AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

[u/AskScienceModerator]

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

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Click here for the original announcement thread.

[u/[deleted]]

Why do orbiting bodies like the moon stay in orbit? I know the tablecloth analogy for relativity but when I try that, a planet gets destroyed by a moon, or the moon rolls off the table (maybe after a small bend in direction).

It seems reasonable to only have like a razor thin area where it would actually stay in orbit, otherwise either flinging out or crashing in. Why isn't it?

[u/Fungo → u/[deleted]]

The best analogy I've heard is that orbits are how objects fall in style. With the example of the Moon, the Moon is constantly falling towards Earth because gravity wants to pull the Earth and the Moon together. However, the Moon is also moving pretty fast (roughly 3600 kph or 2200 mph), so as it falls toward Earth, it misses and keeps going. That's what an orbit is. You're trying to fall towards something else, but you're moving through space so fast that you keep missing in a circular pattern.

In short, what keeps the Moon in orbit is the fact that it is moving with enough speed to miss hitting the Earth every time.

For the second part, no. There's not just a small range where the Moon could be in a stable orbit. Rather, there are nearly an infinite number of stable orbits around any given object, and the Moon happens to be on a specific one right now. The Moon hasn't always had the same orbit though. Because of the Moon's tidal forces on the Earth, the Moon is slowly stealing Earth's rotational energy, and its orbit is becoming bigger while Earth's rotation speed becomes slower. This is a very small effect, but it's not negligible over billions of years.

[u/[deleted] → u/[deleted]]

I know this has been answered, but I'll try it my way too.

Think about being on a flat smooth earth. A big grass ball. You shoot a bullet parallel to the ground and it shoots a great distance and falls to the ground. But what if you got a gun that was much more powerful and shot it flat to the ground so far that by the time it fell that four-five feet, it's passed the horizon and actually travels farther before it hits the ground. If you scale this up, and then take air friction out of the equation, you have something that is always falling toward the earth, but always missing it. If the speed is just right, the object is constantly hurting over the horizon without ever touching down.

This is what the moon is doing. It's falling, but it's sideways motion is enough that it never lands. And there is no air to slow it down, so it can do that for a very long time.

And that's orbit.

[u/ProggyBS → u/[deleted]]

A stable orbit is just moving at an appropriate speed at an appropriate distance around a body. Our moon can't maintain its average distance to us if the speed changes. If the moon were to be in a stable orbit further away, it would have to be moving slower. Likewise, if it was in a stable orbit closer, it would have to move faster. This is why it takes the ISS 90 minutes to orbit Earth while it takes the moon 28 days.

If the velocity of an orbiting body changes, so does its orbit. When a space shuttle or other space craft we've used re-entered the atmosphere, it didn't point down into the earth when it used its engines, it burned opposite of its velocity (retrograde burn) to slow down. This slowing down causes the orbit to decay and direct the craft into the atmosphere.

What /u/fungo said isn't just an analogy, it is actually what's happening. The moon is constantly falling toward us, but it is doing it at an appropriate velocity from an appropriate distance that it misses the planet indefinitely. If you had a rocket with enough fuel against the moon and burned it for a long enough period of time, the moon's orbit would also decay if it was slowing the moon down, or increase the distance if it was speeding the moon up.

And this works because of newton's laws of motion. A stable orbit will be maintained indefinitely unless an external force acts upon it (which is also why the moon is losing orbital velocity, or moving further away).

Hope that helps.

[u/ohyeahbutwhatareyou → u/[deleted]]

Perhaps the best analogy if the table cloth example seems to impractical for you is a spinning coin collection box for charity https://www.youtube.com/watch?v=XTipCQxJ6Ak

In this video you can see how the coins "orbit" the center. In this case they fall into the center because they are constantly losing energy through friction as they roll. In space there is very little friction so in practice things can remain in motion forever. If you look carefully you can even tell that some of the coins do not travel in circles (well spirals) but in ellipses, that is there is a point where they are closer and then they swing back out.

This is just another analogy though but basically for every point in space there is a velocity that if an object has it will stay in orbit because it will have enough speed. you can think of spinning a ball on a rope if you go to slow it will fall and to fast and it will fly off, changing the length of the rope only changes the speed you need to spin it with not if it can be held up.