Orbits in question

I performed a search and could not find a similar question so I pose it here for the teeming millions. Just how does an object stay in orbit, wether its a tiny man-made shuttle or our own planet earth? Would at some point our earth break away from the sun or crash into it? It seems silly, but if its the sun’s gravity keeping us in orbit, why doesn’t it just eventually suck us in, within a a shorter time span than a few billion years? (same goes for sattelites and ships around earth) It sort of reminds me of a roulette wheel, does our speed keep us a constant distance away? I ask because for one I do not understand how this works, and I did read in one of Cecils replies that scientists think the moon was closer millions of years ago, and that Russian sattelite that crashed to earth. Since these examples are opposite, how would you know if an object in orbit was to break away or fall?

One way I’ve heard orbits described is, the satellite is “falling around the sun”. Let me give you a two-dimensional example, treating space as if it were a map of a familiar area. Say the sun is in Kansas City, and the earth is in Dallas. The sun is pulling the earth due north, right? The earth retains its westward momentum though, assuming it’s been previously orbiting clockwize. So by the time the sun has pulled the earth the whole way home (ca. 500 miles, I think), the earth has already moved about 500 miles to the west. So instead of falling into Kansas City, the earth is now happily located in Denver or something, on a northwards trajectory (having been accelerated to that trajectory by the sun’s gravity). Repeat as necessary to take the earth through Minnesota, West Virginia, etc.

All bets are off if the earth hasn’t been orbiting previously. If you were just sitting in Dallas, you’d be pulled into the firey inferno of Kansas City. However, in space there is very little to slow the earth down - no atmosphere, no stoplights, just a little bit of space dust which doesn’t do a whole lot.

Mess with the earth’s cruising speed, or the sun’s mass, and the orbit will be distorted and acquire an eccentricity. The sun will quit being the center of the earth’s circular orbit, and start being one of two focal points in the earth’s elliptical orbit. Increase the earth’s road speed enough, or make the sun much smaller and less attractive, and the earth won’t come back at all; instead, it’ll just sail past the sun with a slightly curved (hyperbolic) trajectory.

Your point in the OP about the earth falling into the sun is actually inevitable, considering that pesky space dust I’ve mentioned previously. The sun isn’t getting any smaller (at least in the long haul; it’ll eventually get much bigger), but the earth is slowing down somewhat. Earth’s orbit is slowly spiralling inward, and follows a course into the surface of the sun if extrapolated far enough (I think the sun, as a young red giant, would swallow us up long before then, so don’t worry).

Any similarity in the above text to an English word or phrase is purely coincidental.

If you you didn’t like my metaphor, then go spin a yoyo around your head.

Seriously though. The yoyo stays at a constant distance from you. You may not conceive of your actions as accelerating the yoyo, but in fact you do have to keep a steady tension on the thread, just as you would if your were dragging the yoyo across a high-friction surface, like a carpet. (Dragging it across the ice doesn’t count, since with one sharp tug the yoyo would start sliding toward you on its own momentum.)

So a steady pull on the yoyo’s string works a lot like gravity. If the yoyo isn’t spinning around you, or glued to the far wall, it will eventually be reeled all the way in and burn up in a tremendous atomic fire. Wait a minute, that last bit doesn’t belong in this metaphor. Anyway, by giving a little initial perpendicular momentum to the yoyo, you prevent from ever reaching, since it will simply run off at right angles to the position you are pulling, by the time it should have reached your hand.

If your string is inelastic, and you speed up the perpendicular momentum, you’ll feel greater tension on the string, just as the sun would have to have more gravity to keep earth in a circular orbit if earth were to speed up. You can simulate elliptical orbits with an elastic cord, but by now you’ve probably smashed all your fine china, so I’ll stop.

Its all about speed, baby.

The gravitational field of the sun, the earth, any body in space theoretically extends to infinity. However, that gravitational field decreases so rapidly with distance that only a finite amount of energy is required to overcome the effects of gravity and allow an object to coat to an infinite distance without “falling back”. This phenomena is what is known as an “orbit”.

The speed needed to acheive this phemonena is called *escape speed[/]. A space ship, satellite, cannonball, anything that can achieve escape speed in any direction will travel on a parabolic escape trajectory. This is regardless of the direction and the distance from the gravitational body, mind you. You can fire a cannonball horizontally, and if it exceeds its escape speed at that altitude, it would achieve orbit.

Let me elaborate. Theoretically, as an obbject’s distance from the central body approaches infinity, its nessecary escape speed approaches zero. Basically, the farther away you are from a gravitational body the less speed u need to attain an orbit around it. Easy enough? Ok lets move on.

You can calculate this escape speed by writing an energy equation for two points along the escape trajectory (horizontal, vertical, or anything in-between). The first point you take along a distance, r from the center where the local escape speed is Vesc, and then at infinity where the speed will be zero. I wont bother with the actual equation here, but what happens is that the farther away you are from the central body you are trying to escape from, the less speed it takes to escape its gravitational field. For example, Escape speed from the surface of the earth is about 36,700 ft/sec whereas if u launched a rocket from a point 3,400 miles above the earth, the escape speed will only be about 26,000 feet per second.

There, if you followed that you are well on your way to being a rocket scientist :slight_smile:
“Wow… that degree in Aerospace Engineering is fanally doing me some good!”

It is called an equilibrium.

Given two objects in space, each has a vector of motion of it’s own, and each has a gravitational force acting on it, from the other one. The first vector and the vector of change in motion caused by the gravitational force combine, and the motion of both objects change. The smaller object will have its motion changed more, and if it is very much smaller, the change will be much larger.

Since the facts of motion, and attraction remain, in every position, each instant of time has a separate resultant for the resolution of the forces. This constantly changing but constantly balanced relationship, or equilibrium, is called an orbit. The changing and original motion will cause the two objects to travel in an elliptical path, with the center of gravity of both objects at one of the foci of the ellipse.

The more complicated cases of real objects in space include dynamic change by other objects, including dust and gasses, which attract, collide, and interact in a very complex manner. Generally it is possible to approximate the orbit of objects by considering only the largest near objects, or the nearest large objects. Sometimes when you do that, your Polar Lander turns into a Polar Crashlander.


POLITICIAN, n. - An eel in the fundamental mud upon which the superstructure of organized society is reared. When he wriggles he mistakes the agitation of his tail for the trembling of the edifice. As compared with the statesman, he suffers the disadvantage of being alive.
Ambrose Bierce (1842-1914?)

In other words, you’re moving forward and you’re falling down simultaneously. If you’re going forward at just the right velocity, you’ll fall past the curve of the object you’re orbiting. If you’re going too slowly, you’ll eventually hit the surface at the end of a curving path. If you’re going too fast, you’ll fly off into space.

This works, of course, only if the object you’re orbiting is spherical. If the object is irregularly-shaped, like an asteroid or one of the moons of Mars, you have to constantly correct your orbit. Or orbit at a very high altitude around the object’s center of mass.

Did you know NASA has photographed an asteroid that looks like a peanut? Try orbiting that thing!

Fighting my own ignorance since 1957.

Much obliged for the info guys… it must have been some incredible amount of precision for our earth to somehow get into an orbit around the sun and stay there for millions of years, much less 8 other planets and I dont know how many moons. Just one thing though, if I remember right, waaaay back in high school astronomy, the planets are in an eliptical orbit, not circular. Is this correct? On that note, would that be the cause of our eventual collision with the sun? And is it not the cause of our seasons? I know… more questons… but I gotta ask those that know!

If you’re somewhere in the solar system and hurl a rock, it will either 1)fly out of the solar system, 2)fall into the sun, or 3)go into an elliptical orbit. If the rock initially has no kinetic energy (stationary relative to the sun) or is heading straight for the sun, it will fall. If it has some initial velocity ‘sideways’ (not straight towards or away from the sun), it will go into an elliptical orbit - the more energy, the larger the orbit. If you give it even more energy, it will fly away from the solar system. Note that there is a HUGE range of energies where the rock ends up in orbit.

Another way to think about it is that orbits are stable. “Stable” in physics means it takes energy to raise OR lower the orbit. If you were to suddenly stop the earth - i.e. kill its orbital speed - it will fall into the sun. But do you have any idea how much energy it takes to stop 6x10^21 tons of mass moving at 30 kilometers per second? Something like the total energy outut of the Sun over several months. If you almost but not completely kill the orbital speed, the Earth will enter a very elliptical orbit, bringing you very close to the surface of the Sun at the ‘low’ point of the orbit and about our current distance at the ‘high’ point of the orbit. Similarly, it takes a huge amount of energy to escape from the Sun’s gravity field.

Yes the Earth’s orbit is eliptical, but not that eliptical. If you were to draw it to scale on an 8x10 sheet of paper it would appear perfectly circular to your eye. The seasons are caused by the Earths slight tilt on it’s vertical axis.

It’s just gravity and momentum doin’ that thing they do so well! Sure looks nice, though. I wouldn’t be too in awe of some of those moons, though; some of them ended up as rings around our bigger planets.

Nah, that’s gonna be caused by the sun exploding. In a couple billion years.

Actually, if you have a small moving object and a large one reasonably near it and nothing else significantly interfering, you’ll almost always get either an orbit or a one-time pass-by. Actually crashing into the center is quite difficult. (Essentially, you have to have an orbit that is so lopsidedly eccentric that part of it is inside the orbited body.)

The only reason that artificial satellites eventually fall is that they’re so close to the Earth that they’re still inside the outer atmosphere – they eventually run out of energy due to friction. The mathematics far and away prefers orbits.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

Somebody said orbiting around a peanut would be difficult. Not true (well, assuming were talking about one honking big peanut). The peanut still has mass, and a center of gravity, and you can orbit around the center of gravity no matter what shape the object is (assuming you dont run into a very tall mountain or some such).

Well, it’s true that any sort of oddly shaped object has a center of mass, but that doesn’t imply that its gravitational field is smooth. It’s quite possible for orbits around objects with “lumpy” gravitational fields to be unstable over short timespans. (Of course, the farther you get from the object, the less pronounced this effect is and the better you can approximate the object as a point mass).

This was a big deal for Clementine - it was in a low orbit around the moon, and had to be corrected on a (roughly) biweekly basis to prevent biffing into the green cheese :-). IIRC, they were seeing weekly altitude differences on the order of 5 to 8 km due to the moon’s “lumpy” density distribution, and that’s quite a lot if you’re only 25 km up.

peas on earth

      • If I remember rightly, the asteroid that was found to have a smaller asteroid orbiting it, is kinda peanut-shaped. Me thinks, Ida and Dactyl, maybe? - MC

Try a few links:

First link is to the home page of some of NASA’s photos of all the planets, asteroids and comets.

Second link gets you a list of all available photos of asteroids, comets, the Shoemaker-Levy collision with Jupiter and of meteorites.

Third link is to a photo of Dactyl & Ida. It ain’t peanut-shaped, but neither is it spherical. More like an Idaho potato. (Large photo, too. Should take up your whole screen.)



The Earth didn’t ‘somehow get’ into orbit around the Sun. It was orbiting long before it was a planet. Our solar system started out as a huge spinning disk of debris. As the eons ticked by, the larger chunks started attracting debris to themselves. This process of ‘snowballing’ continued, until the ‘lumps’ were large enough that the sheer force of their gravity forced them into spheres.

Our Moon is an example of the same process. At some point an enormous impact ejected a massive field of debris into orbit around the Earth. The largest chunk of that debris gradually accumulated enough of the rest that it formed a sphere which we call “Moon”.

Saturn’s rings are a ‘failed’ example of this process. In that case, the debris is homogeneous enough that it fails to ‘clump’ enough to form.

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Stephen: Saturn’s rings are a ‘failed’ example of this process. In that case, the debris is homogeneous enough that it fails to ‘clump’ enough to form.

Actually, it’s not their homogeneity that keeps Saturn’s rings from forming a moon. It’s that they are too close to the planet. There’s a given distance for each planet where debris either accumulates and makes a moon, or keeps getting gravitationally stressed to keep from forming large masses.

I did state that incorrectly. I guess my point is that the rings of Saturn are the type of debris field that, except for the proximity and gravitational forces (which sustain homogeneity) would tend to form planets/moons/etc. Of course, if the circumstances were different, I wouldn’t have the example to point to.

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