Sorry for the terse title, but a complete sentence using the keywords would have been just too long.
Comet ISON is on a hyperbolic trajectory. If it survives it’s Sun graze, it’s heading out and won’t come back. This is pretty easy to establish.
I have also read multiple places that it originated in the Oort cloud.
Are we sure about that?
How does a comet of this size go from a circularish orbit in the Oort cloud to hyperbolic path, esp. one that grazes the Sun so close?
There are two ways it’s Oort orbit could have changed: Collision and gravitational interaction. Keep in mind that a hyperbolic path requires more energy than an orbital one.
Collision. Usually results in a loss of energy, especially “soft” objects like comets. ISON might have been knocked off a larger piece by a still faster moving quite large piece. This is going to be tough to do. Especially given that the resulting path is going to be aimed towards the Sun. I.e., whatever gave it it’s current energy had to have a sufficiently elliptical orbit that we’d know about it.
Gravitational interaction. Now, you can get pretty good increases in speed with a grazing path on the right side of a much heavier object. But to end up heading in so close to the Sun, the “right side” is very limited unless the other object is very heavy. I.e., I think large planet-sized is going to be required. And there doesn’t seem do be another Neptune out in the Oort cloud.
Multiple interactions are even trickier. Other good sized objects are very far apart out there. Doing the Oort cloud pinball game without coming into the inner Solar system in the middle of it would be strikingly difficult to imagine.
So, this leaves option 3. (3?) ISON didn’t come from the Oort cloud. It’s an extra-Solar system visitor. But then … such objects are extremely unlikely to have a path that lines up nearly hitting the Sun.
So, what is the Straight Dope on ISON and the Oort cloud?
The Oort cloud is very far out there at more than a light year. As such Alpha Centauri has about 1/9 the gravitational attraction on a Oort object at closest approach as does the sun. That is definitely enough to disturb an orbit. An there could be Oort objects even farther away.
And the chance that any one Oort object might get disturbed and come close to the sun might be small, but there are lots of them.
When you’re as far out as the Oort Cloud, it only takes a very small change in velocity to reach any orbit which intersects your present position. One small nudge will send you into a very long ellipse; a different small nudge will send you into a hyperbola (which might or might not come closer to the Sun before going out).
I am going to throw out the idea of a rogue planet or black dwarf. ISON may not have been an interstellar object, but it is now. And may have been thrown out of orbit by another interstellar object.
[/ wild hypothesising]
Another explanation for an almost dead-on orbit - the object had almost zero initial velocity relative to the sun. As it slowly drifts closer to the sun, it picks up velocity until it’s plunging almost directly into the sun. However, given the time scale - remember you can cover a long distance over a few billion years even if you are only going a few thousand miles an hour.
I guess the thing that is bugging me is the delta-V involved. The change in speed needed to go from an Oort orbit to escape is really small. But the change is direction is quite dramatic. If you look at the Wikipedia article’s diagram of the orbit, err, path, it’s nearly a 90 degree difference from a circular orbit at the Oort cloud range. It had to have picked up most of that change in direction just recently. To get that dramatic of a change in angle for something whose absolute speed had to be pretty hefty to begin with is quite interesting. And to do it all in one go doubly so. Alpha Centauri ain’t going to cause that.
A “typical” comet gets bumped around in the Oort cloud, eventually gets an orbit that brings it in near one of the gas giants and then a real perturbation happens. A lot of visible comets owe their orbits to passing by Jupiter eventually. But that is eventually. After quite a few passes. ISON has such an interesting orbit, err, path, on it’s first go into the main part of the Solar system.
ZenBeam: I’m not entirely sure that’s right… An object beyond the orbiting object (from the point of view of the Sun) could also provide a change in the object’s velocity that led it to fall Sunwards.
Suppose an object is in a perfectly circular (and very slow!) orbit around the Sun at Oort distances. Another object speeds by it, outside of that orbit, but moving retrograde with regard to that orbit, pulling the orbiting object “backwards” just a little, cancelling part of its orbital velocity. Now…it can only fall sunward.
Such a perturbation does not need to come from an object between the orbiting thingy and the Sun…
I think. I’ve done some orbital math. (I’ve gotten through most of Bate, Mueller, and White, Fundamentals of Astrodynamics.)
The Oort Cloud isn’t a monolithic body with all components moving perfectly in sync (nothing is). Slight perturbations, accentuated by nearby bodies, could give just the tiniest shove to an object in a sun-wise direction. Wouldn’t have to be a globular or gaseous, grazing galaxy, just the gentle jostling of other Oort objects.
Remember that time and acceleration vastly multiplies small differences in orbits and trajectories. Given enough time, an object falling towards the Sun at a meter per minute at first could become a rapidly moving object a million years later when Earthlings spot it and say, “Look at that muhfucker go!”
I’ll leave it to the modern math mavens to crunch the crucial calculations.
That’s entirely correct, but then you end up with the comet in a bound orbit.
Comet ISON is in a hyperbolic orbit, so it can’t have close to zero velocity at Oort distances. It had to initially be moving fast enough to escape, but with that velocity mostly towards the Sun, rather than some other direction.
Related questions.
How confident are we that it actually is a hyperbolic orbit?
How confident are we that this body did indeed originate from the Oort cloud (in the recent past)?
It would seem to me that if the answers to both questions is, “we are very confident”, then it is not too much of a stretch to calculate what kind of acceleration would have been applied to the comet to turn it from its near circular and very slow Oort cloud orbit to its current trajectory. That would give an indication as to what kind of mechanism may have caused its trajectory change.
(Not saying that the math would be easy or that there wouldn’t be some assumptions and inaccuracies in the calculations, but it would surely give a ball-park figure – as good as we ever get in astronomy.)
“Close” is relative. “initially moving fast enough to escape” is not close to to zero relative to the velocity needed for a roughly circular orbit. It has to be larger.
But for the scenario Trinopus posited, the comet would have close to zero velocity, small relative even to the small velocity of a circular orbit at that distance.
ETA: I had to look it up, but surely you know off the top of your head that escape velocity is sqrt(2) times the circular orbit velocity.
ZenBeam: You are correct; I was missing part of the assumption.
The bit you cited, escape velocity is sqrt(2) times circular orbital velocity, comes nicely out of the very useful “Vis Viva” equation. In a circular orbit, r = a. In a parabolic orbit, a is infinite.
On the other hand, Chronos is also right: at Oort distances, the two velocities are very similar. If the guy’s circular orbital velocity is 2 m/s and it gets bumped to escape velocity of nearly 3 m/s – well, we might be able to do that with a modern Ariane rocket, if the asteroid isn’t too awfully big!
OK, OK, let’s get some actual numbers in here. Assuming a nice round 1 ly distance for the Oort Cloud, the circular orbit speed is about 120 m/s (which is incidentally higher than I expected). Escape speed is, as ZenBeam says, sqrt(2) times that. For an object to go from a circular orbit to escape speed straight “down” (it wouldn’t actually be straight down, since it doesn’t hit the Sun, but close enough) would require a delta-V with magnitude equal to sqrt(3) times circular orbit speed, or about 200 m/s.
Looking at animation of Voyager flybys of Jupiter may be clouding my thinking.
These probes have their trajectory changed by modest angles (but gain a lot speed). And they are skimming quite close to a really huge object that presumably doesn’t exist in the Oort cloud. It was hard to imagine how to get a much sharper angle from a much smaller object. Especially since ISON and the other object are likely closer in size.
But I think a key might be the relative speed of the two objects involved in the slingshot. The Voyagers et al., were moving fairly fast compared to Jupiter and this made getting a big change in angle hard.
If ISON and whatever it danced with were moving at a very low speed compared to each other, a near miss seems to have the capability of making a greater change in direction.
Given the huge distances involved and the random perturbations the Oort objects go through, having a couple of good sized objects finding themselves in nearly the same orbit and eventually brushing by each other seems doable. (Unlike the “local” part of the Solar system where most objects in similar orbits other than Trojans and satellites got swept up eons ago.)
If ISON gained speed then it’s friend lost speed and would be in a much more elliptical orbit. Perhaps it will eventually get perturbed enough to be seen closer in. I wonder if we’d be able to show that it was related to ISON. A lot of asteroid groups have been found via orbital similarities.
You are (at first) ignoring the component of time. Given a billion years +/- a few measly million, a tiny perturbation can become a large deflection.
It’s a little like compound interest for your bank account. Invest a dollar in the year 1 CE at some rate of interest and see what happens in 2000 years. Or do the exercise of putting a single grain of rice on a chessboard square and double the number on the next square and the next until all squares are accounted for. There’s not enough rice in the world to supply what you need. In short, things can add up faster than you intuitively anticipate.
I don’t get why you posted this. ISON was (relatively) fresh in from the Oort cloud. While it no doubt has had various interactions in the long ago past, those don’t really matter. It’s the recent one that counts. The one that significantly changed it’s path into a death dive towards the Sun.
Just ask yourself: if ISON has always gone thru just tiny interactions, what was it doing just before the last little one, when it was still humming more-or-less around the Oort cloud?
It is extremely unlikely, as I pointed out earlier, that this major change was spread out over 2 or more interactions given the incredible sparsity of the Oort cloud objects.
It is not like a regular comet. Dipping closer in towards to planets over several orbits until it passes by Jupiter and becomes a typical comet.
I mean, the whole point of the OP was the hyperbolic path of the comet. This is a one time thing. It had a one time cause.
If you don’t realize this, then you aren’t getting the point of the question at all.
Just noticed this article: Scientists baffled by new pictures of Comet ISON which seems to say that the comet may have broken apart and evaporated upon its close encounter with the neighborhood star, unless it didn’t. Either it’s still there or it isn’t, or both. Who knows?