Why don’t they fly off into space? Why don’t they fall into the sun?
Because the force that would cause them to do the first of these is counterbalanced by the force that would cause them to do the second, and so they’re in equilibrium between the two.
ETA: And this is stable. If they drop slightly towards the Sun, then they speed up - which increases the force trying to make them fly off in a straight line. Also the other way around. You get a (slightly) elliptical orbit as a result - for instance, the Earth’s distance from the Sun varies between about 91 million and 94 million miles.
The illustrations in Newton’s old book illustrates the princible well: Explainer: How do satellites orbit the Earth?
Anything in an orbit around something else is constantly “falling towards”* that something else, and constantly missing.
*sometimes the distance increases, but the acceleration is always towards the object one is orbiting.
As the folks above said, they’re in a delicate but stable balance between distance and speed which keeps them there.
The “why” part of that gets a little deeper. Gravity exists and pulls the planets towards the Sun (and vice versa). We’re all pretty familiar with gravity from our day to day life. It’s always there and it always works consistently.
And meanwhile, the vacuum of space provides very close to zero drag against the planet’s motion. This 99.99999999999% frictionless motion is something we never experience in our daily lives and seems kind of counterintuitive at first. Every moving thing we encounter day to day quickly grinds to a halt unless being actively powered against friction. That is NOT the reality out in the vacuum of space.
Given the existence of these two facts, one intuitive and the other not …
Once there is some planet moving in some direction with some speed near some star, it will do one of three things: 1) Fly off into space fairly soon. 2) Fall into the star fairly soon. 3) Become trapped in a stable orbit around the star for the long term.
What we see in our solar system today are the survivors after the other bodies whose motions weren’t stable enough already did a 1) or a 2). Once in awhile a chunk comes sailing in from the far reaches of the solar system and does a 1).
Simply … gravity is like a string between the two objects …
Think of it this way; when you throw a baseball, it falls to the ground because it doesn’t have enough energy to keep going above the horizon. However, if you could throw it fast enough (and there were no atmosphere to cause drag) it could go well above the horizon in a high or flat elliptical arc. If you add enough energy and point it in the right direction, the entire arc will be above the horizon at all points, making a complete ellipse around the planet. (Since you’re throwing this at an altitude of 5 ft above ground level this is really hard, but we’ll address that in a second.) At this point, the ball is going so fast that even when it falls (is pulled by gravity) it falls above the horizon. And that’s what keeps objects from falling into the sun; they have so much kinetic energy and a momentum vector pointing tangent to its path at any point that it can’t “fall into the Sun”, and in fact it is very, very difficult to send even small objects into a solar-intercept trajectory as they have to shed all of that velocity.
As for throwing objects into orbit from the surface without another impulse to circularize the orbit, you can’t because your trajectory will want to reintersect the point from which you launched it (in a non-rotating coordinate frame). Unless this trajectory is very flat and very fast, that means the ellipse will intersect the planet. Hence, why you can’t just launch satellites or other items without propulsive capability from a ground cannon or railgun or whatever into orbit even if such a technology were workable to begin with.
Stranger
One thing to note is that one planet around one sun is a stable situation. But two or eight or nine and stability is not guaranteed. Especially when one or two of them are much larger than the others. They perturb each other and a giant planet will perturb the others more. I once read about a computer built just to study orbits and the outcome was that the 8 inner planets seem to be stable for the next 10 years, but Pluto was not. Another reason to make Pluto a non-planet. And we will (literally) be toast in 5 billion years anyway.
I know this is the basic concept that that is taught in physical science class, but it is so very wrong it really should never be repeated any more than “there is no gravity in space”. Gravity isn’t a “thing”; it is a tensor property of the metric of the underlying fabric of space-time. Mass causes this tensor to be curved, and this curvature causes the ostensibly “straight” lines (geodesics) along which an object with a fixed total energy with respect to the central mass would travel to be curved to an observer who is well away from the mass. That one object orbits another (or more precisely, two objects orbit each other about their common center of mass) isn’t because there is some hidden connection between them; it is because the presence of mass causes this curvature. This is really quite a simple concept and easily demonstrated by reducing four dimensions to three (e.g. the bowling ball on a bed sheet analogy), and there is no reason completely obscure the essential concept by inserting a mysterious “string” connecting the two objects together.
Stranger
While Jupiter (and to a much lesser extent Saturn) does perturb the orbits of the inner planets to a small but measurable degree, this doesn’t make them unstable, and in fact the orbital perturbances are fairly predictable over a span of time many orders of magnitude greater than a decade. The JPL HORIZONS Online System provides solar system ephemeris data for planets for epochs going out for tens of centuries with a high degree of precision. Smaller bodies are harder to predict, especially in areas like the asteroid belt or the minor moons within the Jovian and Saturnian systems in which the orbital resonances of larger bodies may create complex gravimetric conditions for smaller ones in a quasi-chaotic fashion.
Pluto was partially downgraded to a minor planet in part because of the potential lack of long term stability of its orbit, but it is also smaller than some other bodies and because we now know it to be part of a larger population known as Edgeworth-Kuiper Belt Objects. At the time it was originally found it was thought to have much greater mass, which explained some of the odd observed motions of Uranus and Neptune, but later measurements showed it to be not nearly massive enough to cause these observations, and more detailed models of planetary motion reduced the anomalous motion of the outer planets to within observational tolerances.
Stranger
The real problem with describing gravity as being like a string is that the force law is completely different. If you have two objects connected by a string, and you bring them closer together, then the force will nearly vanish, while if you pull them further apart (which will be very difficult), the force will get much stronger. With gravity, meanwhile, the closer you get the bigger the force, and it changes in a nice smooth, continuous way.
Have a bit of nit to pick with your ETA, as it’s not really an explanation for a stable orbit. Any unperturbed orbit is intrinsically stable. The reason a small body in orbit around a large one reaches maximum velocity at its lowest (closest) approach and minimum velocity at its furthest is that it’s just exchanging potential and kinetic energies – less of one means more of the other because its specific orbital energy is constant. I don’t see this as really having anything to do with stability. A working definition of orbital stability, excluding special situations like Lagrange points, might be the range of orbital energy that the body can tolerate relative to external perturbations before its closest approach causes it to either fall into the parent body or accelerate beyond escape velocity.
Okay, then simply, the planet moves in a straight line, it is space itself that is curved.
The OP wanted the simplified answer.
They will fly off into space - the fate of most of the orbiting bodies is to be lost … The moon keeps moving farther and farther from earth… enlarging its average radius of orbit.
They are temporarily steered around the central body (star, planet) by gravity.
The orbit is not stable. The minor perturbations kept making the orbit more elliptical or to increase the average radius of the orbit - make it larger to the point of not existing.
But there has to be a source of such perturbations that add or deplete the orbiting object’s specific orbital energy, as I said earlier. That isn’t intrinsic in orbital mechanics. The only reason the moon is moving away from the earth is that its orbital energy is increasing due to the tidal effects on earth stealing the earth’s rotational energy and imparting some of it to the moon’s orbital energy, raising its orbit.
And further, there’s not an infinite supply of that tidal energy. The Earth will not push the Moon away to the point that the Moon escapes. The Moon’s orbit IS stable in the loose sense that nothing in the Sun / Earth / Moon system is going to drive the Moon out of orbit. It isn’t stable in the sense of “never changing any parameter even a smidgen.” But no orbits are.
Bees.
Great…
big…
Bees.
Across the universe, some planets will have done both these things. By definition the ones we see now are the survivors, those for whom the inward force (gravity) and outward force (“centripetal force”) balance, or very nearly do.
Oh? And what keeps *them *from flying off? Huh? Huh?
this was a good video I came across recently. Note that the one-directional spin is an outcome that comes after a long period of collisions, just like with things that are too fast or too slow for stable orbit
They are constantly falling into the Sun, but they keep missing and falling past it.
Here’s a simple answer.
Any rock (or big clump of rocks) whizzing through space near the sun has three possibilities:
(1) fall into the sun
(2) fly off into space
(3) loop around the sun over and over
All the ones who were in category (1) are now inside the sun, so you can’t see them.
All the ones who were in category (2) are now gone away, so you can’t see them.
So all the ones you can see are in category (3). But there’s no guarantee they’ll stay that way. In fact, it’s highly likely that sooner or later everything will shift to either (1) or (2) but it may take billions of years before that happens.