Orbits

[JacobEvans]

Simple question.

If orbits are cause by objects "falling" towards another object due to gravitational forces at a velocity that allows it to fall almost indefinitely, why don't objects just fall straight into the object they orbit?

Kinda dumb question, but I was asked this, and actually found I couldn't answer this on the knowledge I had, and was forced to admit I didn't know.

 

[darthrender2010]

most do, which is actually why we have objects larger then a grain of sand. The reason planets and other solar satellites don't "fall into" the sun is because they were formed from a cloud of dust circling the star as the star itself was "born". After this planetoids also began forming from particles bumping and attaching onto each other through processes, including gravity.

Edit: for lunar satellites such as the earth's moon the scientific consensus (if I remember correctly) is that it was another body that collided with the earth at a trajectory that allowed it to spin around the planet, collecting the particles from the collision that didn't fall down to earth while orbiting it. Because of this the moon is very slowly escaping earth's orbit. Most of Jupiter's moon's I'm guessing happened similar to the planets and other satellites, though I'm not sure, while the martian moons are captured asteroids that came in at a trajectory that allowed them to be caught in the orbit of mars (I would assume this means they have degrading orbits, but I haven't checked this).

A VERY basic description of the answer, but it seems that's what you need. Also, this may not actually be correct, but I think that's what I remember from my entry level astronomy university class which I took a few years ago as well as youtube videos by Andromeda.

 

[JacobEvans]

Well why was the disk that formed the planets rotating to begin with?

 

[Netheralian]

I think you are ignoring inertia. If you had two stationary bodies that were instantly gravitationally bound to each other then they would fall directly towards each other. However as nothing in the universe is stationary, then the relative motion between the two bodies (which initially most likely won't be in the same direction as the gravitational vector) needs to be taken into account as they become gravitationally bound.

 

[darthrender2010]

Protostars typically form from molecular clouds consisting primarily of molecular hydrogen. When a portion of a molecular cloud reaches a critical size, mass, or density, it begins to collapse under its own gravity. As this collapsing cloud, called a solar nebula, becomes denser, random gas motions originally present in the cloud average out in favor of the direction of the nebula's net angular momentum. Conservation of angular momentum causes the rotation to increase as the nebula becomes smaller. This rotation causes the cloud to flatten out,much like forming a flat pizza out of dough,and take the form of a disk. The initial collapse takes about 100,000 years.



The nebular hypothesis of solar system formation describes how protoplanetary disks are thought to evolve into planetary systems. Electrostatic and gravitational interactions may cause the dust and ice grains in the disk to accrete into planetesimals. This process competes against the stellar wind, which drives the gas out of the system, and accretion, which pulls material into the central T Tauri star.

there ya go

 

[JacobEvans]

thanks guys! I hate how I still get stumped by the stupidest things!

 

[Zylstra]

Why do orbits tend to be elliptical and why do the objects not get pulled closer and closer to the sun every time they reach the maximum distance from the object they orbit and get pulled back towards it? Shouldn't toe orbit become more and more elliptical until the object gets pulled in?

 

[Netheralian]

Why do orbits tend to be elliptical and why do the objects not get pulled closer and closer to the sun every time they reach the maximum distance from the object they orbit and get pulled back towards it? Shouldn't toe orbit become more and more elliptical until the object gets pulled in?

It comes back to initial velocities again - achieving a circular orbit is unlikely because your initial inertia is never going to be exactly in the right direction such that a circular orbit results. And any perturbations would then also put it out of circular even if it managed to be in such an orbit in the first place.

As a body reaches apoapsis (the furthest point) it is going slower, but the angular momentum is still conserved (because it is futher out). So it does get pulled back to the gravitational body at which point (at periapsis) the body has a maximum velocity (but still the same angular momentum). The only way you can get the orbit to become more elliptical is by reducing its angular momentum through something like drag.

Sorry if that is really vague...