Could Earth capture a new, small moon without it hitting us and killing us all?

Suppose that a rogue planetoid were to enter our solar system from parts unknown and barrel towards us. To make this specific, let us suppose this object is about the size of Ceres, the largest object in the asteroid, so it’s about 470 kilometres in diameter. We’ll call it Achilles because I can’t think of any celestial object called that and it’s mythological. The Moon is about 3500 km in diameter, so while our new friend is way large enough to be spherical, it is quite a lot smaller than the Moon.

If it HITS the Earth we’re all dead meat. But suppose it does not, it just gets close. Is there some way Earth could pick this thing up as a second moon without it smashing into us and blowing us all to smithereens? Is a contactless capture possible?

Would it be possible for such an object to assume a stable orbit inside the Moon’s orbit?

The object would have to be going at just the right speed and angle, which is extremely unlikely, but it is theoretically possible.

Of course not as big as Ceres, that would not go unnoticed, but it happens all the time. Earth has often a second moon, generally in an unstable orbit. If you search for “Earth’s second moon” you will get plenty of hits, here is one:

And we do this kind of thing deliberately, rather than accidentally, all the time, not just around the earth but around other bodies in the solar system. That’s on top of natural satellite capture for the gas giants. So proof of concept isn’t a problem at all.

Sounds like a great idea for an action movie to me…

Don’t we have to actually burn some delta-v to get our artificial satellites into orbit?

A rock with no propulsion ending up in anything but an absurdly eccentric elliptical orbit is much harder, no? But, yes, still theoretically possible.

We do, yes, but the point is that given the right combination of speed and angle, it can happen.

We make those factors happen deliberately in the case of artificial satellites but there’s nothing stopping them from occurring naturally. And indeed, we know the gas giants capture asteroids. Earth does, too.

The difference for the OP would be the size of the body and specificity of any orbital parameters, not the underlying physical principles.

As with many other scientific hypotheticals, like the possibility of a dinosaur civilization or the chance that your pants will suddenly jump a foot from your body and start twerking, this does not violate any fundamental laws (except possibly the stable orbit between us and the moon part) but is so improbable that the mere lifetime of the Earth has a infinitesimally low chance of witnessing it.

Trivia point: Achilles is the name of the first Trojan asteroid discovered, in 1906.

By “capture” does the OP mean the earth itself drawing it into orbit naturally, or are we talking about humans drawing it in and putting it in a stabilized orbit?

No strictly two-body gravitational interaction ends up with a capture. However Earth has a convenient thied body hanging around, so it’s possible though not likely that a new moon might interact with the Earth and curent Moon to leave Earth with two moons (with the current one in a slightly different orbit)

Note that the planet Neptune has an unusual moon, Triton, which was almost certainly captured from an independent orbit at some point. This capture involved a great deal of disruption to the system of existing moons around Neptune, and left Triton in a retrograde (backwards) orbit, and another moon (Nereid) in a highly eccentric orbit. Indeed, the capture of Triton may have involved at least one collision.

If we managed to capture another large moon from somewhere, this would almost certainly disrupt our existing moon’s orbit, and quite possibly have serious repercussions on our own world.

Achilles is the first Jupiter trojan asteroid discovered in 1906. Many other such trojans are named after other characters in the Trojan War stories. (Hence the general name for such objects at stable Lagrange points in the Solar System.)

Inside the Moon’s orbit, stability in any long term sense is impossible. Even far outside the Moon’s orbit it will end up just wandering off eventually. And good riddance, I say. A 2nd moon, who needs it?

Depending upon how elastic your definition is, Earth has several quasi-moons, the biggest of which is Cruithne (actually 3753 Cruithne), discovered in 1986.

There are others that are similar:

There’s capture, and then there’s stable capture.

With just the Earth, capture is impossible.

With the Earth and the Moon both, capture is possible, just unlikely… but even if the Earth and the Moon did, between them, manage to capture a third object, the resulting system couldn’t be stable. Most likely, the low-mass newcomer would stick around for perhaps a few dozen orbits, and then get ejected again, or possibly (though less likely) collide with the Earth or Moon. It could, in principle, stick around for longer, and there’s no upper limit for how long it could last, but eventually, something would have to fall apart.

Alternately, if you start with a system with a lot of things in orbit, and you add one more, or if you already have something in orbit comparable in mass to the newcomer, it’s possible that the newcomer does end up in a stable orbit, but that that some other pre-existing object ends up being either ejected or crashing.

We can put satellites into stable orbits, either around the Earth or around other planets, but it requires rocket burns to do that. Although, come to think of it, there is one natural scenario that could happen: You could have an object that’s partly rocky and partly made of ice (water or carbon dioxide), and the ice evaporates when heated, producing jets. It’d take a heck of a lot of luck, but that could end up nudging the new object into stability. The same jets that make it stable could also make it unstable again… but maybe it runs out of volatiles, at a point when it’s stable. Fantastically unlikely for any given single object, but over billions of years, I wouldn’t bet against it having happened once or twice.

The thing that really makes this scenario unlikely is that it’s an extra-solar object. Extra-solar objects are falling into the system from essentially infinity, so they’re going to have lots more velocity than things already in solar orbit.

In order for an object to fall into orbit around one of the planets, it needs to lose a lot of velocity. Even objects already in solar orbits need to lose lots of velocity, which is why they are rarely captured. You occasionally hear about some rock passing within the orbit of the Moon, but they just sail on past. The few temporary moons just make a loop or two around the Earth and then escape again. They haven’t really lost enough velocity to make those temporary orbits permanent. To stay in orbit, that excess velocity has to be transfered to some other object, such as the Moon. That could happen, but it takes a special and very unlikely condition.

Anyway, the extra velocity that extra-solar objects have make it doubly unlikely they’ll fall into Earth orbit. So essentially the answer is “No, it won’t happen.”

The OP specified that we’re asking about something “about the size of Ceres”, which is WAY bigger than any of the artificial satellites that we’ve put up there. So my question concerns the collateral damage. I imagine, for example, that our orbital speed around the Sun would either speed up or slow down, I know not which. Is Ceres small enough that the change would be noticed only by astronomers who know the length of a year down to the millisecond? Or would ordinary folks have to get used to a new system of time?

That’s pretty wild!

I mean the Earth captures it, not us. It becomes a stably orbiting moon. You can look up and see it at night.

Surprised no one mentioned this yet:

Yes if it hit the moon instead of the earth.

Of course that might cause the moon to alter its orbit.