# Question Regarding the Orbit of Comets

## Main Question or Discussion Point

Recently, I attended a lecture at my university entitled "The Life and Death of a Star-Grazing Comet" presented by John Raymond, an astrophysicist at Harvard Smithsonian. In this lecture, he talked about Comet Lovejoy and how it allowed us to help study the corona of the sun. As the comet was so big, it survived going through the corona of the sun and allowed us to understand it (since we saw it come out the other side before it vaporized about a day later).

However, when he showed a picture of the orbit of the Kreutz family of comets (attached) depicting an extremely elongated orbit as it approaches our sun. From what I have always believed, the earth is gravitationally "locked" to the sun; that is it is orbiting the sun and the sun is the major body influencing the Earth's motion (keeping it going around the sun and not another body). I was wondering how a comet can get such an elongated orbit and still be gravitationally locked to the sun in the same way, that is we can predict when it will return because it has a very specific orbital period.

If something is going so far out in the solar system, why does it not get "locked" to another body that it gets closer to on the way to and from the sun? If the orbit is so elongated, why doesn't it fall into orbit of another large body somewhere else in the solar system? Is it because of where they come from, as in nothing out where they come from is massive enough to hold a comet in their orbit?

Edit: in the picture, as I'm sure you guessed, the thing, very elongated lines coming from the bottom of the page are the orbital paths of the Kreutz family of sun-grazing comets.

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tiny-tim
Homework Helper
Hi Yosty22!
If something is going so far out in the solar system, why does it not get "locked" to another body that it gets closer to on the way to and from the sun? If the orbit is so elongated, why doesn't it fall into orbit of another large body somewhere else in the solar system? Is it because of where they come from, as in nothing out where they come from is massive enough to hold a comet in their orbit?
It will get "locked" if it gets close enough to a planet.

(lots of comets are caught, or at least have their orbits altered, by Jupiter)

But there aren't any planets out there, away from the plane of the ecliptic!

Bandersnatch
The Sun is so much more massive than the rest of the bodies in the solar system that it's gravitational influence dominates almost everywhere.

For a body to be captured by some other body's gravity(or "locked", as you say), it has to pass close enough to it that the gravitational field of the small body becomes stronger than that from the Sun. But since the Sun is so massive, that sphere of influence(called Hill Sphere - http://en.wikipedia.org/wiki/Hill_sphere) is very small.

You can get a feel for the distances involved by analysing the gravitational field equation:

$g=G\frac{M}{R^2}$

where g is basically the acceleration experienced by any body at distance R from the mass M, towards that body.

So, the gravitational field of e.g. Jupiter, which is ~1/1000th as massive as the Sun, would become stronger than our star's only if the distance to Jupiter is ~31 times closer than to the Sun.

Thanks for the reference on the hill sphere, its a term I hadn't heard before not even in my astronomy text books Grrrrr. Not too surprised on the text book thing though lol astronomy is a broad science.