix, Kerberos and Hydra.

Kerberos... Kerberos... It seems like a place for kerbals?!?

[NovaSilisko]

No, Pluto's the dwarf planet. Charon is just considered its moon, since Pluto has the larger mass.

[Mr Shifty]

Part of it. The other part is that being an Oort cloud object, it has had a lot of opportunities to capture moons.

Isn't Pluto a Kuiper Belt object? The Oort cloud is out at about a light year from Sol.

[stupid_chris]

Isn't Pluto a Kuiper Belt object? The Oort cloud is out at about a light year from Sol.

Yeah, Pluto is a Kuiper belt object. The Oort cloud stretches as far as 0.8ly

The Kuiper Belt is actually pretty small when you put it next to the Oort cloud.

[Rjhere]

Nova, you're partly correct, partly wrong.

You are correct on the part that Pluto has the bigger mass, but Charon is about 2/3rds the size of Pluto, and Charon's mass is large enough to place the barycenter between the two bodies, and not inside Pluto.

This is according to the centre of mass definition of a double planet, I just wanted to point that out.

Although it was laid out for discussion in the mid 00's, the subject was just lain aside, and never a final answer was published.

[magnemoe]

The Moon is actually pretty far away from the Earth as far as moons go. It's orbiting at a distance of 40% of Earth's sphere of influence radius. So there's not much room for anything orbiting farther away. And since its mass is relatively close to the Earth's (more than any other moon of a planet) there would be huge tidal forces for any other orbiting body. I don't think any orbit in the Earth-Moon system would be stable over millions of years.

If you look at the Pluto-Charon system, the other discovered moons orbit only at 3 or more times Charon's distance. That would be impossible in the Earth-Moon system since 3 times the Moon's distance is outside the Earth's sphere of influence.

How long would something inside the moons orbit last, optimal distance, high enough to avoid drag while low enough to minimize moon's gravity.

Downside is that it would be extremely unlikely to get captured where.

Another problem is that the moon moves outward, some theory that we had an second moon at moons L4 or L5 point however as moon moved outward it became unstable.

[metaphor]

A natural satellite can't orbit closer than about 15,000 km from the Earth or it would break up from tidal forces. Farther than that, the gravitational pull of the Moon would be pretty significant compared to the Earth's pull, so over many orbits it would be unstable due to the Moon's tidal force. So long-term natural satellites can't really exist in the Earth-Moon system.

There have been a lot of searches for Earth natural satellites but none have been found so far. A more successful venture might be looking for moons around Venus.