Why Do Comets Have Elliptical Orbits While Planets Are Circular?

[madphysics]

If the planets are forced into circular orbits by inertia and gravity, why are the orbits of comets oval shaped? What keeps them from entering the sun.

 

[russ_watters]

Planets are not forced into circular orbits by gravity. Eliptical orbits are perfectly stable on their own. The chaos of the early solar system is likely why the planets settled into nearly circular orbits (.017 is still a noteworthy eccentricity). Objects in very eliptical orbits would have crossed paths with each other and led to collisions. There is a limit to how circular they can be, though, due to interactions between the planets.

What keeps the comets from hitting the sun is the same thing that keeps any object in any orbit from hitting what it is orbiting: it has a tangential component of velocity, so when it falls toward the object it is orbiting, it always misses.

 

[madphysics]

Why do they miss? And even if they do, what keeps them from orbiting closer and closert till they hit the orbital point. Does it work something like a slingshot? The gravity pullsit in, but when it misses, the inertia from the gravity keeps it moving far beyond the orbital point.

 

[Sojourner01]

Why do they miss? And even if they do, what keeps them from orbiting closer and closert till they hit the orbital point

Conservation of energy and of momentum. This is simply not the way objects behave.

Consider it this way - orbits are periodic; they repeat themselves indefinitely assuming the total orbital energy is conserved. If the orbit could ever result in collision with the sun, it would have already done so. Comets don't quite follow this, because their orbits are influenced by the planets and at any given orbit, the planets are highly unlikely to be in the exact same configuration that they were before - but as far as the sun is concerned, comets that haven't already crashed into the sun are unlikely to do so.

 

[russ_watters]

Why do they miss?

Why do you miss when you throw something at something? You miss if the velocity vector is not directly toward the target. Even with gravity, you will always miss if the velocity vector is not directly toward the target, when considering the target a point mass. When the target isn't a point mass, you have to calculate the closest point of approach and see if that is inside the radius of the target.

In other words, when the space shuttle does a de-orbit burn to re-enter the atmosphere, astronautics types like to say that the shuttle re-shapes it's orbit so the perigee is below the surface of the earth. But it is still technically an eliptical orbit.

And even if they do, what keeps them from orbiting closer and closert till they hit the orbital point.

Conservation of momentum. An eliptical orbit is not a spiral, it is an ellipse.

Does it work something like a slingshot? The gravity pullsit in, but when it misses, the inertia from the gravity keeps it moving far beyond the orbital point.

Yes, that's basically it. As it falls toward the planet, it speeds up, so it is tougher for the planet to alter the trajectory further. It's momentum carries it away in a symmetrical path to it's approach.

 

[madphysics]

The theory involving binary star systems is that they circle each other until the collide. The same is the theory for galaxies, like the Andromeda and Milky Way galaxy. So what's the foundation for the belief that they won't hit each other?

 

[Sojourner01]

Where did you get the idea that binary stars must necessarily collide?

Objects in orbit around a common centre of mass will gradually lose angular momentum, but I can't see this being significant in most cases.

I shall turn the question on its head: How do you propose to disrupt a stable orbit in order to shift an entire star directly into the path of the other?

 

[madphysics]

Gravity

Although that doesn't make much sense, as no new source of gravity would suddenly be introduced.

However, what of the galaxies? They move through the universe by the gravity of other galaxies, so it stands to reason that they are almost always being effected by new ones, but is it possible to have a stable satelite of a smaller galaxy orbitinga larger one?

 

[Sojourner01]

That's a difficult one, galaxies by nature aren't even remotely close to being solid, since they're very diffuse and stars can move about within them. There isn't a perfect analogue to how galaxies behave under gravitational attraction to one another. I suppose you could imagine them as pouring like a liquid into one another under influence of their own gravitational fields. To be honest though I don't know a great deal about galactic physics.

 

[russ_watters]

The theory involving binary star systems is that they circle each other until the collide. The same is the theory for galaxies, like the Andromeda and Milky Way galaxy. So what's the foundation for the belief that they won't hit each other?

You have it backwards - there needs to be extra effects in there like tidal friction to cause objects orbiting each other to spiral in or out. Conservation of energy/momentum prohibits it otherwise. An orbit of two point masses with no other perturbating effects is completely stable.