Orbit, or course, is a special balance between the
velocity of an object, and the gravitational pull
on it.
Why does so much stuff seem to naturally be in this
state?
Most planets have moons, the planets, asteroids, and
lots of flotsam orbit the sun. Lots of stars seem to
have planets in orbit - all in a state of equilibrium.
Did we really get that lucky, or is there an
explanation?
That should be
“Most planets have moons; the planets, asteroids, and
lots of flotsam orbit the sun.”
Of course there is an explanation. When the solar system was young, there was much more stuff circling the Sun, but over time, everything that was too close/too slow fell into the Sun, or was accreted into the Planets. What was left were those bodies in your state of equilibrium. No luck, just physics.
And someone will no doubt point out that solar orbit really isn’t a state of equilibrium. Eventually, it all ends up in the Sun.
An object in orbit isn’t quite the delicate equilibrium you might think. If some object is in orbit around a planet or sun, and something causes a change in its motion (booster rockets on a spacecraft, or a collision between rocks) it will just change to another orbital path where the forces are in balance. That could be a higher or lower orbit, or a more or less eccentric ellipse. And that object will stay in orbit until it gets enough energy to break free, or it just decays down and becomes part of the planet it was orbiting.
That explains why things stay in orbit, which I hope answers your question. Or were you mostly wondering how so many things got into stable orbits to begin with?
Hmm.
First off, yeah, I know I’m talking about relative
equilibrium, but I thought most things were drifting
away from the sun, and the planets.
I guess I thought that booster rockets had to do their
work carefully. Not only project you further from that
which you were orbiting, but at the right trajectory and
speed so that you would go the new correct speed to be in
a higher orbit.
Also, yes, I am wondering how so much stuff got into
orbit in the first place. When the sun started this
crazy ride, how did the stuff start accelerating at right
angles to the obvious path (the obvious path being the
one where you fall straight into the sun).
ah. The solar system started out as a nebula that collapsed under its own gravity. The collapsing material had an overall rotation to it (stuff going the other way was eventually pulled along with the majority of the material due to collisions & gravity). As more mass collected toward the middle, the rate of rotation increased (just like a figure skater who pulls in his/her arms in order to spin faster). Due to friction/collisions and conservation of angular momentum the spinning mass flattened out into a spinning disk. Most of the mass was in the center (which became the sun) and the stuff in the outer part of the disk built up into the planets, asteriods, comets, etc. (via gravity, collisions). There used to be a lot more proto-planets, but they collided with other planets or were ejected from the solar system. Our 9 planets are the lucky survivors. It is thought that our moon was formed when a Mars-sized planet crashed into the early Earth. An Earth sized planet probably whacked Uranus onto its side (its rotational axis is about parallel with the plane of the solar system instead of perpendicular to it).
I don’t think this is necessarily so.
Getting something in exactly the orbit you want is, in fact, a difficult task and the booster rockets do have to work very carefully. Getting it into any old orbit is easier, but you still have to have the right combination of altitude and horizontal velocity to achieve orbit. If you miss by too much the payload either falls back down or escapes entirely.
Although they are usually perceived as a brute force mechanisms, booster rockets are really high-precision instruments. They just happen to be very large and noisy.
A while ago, I was testing a theory that I had, and I made an interesting computer program, by modifying one I had made before that was a simple 3D engine that drew spheres.
The way it worked is, at the beginning of the program, a bunch of multicolored spheres were spawned from a single point going in random directions (a big bang simulation). I made it so that the spheres were all attracted to each other by gravity, and the force was equal to the inverse of the square of the distance, which was done via the pythagorean theorem (I’m sure there’s other equations involed in the real gravity math, but this is accurate enough for me.)
So I ran the program, and what happened? Well, as I kind of expected, the spheres started orbiting each other! I also later modified the program so that there were bigger spheres and smaller spheres, and the smaller spheres started orbiting around the bigger ones! It was an amazing sight.
The orbits weren’t exactly circular, but I’m sure had I ran the program for a million years, they would’ve formed a generally stable system.
The way I think about it, there are 3 basic possibilities:
Of course, once 2. or 3. happens, we never see Rock again. After 4.5 billion years, we’re left with only 1., and the occasional comet that breaks up when it finally goes to close the Sun too many times.
The reason why so much stuff is left in orbit is pretty much what Phobos said. I’d add that if you take two masses and let them go at each other, unless they’re going straight at each other to begin with, they’ll start to revolve around each other.
You can try it out with magnetic marbles - roll them at any angle toward each other, but not quite close enough for a collision. When they get within reach of each other’s magnetism, the paths will bend in, and they’ll “orbit” each other for a second before they hit. So except for a few cases where the masses are headed straight in, or the masses are huge related to the velocities, there will be some revolution before impact. Orbits are just a really long term case of this.
The orbits of the planets and moons of the solar system are not perfectly circular either (they’re slightly elliptical). Elliptical orbits can be stable too.
Getting things in an elliptical orbit isn’t too tough: You just have to have a low enough energy that you don’t escape entirely, and you can’t be aimed directly at anything else. The radius of a planet’s orbit is much larger than the radius of the Sun, so this is actually pretty easy. OK, so now you’ve got a bunch of stuff in highly eccentric orbits. Why are all the planets’ orbits so close to circular? Because friction with the interplanetarty medium (not quite a perfect vacuum), relativistic effects, tidal effects, and pretty much anything else that will tend to decay an orbit, will do so mostly by bringing the far end of the orbit closer in, making it more circular.
KJ, your program was pretty much dead-on, as long as your masses were all the same. You’re ignoring GR and frictional effects, but were I in your shoes, I would ignore them, too… they’re usually negligable.
not all orbits are stable, the moon is getting further away and will eventually get flung off. phobos? one of mars’ moons will eventually crash into mars. Is there such a thing as a truly stable orbit, if so how tight a tolerance does this have?
and a hijack here - is the 2 so called moons orbiting mars really moons or are they just meterors. I am guessing that sinced mars is comparatively close to us, the ‘moons’ were found and called moons before we really knew what a moon is.
Phobos and Deimos are odd spud-shaped things, and probably once were asteroids, but they’re big enough and orbit Mars so we call them moons. Just like with the whole “Pluto isn’t really a planet” sillyness, it’s not worth worrying too much about the definitions. Pluto isn’t any more different from Earth than Jupiter is. Similarly, there’s plenty of variety among the moons in the solar system. Former asteroids like Phobos apply just as easily as Io, Europa or Titan.
[nitpick]
Meteors are the streaks of light we see in the sky when dust particles burn up in the atmosphere. Asteroids are the rocks that mostly orbit between Mars and Jupiter…
[/nitpick]
BTW if you’ve got a Mac and want to play around with model gravitational systems, try “Gravitation Ltd 5.0” which you can find here. Lots of fun.
[Meteors are the streaks of light we see in the sky when dust particles burn up in the atmosphere. Asteroids are the rocks that mostly orbit between Mars and Jupiter…
[/nitpick] ]
I thought that was a meteorite
[BTW if you’ve got a Mac and want to play around with model gravitational systems, try “Gravitation Ltd 5.0” which you can find here. Lots of fun.]
no but I will probally look for a pc equivalant
Meteorites are meteors that didn’t burn up completely, and made it to the Earth’s surface.
And to complete the terminology, before it hits the atmosphere and starts ablating, it’s a meteoroid.
The universe has been hanging around for a long time. Billions of years. (4.5 to 15 of those billions, take your choice) With stuff in space it has had time enough to either:
A: Hit something
B: Leave the neighborhood
Or fall into an orbit, and get tugged around by every thing else that didn’t either a, or b for a couple of billion years.
Orbits tend to “regularize” over time, given a fairly stable system of objects. A lot of math explains why, and I can only follow it, not teach it.
On a non local basis, though lots of stuff is still milling around bumping into itself in the cosmos. The local predominance of orbital motions is just that, local.
Tris
hey, I’m not that odd! :mad: