Why retrograde?

[Frida Space]

Hi everyone, I had a possibly-stupid-hopefully-not-too-much question.

Why are some comets (e.g. C 2013 A1 Siding Springs) retrograde?

I know that most objects in the solar system orbit on more or less the same plane because of the conservation of angular momentum between the protosolar cloud and the spinning disk from which the planets etc emerged, but shouldn't everything orbit in the same "prograde" direction too?

I think it could be that, since most retrograde objects come from the Oort Cloud, where orbits are extremely slow, it's much easier to change a body's direction of 180 degrees compared to, for example, the inner solar system. However, I'm absolutely not sure whether or not this is the case, and I'd be glad if anyone could confirm my theory or explain to me the correct one. Thanks!

[Rune]

Hi everyone, I had a possibly-stupid-hopefully-not-too-much question.

Why are some comets (e.g. C 2013 A1 Siding Springs) retrograde?

I know that most objects in the solar system orbit on more or less the same plane because of the conservation of angular momentum between the protosolar cloud and the spinning disk from which the planets etc emerged, but shouldn't everything orbit in the same "prograde" direction too?

I think it could be that, since most retrograde objects come from the Oort Cloud, where orbits are extremely slow, it's much easier to change a body's direction of 180 degrees compared to, for example, the inner solar system. However, I'm absolutely not sure whether or not this is the case, and I'd be glad if anyone could confirm my theory or explain to me the correct one. Thanks!

Stuff has had a lot of time to maneuver around the solar system, with plenty of 3-body interactions going on messing with the orbits. Statistically, pretty much everything that could happen, happened, including some objects having their orbits turned the whole way around.

That's how we measure how many things there are out there without having detected them, BTW, looking at the probability distribution of each orbit: the likelier an orbit is, the more objects of that class we expect. So if you get some numbers on any category (say, main belt asteroids >100kms across), and if your p is good enough, you can take a good guess at how many 1mm peebles orbit retrograde inside Mercury's orbit, for example.

Rune. The power of big numbers!

[Frida Space]

Thanks for your answer!

[-Velocity-]

I think it could be that, since most retrograde objects come from the Oort Cloud, where orbits are extremely slow, it's much easier to change a body's direction of 180 degrees compared to, for example, the inner solar system. However, I'm absolutely not sure whether or not this is the case, and I'd be glad if anyone could confirm my theory or explain to me the correct one. Thanks!

That's what I've always thought. Once you've been bumped out to the Oort cloud, at aphelion, it only takes a small delta-V nudge to flip you retrograde or hyperbolic.

[lajoswinkler]

Extremely far objects have unfathomably long orbiting periods and their speeds far away are very small. Order is closer to the Sun. Far away, things never had the time to revolve enough times to bump into each other and leave a dominant direction of orbiting.

Also, it's quite easy to reverse their orbits, give their tiny speeds. One passes next to each other and momentum gets stolen... voila.

[-Velocity-]

Stuff has had a lot of time to maneuver around the solar system, with plenty of 3-body interactions going on messing with the orbits. Statistically, pretty much everything that could happen, happened, including some objects having their orbits turned the whole way around.

That's how we measure how many things there are out there without having detected them, BTW, looking at the probability distribution of each orbit: the likelier an orbit is, the more objects of that class we expect. So if you get some numbers on any category (say, main belt asteroids >100kms across), and if your p is good enough, you can take a good guess at how many 1mm peebles orbit retrograde inside Mercury's orbit, for example.

Rune. The power of big numbers!

I doubt that within like 50-100 AU there's enough gravitational perturbations to force many or any objects into retrograde orbits. I suppose it might be possible to imagine a highly unlikely scenario where such a thing happens. Also consider that the closer you are to the Sun, the wider the range of velocity at aphelion that will result in you actually colliding with the Sun. You have to get through that range in a single interaction. A close passage by a planet is probably enough, but that is highly, highly unlikely. You also have to go through a series of solar system encounters where you use planetary gravity to brake your orbital velocity around the Sun to near-zero. That is probably even MORE unlikely. The only retrograde solar system objects I'm aware of are long-period comets, and the reason these exist is because their aphelion is so far away that they can be forced into a retrograde orbit in a single, low-delta-V encounter.

I suppose we might find an example of an object in a retrograde orbit with a perhelion of only a few hundred AU, who knows, maybe less, but if so, it would probably have started out as a long-period comet that went retrograde, and then had its aphelion reduced by subsequent gravitational interactions with planets.

Edited March 13, 2015 by |Velocity|

[softweir]

I doubt that within like 50-100 AU there's enough gravitational perturbations to force many or any objects into retrograde orbits.

A Close Encounter with a dwarf star

Might that do the trick?

[lajoswinkler]

Oort cloud is at roughly 1 lightyear distance. Plenty of chaos there.

[-Velocity-]

A Close Encounter with a dwarf star

Might that do the trick?

No, I said within 50-100 AU, not 0.8 light-years, which is like 50,000 AU. At tens of thousands of AU, there are plenty of gravitational perturbations from things like passing stars or molecular clouds to disrupt objects into retrograde or hyperbolic orbits. My point is that interactions between objects that are relatively close to the Sun (and stay relatively close to the Sun) are unlikely to transform an objects orbit into a retrograde orbit, the delta-V change required is probably just too high.

That said, I wonder if a gas giant could modify a close passing long period comet- especially one that's already "sun-grazing"- into a retrograde comet in a single interaction?

Anyway, I believe that the preponderance of retrograde long period comets is entirely or almost entirely explained by what the original poster believed, namely that it takes only a very small delta-V perturbation at like 10,000+ AU to transform your orbit into a retrograde one.

Edited March 13, 2015 by |Velocity|

[Frida Space]

That's what I've always thought. Once you've been bumped out to the Oort cloud, at aphelion, it only takes a small delta-V nudge to flip you retrograde or hyperbolic.

Extremely far objects have unfathomably long orbiting periods and their speeds far away are very small. Order is closer to the Sun. Far away, things never had the time to revolve enough times to bump into each other and leave a dominant direction of orbiting.

Also, it's quite easy to reverse their orbits, give their tiny speeds. One passes next to each other and momentum gets stolen... voila.

Anyway, I believe that the preponderance of retrograde long period comets is entirely or almost entirely explained by what the original poster believed, namely that it takes only a very small delta-V perturbation at like 10,000+ AU to transform your orbit into a retrograde one.

So maybe my initial theory wasn't so bad after all. Thanks everyone!

[Rune]

I doubt that within like 50-100 AU there's enough gravitational perturbations to force many or any objects into retrograde orbits. I suppose it might be possible to imagine a highly unlikely scenario where such a thing happens. Also consider that the closer you are to the Sun, the wider the range of velocity at aphelion that will result in you actually colliding with the Sun. You have to get through that range in a single interaction. A close passage by a planet is probably enough, but that is highly, highly unlikely. You also have to go through a series of solar system encounters where you use planetary gravity to brake your orbital velocity around the Sun to near-zero. That is probably even MORE unlikely. The only retrograde solar system objects I'm aware of are long-period comets, and the reason these exist is because their aphelion is so far away that they can be forced into a retrograde orbit in a single, low-delta-V encounter.

I suppose we might find an example of an object in a retrograde orbit with a perhelion of only a few hundred AU, who knows, maybe less, but if so, it would probably have started out as a long-period comet that went retrograde, and then had its aphelion reduced by subsequent gravitational interactions with planets.

You get exactly the idea behind my post, without getting it

Of course it is a very low probability event to reverse direction in the Oort, fall to the inner system, then circularize at a low altitude. Which is why only a few objects will have done it, most probably in the peeble or grain size. And the probability of them doing such an unlikely thing, compared to anything else, gives us a very good estimate of how many we should expect to find with those orbital parameters. Still, there will be some, there is a lot of gravel out there.

Rune. It still amazes me how we can backtrack through millions of years of planetary billiards, though, and still get usable results.

[YNM]

Could've been a point for the migration of gas giants. When these things move off from the inner side to the outer side, they increase angular momentum - but angular momentum have to be preserved in the system ! This causes a lot of bodies to be moved into a lower angular momentum orbit, either a smaller orbit (closer to Sun), a very elliptical orbit, or even, retrograde orbit.

Or, it could've been an ISM dust. Such things might exist. If you want to be sure, just do the tisserand value.

[Bill Phil]

There's quite a large amount of mass around the gas ad ice giants... I would expect some of them to displace objects.

Plus, not all objects that form in the solar system will be in the exact same plane. Lighter objects are more likely to move...

[Tommygun]

Are there ever any clues to an object's orbit that might suggest that an object is extra solar or is there just too much randomness?

[PakledHostage]

So maybe my initial theory hypothesis wasn't so bad after all. Thanks everyone!

Fixed that for you.

[Frida Space]

Fixed that for you.

Thanks! English is one tough language

[PakledHostage]

Hey, I know that English isn't your first language but it is an important distinction to make. Especially on a science forum. There are those who exploit the common misunderstanding of what a "theory" is in the scientific context to further their ideological objectives.

[Frida Space]

Hey, I know that English isn't your first language but it is an important distinction to make. Especially on a science forum. There are those who exploit the common misunderstanding of what a "theory" is in the scientific context to further their ideological objectives.

You're absolutely right, and I'm glad you corrected me in the first place. Thanks!