**Ask a monkey a physics question thread**

[CoccoBill]

An orbit is moving forward fast enough that you miss as you fall toward the Earth. If you're orbiting the earth at an altitude you would consider in space, you're in free fall like a skydiver; but you've got so much forward velocity and no air to hold you back that the Earth moves out of the way below you.

You fall to the Earth and perpetually miss. You just move forward so fast, the Earth seemingly moves out of the way.

That's all quite intuitive. What isn't however, is that wouldn't even a slight change in velocity, even the tiniest bit of acceleration or deceleration cause the planet to either get loose or crash? How come this doesn't happen? What creates this universal (ha!) tendency for all objects to just play nice and circle each other at exactly the right speed?

Well, it's a logical fallacy. The ones with the incorrect speed have indeed already left the orbits or will do so later. None of the orbits are stable, for example the moon is moving away from earth at 3.8cm/year, and will eventually break loose. More about it here: Curious About Astronomy: Is the Moon moving away from the Earth? When was this discovered?. Ok, some orbitses(? orbii? wtf) are quite stable, such as earth's orbit around the sun. There are tiny oscillations in the elliptical orbit, but none of them suggest we're moving neither towards nor away from the sun. There are, however, a couple other forces at play. One is the same tidal interaction as between the earth and the moon, just quite a bit weaker. It turns out that the yearly increase in the distance between the earth and the sun from this effect is about one micrometer (a millionth of a meter, or a ten thousandth of a centimeter). Another effect is that due to the sun being powered by nuclear fusion, it's continuously losing mass, causing earth's orbit to widen. However, over the lifetime of the sun, about 10 billion years, it will only lose about 0.1% of its mass, causing the earth to move out by a total of about 150,000km, that is, about 1.5cm/year. So, we're not breaking loose anytime soon.

[MadMojoMonkey]

... wouldn't even a slight change in velocity, even the tiniest bit of acceleration or deceleration cause the planet to either get loose or crash?

Most accelerations would only result in a different shape of orbit (a change in the eccentricity of the ellipse), and a different orbital period. Only when 2 bodies are moving near their escape velocity and some interaction accelerates them to or above the escape velocity would they break orbit. Anything which reduces the relative speed would result in another "stable" orbit, or a collision.

Well, it's a logical fallacy. The ones with the incorrect speed have indeed already left the orbits or will do so later.

There is a lot of random motion is the early formation of a stellar system, and the localizing of the masses causes more and more instability in the surrounding areas. Planets "clear" their orbits, either by accretion, or by "swinging" smaller bodies away from the orbit. Most of these swings leave the smaller body moving out of the solar system.

None of the orbits are stable

The notion of stability is clear in the 2 body problem, but that's not the whole picture. There are thousands of bodies in the solar system. That's not really the issue here, as you've stated. The real issue is that we've been talking about stability without giving a time frame.

"In the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than 10–20 million years, calculations become speculative: Pluto's tiny size makes it sensitive to small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit." ... "This does not mean Pluto's orbit itself is unstable, but its position on that orbit is impossible to determine so far ahead."
- Wikipedia "Pluto"

Your info about the orbits of the moon and Earth are solid, to my knowledge.

Another effect is that due to the sun being powered by nuclear fusion, it's continuously losing mass.

The mass loss is not strictly due to fusion, merely that its temperature is above absolute zero is enough. Anything with a non-zero temperature is radiating heat (by way of releasing photons through blackbody radiation). Heat is a measure of energy. The object releases energy, therefore the object loses mass, QED.

The mass loss rate of the sun, based on E = mc^2 (m = E/c^2), and dividing both sides of the equation by time is:

Solar photon energy output at Earth: 1,368 W/m^2
Earth's mean orbital distance: 149.6 x 10^9 m
Surface area of a sphere with that radius: 4*pi*r^2 = 2.812 x 10^23 m^2
Total photon power output of the sun: 1,368 W/m^2 x 2.812 x 10^23 m^2 = 3.847 x 10^26 W
Speed of light: 2.998 x 10^8 m/s
Speed of light, squared: 8.988 x 10^16 m^2/s^2
Solar mass loss rate: 3.847 x 10^26 W / 8.988 x 10^16 m^2/s^2 = 4.281 x 10^9 [units]
W = J/s = kg m^2 / s^3 ; [units] = kg/s

Solar mass loss rate: 4.281 x 10^9 kg/s

That's over 4 billion kilograms per second in light alone! Oh yeah, the sun's mass is roughly 2 x 10^30 kg, so dropping 10^9 kg/s is nothing at all.

The fusion processes release veritable torrents of neutrinos, which also contribute to the mass loss rate. Neutrinos are incredibly abundant, but are nearly mass-less and interact so weakly that they are difficult to detect. In fact, billions of neutrinos have sped through your body at nearly the speed of light during the time it took you to read this sentence. If it's night time, those neutrinos passed through the planet before they passed through you. How cool is that!?!

Even still, as you've stated, the total mass loss rate is negligible when compared to the sun's mass. More relevant is the natural evolution of a star like our sun. While remaining constant in temperature, the sun's size and luminosity increase over time. In about 1 billion years, this increase will be enough to boil the earth's oceans.

... but what if global warming is a fraud, and we make the world a better place for nothing?

[Icanhastreebet]

That's all quite intuitive. What isn't however, is that wouldn't even a slight change in velocity, even the tiniest bit of acceleration or deceleration cause the planet to either get loose or crash? How come this doesn't happen? What creates this universal (ha!) tendency for all objects to just play nice and circle each other at exactly the right speed?

Dunno if this was answered, if an object speeds up it just moves into a larger orbit(this is happening to the moon as we speak). If it slows down it goes into a smaller orbit. This happens with ~meter sized objects in the asteroid belt so their is literally almost no meter sized objects in the belt. Basically as if the object rotates the same way it goes in revolution it'll be pushed out because the object is the warmest on the rear side(This works like a mini rocket). If it rotates the opposite way of it's orbit it slows down and falls toward the sun.

So basically everything is going at "exactly" the right speed because if it was going any slower/faster it would be in a different orbit where it would be exactly the right speed.

Also no one mentioned the earth formed within 100k years after the sun and the whole solar system was VERY stable after the first few 100k years maybe even sooner. That's the best guess anyway from my understanding.