But that would smash incoming object to smithereens. A good chuck of Moon would be smithereened too. Some of those smithers would end up hitting the Earth, potentially a bad thing, depending on the size of the smither. However, it’s possible that some of the smithers would coalesce into a new body orbiting the Earth or the Moon at least temporarily. Unlikely to be in a stable orbit.
Absolutely true.
And there’s also “stable” and “stable” for different degrees of “stable”.
Captures usually follow orbits like this:
But then you have the orbits of Phobos and Deimos around Mars, which are very close to circular. There’s some debate about how Mars ended up with two moons, but capture seems like one of the likely theories.
As far as stability goes, their orbits are stable enough that most folks think of them as stable, but in the long term I think one or both of them is predicted to smack into the surface of Mars sometime within the next 50 to 100 million years (IIRC). They have short term stability, but not long term stability.
If our moon managed to nudge the captured object into a reasonably circular orbit that would last even half a million years, I think a lot of people would call that “stable”, at least to some degree.
Tidal forces tend to circularize orbits over the long term.
In the very long time, every orbit is unstable, due to tidal effects (as in the case of Phobos and Deimos) or gravitational radiation.
Neither one is going to auger into Mars. Phobos, however, has a steadily decreasing orbit and it’ll hit the Roche limit (astronomically) soon. At that point it’ll break up and become a ring. Stuff from the inner part will likely rain down on the planet, so the ring will be astronomically temporary. (That happens with Saturn’s rings, BTW.)
There’s a hypothesis that this has already happened two or three times with Phobos. After it breaks up, the parts of the ring outside the Roche limit coalesce into another, much smaller body and the process repeats. However, this is just an idea. I don’t think they have anything in the way of proof.
So do gravitational radiation, trace air resistance, interactions with large numbers of other objects in orbit around the same primary, and most other real-world deviations from Kepler. Basically, most “extra” forces will tend to oppose direction of motion, and will tend to be stronger when the object is going faster and/or closest to its primary (which are the same time). Something that slows down an object near its periapsis (near point) will decrease its apoapsis (far point), so while both decay, the apoapsis decays quicker, and so the orbit becomes more circular.
Unless its already in a circular orbit around the sun , pretty much matching earth, then its not going to just fall into orbit…
But there is the slingshot effect, this transfers momentum from an object in orbit, eg earth, to your object … or the other way… if you want to wipe out speed, transfer it to the earth…
So an object that comes through earths orbit plane can slingshot earth a number of times and thus get its speed dropped enough to enter into the solar orbit at the same radius and in same plane as earth… thus following earth around. then it can easily enter into earth orbit, or do orbits in pretty much sequence, earth ,sun,earth ,sun… there has been an object that does it, it does a U turn at earth… and then orbits sun the other way around until it slingshot U turns on earth… or just do a few earth orbits and return to the solar orbit for heaps.
The object falling toward the sun can be slowed down by other bodies, such as jupiter,saturn,mars, venus, mercury, or hitting gases to do with the sun…
This is not actually possible, at least not stably. Orbits (at least, absent extra dissipative forces like air resistance) are time-reversible, so if an orbit is such that you can get into it, then it’s also such that you can get out of it.
This is called co-orbiting, although objects in such orbits are (mistakenly) called quasi-moons. But the objects do not orbit backwards; they’re all going around the Sun the same way at all times. What happens is that they alternate between being in an orbit with a period that’s a bit shorter than a year with one that’s a bit longer. So when they’re in the shorter orbit, they’ll be catching up with the Earth and at other times falling behind the Earth. When they show the orbit relative to the Earth during the longer orbit, it’ll appear to be going backwards, but it’s still orbiting the Sun forwards.
There are two satellites of Saturn that are co-orbiting: Janus and Epimetheus. When they were first discovered, there was a lot of confusion among astronomers, because they were trying to fit observations of both objects to a single orbit. Didn’t work. Eventually someone figured out there were actually two objects there.
Thanks.
For the case of interaction with other objects - am I right in vaguely remembering that smaller objects get ejected and larger ones get circularized orbits?
Typically, yes, at least to the extent that there is any “typical” for chaotic systems. It’s tough to eject a large object, because that takes a lot of energy, and if that energy just isn’t present in the system, then no amount of chaos will change that.
Analogous to evaporative cooling too, I suppose.
Gee, I hope not.
That analogy hadn’t occurred to me, but on thinking about it, I don’t think it’s a bad one: Some particles with especially high energy leave the system, and so what’s left of the system has a lower “temperature”.
They use that terminology in the Kuiper Belt. Cold KPOs are those in circular orbits with low inclinations, while hot KPOs are in more elliptical/high inclination orbits. Nothing to do with their actual temperatures, which are essentially the same.
Am I the only one who would love to see this as a separate thread with proper Doper treatment?